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2009-09-03
|
Simulating the effect of AGN feedback on the metal enrichment of galaxy clusters
|
We present a study of the effect of AGN feedback on metal enrichment and
thermal properties of the intracluster medium (ICM) in hydrodynamical
simulations. The cosmological simulations are performed for a set of clusters
using a version of the TreePM-SPH Gadget code that follows chemo-dynamical
evolution by accounting for metal enrichment by different stellar populations.
Besides runs not including any efficient form of energy feedback, we carry out
simulations including: (i) kinetic feedback in the form of galactic winds
triggered by supernova explosions; (ii) AGN feedback from gas accretion onto
super-massive black holes (BHs); (iii) AGN feedback in which a 'radio mode' is
included. We find that AGN feedback is able to quench star formation in the
brightest cluster galaxies at z<4 and provides correct temperature profiles in
the central regions of galaxy groups. However, its effect is not sufficient to
create cool cores in massive clusters. AGN feedback creates a widespread
enrichment in the outskirts of clusters, thanks to its efficiency in displacing
enriched gas from galactic halos to the inter-galactic medium at relatively
high redshift. Iron abundance profiles are in better agreement with
observations, with a more pristine enrichment of the ICM around and beyond the
cluster virial regions. From the pattern of the relative abundances of Silicon
and Iron, we conclude that a significant fraction of ICM enrichment in
simulations is contributed by a diffuse population of intra-cluster stars. Our
simulations also predict that profiles of Z_Si/Z_Fe abundance ratio do not
increase at least out to 0.5 R_vir. Our results clearly show that different
sources of energy feedback leave distinct imprints in the enrichment pattern of
the ICM, that are more evident when looking at cluster external regions.
|
0909.0664v1
|
2009-12-31
|
Production of the p-Process Nuclei in the Carbon-Deflagration Model for Type Ia Supernovae
|
We calculate nucleosynthesis of proton-rich isotopes in the
carbon-deflagration model for Type Ia supernovae (SNe Ia). The seed abundances
are obtained by calculating the s-process nucleosynthesis that is expected to
occur in the repeating helium shell flashes on the carbon-oxygen (CO) white
dwarf (WD) during mass accretion from a binary companion. When the deflagration
wave passes through the outer layer of the CO WD, p-nuclei are produced by
photodisintegration reactions on s-nuclei in a region with the peak temperature
ranging from 1.9 to 3.6 x10^9 K. We confirm the sensitivity of the p-process on
the initial distribution of s-nuclei. We show that the initial C/O ratio in the
WD does not much affect the yield of p-nuclei. On the other hand, the abundance
of 22Ne left after the s-processing has a large influence on the p-process via
22Ne(alpha,n) reaction. We find that about 50% of p-nuclides are co-produced
when normalized to their solar abundances in all adopted cases of seed
distribution. Mo and Ru, which are largely underproduced in Type II supernovae
(SNe II), are produced more than in SNe II although they are underproduced with
respect to the yield levels of other p-nuclides. The ratios between p-nuclei
and iron in the ejecta are larger than the solar ratios by a factor of 1.2. We
also compare the yields of oxygen, iron, and p-nuclides in SNe Ia and SNe II
and suggest that SNe Ia could make a larger contribution than SNe II to the
solar system content of p-nuclei.
|
1001.0145v3
|
2010-01-19
|
Analyzing the prices of the most expensive sheet iron all over the world: Modeling, prediction and regime change
|
The private car license plates issued in Shanghai are bestowed the title of
"the most expensive sheet iron all over the world", more expensive than gold. A
citizen has to bid in an monthly auction to obtain a license plate for his new
private car. We perform statistical analysis to investigate the influence of
the minimal price $P_{\min}$ of the bidding winners, the quota $N_{\rm{quota}}$
of private car license plates, the number $N_{\rm{bidder}}$ of bidders, as well
as two external shocks including the legality debate of the auction in 2004 and
the auction regime reform in January 2008 on the average price $P_{\rm{mean}}$
of all bidding winners. It is found that the legality debate of the auction had
marginal transient impact on the average price in a short time period. In
contrast, the change of the auction rules has significant permanent influence
on the average price, which reduces the price by about 3020 yuan Renminbi. It
means that the average price exhibits nonlinear behaviors with a regime change.
The evolution of the average price is independent of the number
$N_{\rm{bidder}}$ of bidders in both regimes. In the early regime before
January 2008, the average price $P_{\rm{mean}}$ was influenced only by the
minimal price $P_{\min}$ in the preceding month with a positive correlation. In
the current regime since January 2008, the average price is positively
correlated with the minimal price and the quota in the preceding month and
negatively correlated with the quota in the same month. We test the predictive
power of the two models using 2-year and 3-year moving windows and find that
the latter outperforms the former. It seems that the auction market becomes
more efficient after the auction reform since the prediction error increases.
|
1001.3176v1
|
2010-02-12
|
Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756
|
Classical chemical analyses may be affected by systematic errors that would
cause observed abundance differences between dwarfs and giants. For some
elements, however, the abundance difference could be real. We address the issue
by observing 2 solar--type dwarfs in NGC 5822 and 3 in IC 4756, and comparing
their composition with that of 3 giants in either of the aforementioned
clusters. We determine iron abundance and stellar parameters of the dwarf
stars, and the abundances of calcium, sodium, nickel, titanium, aluminium,
chromium, silicon and oxygen for both the giants and dwarfs. We acquired UVES
high-resolution, of high signal--to--noise ratio (S/N) spectra. The width of
the cross correlation profiles was used to measure rotation velocities. For
abundance determinations, the standard equivalent width analysis was performed
differentially with respect to the Sun. For lithium and oxygen, we derived
abundances by comparing synthetic spectra with observed line features. We find
an iron abundance for dwarf stars equal to solar to within the margins of error
for IC 4756, and slightly above for NGC 5822 ([Fe/H]= 0.01 and 0.05 dex
respectively). The 3 stars in NG 4756 have lithium abundances between Log N(Li)
2.6 and 2.8 dex, the two stars in NGC 5822 have Log N(Li) ~ 2.8 and 2.5,
respectively. For sodium, silicon, and titanium, we show that abundances of
giants are significantly higher than those of the dwarfs of the same cluster
(about 0.15, 0.15, and 0.35 dex).
|
1002.2547v3
|
2010-03-18
|
High magnetic field scales and critical currents in SmFeAs(O,F) crystals: promising for applications
|
Superconducting technology provides most sensitive field detectors, promising
implementations of qubits and high field magnets for medical imaging and for
most powerful particle accelerators. Thus, with the discovery of new
superconducting materials, such as the iron pnictides, exploring their
potential for applications is one of the foremost tasks. Even if the critical
temperature Tc is high, intrinsic electronic properties might render
applications rather difficult, particularly if extreme electronic anisotropy
prevents effective pinning of vortices and thus severely limits the critical
current density, a problem well known for cuprates. While many questions
concerning microscopic electronic properties of the iron pnictides have been
successfully addressed and estimates point to a very high upper critical field,
their application potential is less clarified. Thus we focus here on the
critical currents, their anisotropy and the onset of electrical dissipation in
high magnetic fields up to 65 T. Our detailed study of the transport properties
of optimally doped SmFeAs(O,F) single crystals reveals a promising combination
of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities
along all crystal directions. This favorable intragrain current transport in
SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a
crucial requirement for possible applications. Essential in these experiments
are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with
sub-\mu\m^2 cross-section, with current along and perpendicular to the
crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed
magnetic fields. The pinning forces have been characterized by scaling the
magnetically measured "peak effect".
|
1003.3612v2
|
2010-03-24
|
Abundances of C, N, Sr and Ba on the red giant branch of omega Centauri
|
Abundances relative to iron for carbon, nitrogen, strontium and barium are
presented for 33 stars on the red giant branch of the globular cluster omega
Centauri. They are based on intermediate-resolution spectroscopic data covering
the blue spectral region analyzed using spectrum synthesis techniques. The data
reveal the existence of a broad range in the abundances of these elements, and
a comparison with similar data for main sequence stars enables insight into the
evolutionary history of the cluster. The majority of the red giant branch stars
were found to be depleted in carbon, i.e. [C/Fe]<0, while [N/Fe] for the same
stars shows a range of ~1 dex, from [N/Fe]~0.7 to 1.7 dex. The
strontium-to-iron abundance ratios varied from solar to mildly enhanced
(0.0<=[Sr/Fe]<=0.8), with [Ba/Fe] generally equal to or greater than [Sr/Fe].
The carbon and nitrogen abundance ratios for the one known CH star in the
sample, ROA 279, are [C/Fe]=0.6 and [N/Fe]=0.5 dex. Evidence for evolutionary
mixing on the red giant branch is found from the fact that the relative carbon
abundances on the main sequence are generally higher than those on the red
giant branch. However, comparison of the red giant branch and main sequence
samples shows that the upper level of nitrogen enhancement is similar in both
sets at [N/Fe]~2.0dex. This is most likely the result of primordial rather than
evolutionary mixing processes. One red giant branch star, ROA 276, was found to
have Sr and Ba abundance ratios. High resolution spectra of ROA 276 were
obtained with the Magellan Telescope/MIKE spectrograph combination to confirm
this result, revealing that ROA 276 is indeed an unusual star. For this star
calculations of the depletion effect strongly suggest that the observed Sr
enhancement in ROA 276 is of primordial origin, rather than originating from a
surface accretion event.
|
1003.4661v2
|
2010-04-12
|
A self-consistent approach to the hard and soft states of 4U 1705-44
|
We analyzed two XMM-Newton observations of the bright atoll source 4U
1705-44, which can be considered a prototype of the class of the persistent NS
LMXBs showing both hard and soft states. The first observation was performed
when the source was in a hard low flux state, the second during a soft,
high-flux state. Both the spectra show broad iron emission lines. We fit the
spectra using a two-component model, together with a reflection model
specifically suited to the case of a neutron star, where the incident spectrum
has a blackbody shape. In the soft state, the reflection model, convolved with
a relativistic smearing component, consistently describes the broad features
present in the spectrum, and we find a clear relation between the temperature
of the incident flux and the temperature of the harder X-ray component that we
interpret as the boundary layer emission. In this state we find converging
evidence that the boundary layer outer radius is ~ 2 times the neutron star
radius. In the low flux state, we observe a change in the continuum shape of
the spectrum with respect to the soft state. Still, the broad local emission
features can be associated with a disk reflecting matter, but in a lower
ionization state, and possibly produced in an accretion disk truncated at
greater distance. Our analysis provides strong evidence that the reflection
component in soft states of LMXBs comes from to hard X-ray thermal irradiation,
which we identify with the boundary layer emission, also present in the
continuum model. In the hard state, the broad iron line if also produced by
reflection, and the continuum disk emission can be self-consistently accounted
if the disk is truncated at a greater distance than the soft state.
|
1004.1963v2
|
2010-04-13
|
Analysis of optical Fe II emission in a sample of AGN spectra
|
We present a study of optical Fe II emission in 302 AGNs selected from the
SDSS. We group the strongest Fe II multiplets into three groups according to
the lower term of the transition (b $^4$F, a $^6$S and a $^4$G terms). These
correspond approximately to the blue, central, and red part respectively of the
"iron shelf" around Hb. We calculate an Fe II template which takes into account
transitions into these three terms and an additional group of lines, based on a
reconstruction of the spectrum of I Zw 1. This Fe II template gives a more
precise fit of the Fe II lines in broad-line AGNs than other templates. We
extract Fe II, Ha, Hb, [O III] and [N II] emission parameters and investigate
correlations between them. We find that Fe II lines probably originate in an
Intermediate Line Region. We notice that the blue, red, and central parts of
the iron shelf have different relative intensities in different objects. Their
ratios depend on continuum luminosity, FWHM Hb, the velocity shift of Fe II,
and the Ha/Hb flux ratio. We examine the dependence of the well-known
anti-correlation between the equivalent widths of Fe II and [O III] on
continuum luminosity. We find that there is a Baldwin effect for [O III] but an
inverse Baldwin effect for the Fe II emission. The [O III]/Fe II ratio thus
decreases with L\lambda5100. Since the ratio is a major component of the
Boroson and Green eigenvector 1, this implies a connection between the Baldwin
effect and eigenvector 1, and could be connected with AGN evolution. We find
that spectra are different for Hb FWHMs greater and less than ~3000 km/s, and
that there are different correlation coefficients between the parameters.
|
1004.2212v2
|
2010-04-20
|
Convective shifts of iron lines in the spectrum of the solar photosphere
|
The influence of the convective structure of the solar photosphere on the
shifts of spectral lines of iron was studied. Line profiles in the visible and
infrared spectrum were synthesized with the use of 2-D time-dependent
hydrodynamic solar model atmospheres. The dependence of line shifts on
excitation potential, wavelength, and line strength was analyzed, along with
the depression contribution functions. The line shifts were found to depend on
the location of the line formation region in convective cells and the
difference between the line depression contributions from granules and
intergranular lanes. In visible spectrum the weak and moderate lines are formed
deep in the photosphere. Their effective line formation region is located in
the central parts of granules, which make the major contribution to the
absorption of spatially unresolved lines. The cores of strong lines are formed
in upper photospheric layers where is formed reversed granulation due to
convection reversal and physical conditions change drastically there. As a
consequence the depression contributions in the strong line from intergranular
lanes with downflows substantially increase. This accounts for smaller blue
shifts of strong lines. In infrared spectrum the observed decrease in the blue
line shifts is explained by the fact that their effective line formation
regions lie higher in the photosphere and extend much further into the reversed
granulation region due to the line opacity rise with the increase of line
wavelength. Additionally the effective line formation depths of the synthesized
visible and infrared Fe I lines and their dependence on line parameters is
discussed.
|
1004.3438v1
|
2010-04-21
|
Nodes in the gap structure of the iron-arsenide superconductor Ba(Fe_{1-x}Co_x)_2As_2 from c-axis heat transport measurements
|
The thermal conductivity k of the iron-arsenide superconductor
Ba(Fe_{1-x}Co_x)_2As_2 was measured down to 50 mK for a heat current parallel
(k_c) and perpendicular (k_a) to the tetragonal c axis, for seven Co
concentrations from underdoped to overdoped regions of the phase diagram (0.038
< x < 0.127). A residual linear term k_c0/T is observed in the T = 0 limit when
the current is along the c axis, revealing the presence of nodes in the gap.
Because the nodes appear as x moves away from the concentration of maximal T_c,
they must be accidental, not imposed by symmetry, and are therefore compatible
with an s_{+/-} state, for example. The fact that the in-plane residual linear
term k_a0/T is negligible at all x implies that the nodes are located in
regions of the Fermi surface that contribute strongly to c-axis conduction and
very little to in-plane conduction. Application of a moderate magnetic field
(e.g. H_c2/4) excites quasiparticles that conduct heat along the a axis just as
well as the nodal quasiparticles conduct along the c axis. This shows that the
gap must be very small (but non-zero) in regions of the Fermi surface which
contribute significantly to in-plane conduction. These findings can be
understood in terms of a strong k dependence of the gap Delta(k) which produces
nodes on a Fermi surface sheet with pronounced c-axis dispersion and deep
minima on the remaining, quasi-two-dimensional sheets.
|
1004.3804v1
|
2010-05-20
|
Co-evolution of atmospheres, life, and climate
|
After Earth's origin, our host star, the Sun, was shining 20 to 25 percent
less brightly than today. Without greenhouse-like conditions to warm the
atmosphere, our early planet would have been an ice ball and life may never
have evolved. But life did evolve, which indicates that greenhouse gases must
have been present on early Earth to warm the planet. Evidence from the geologic
record indicates an abundance of the greenhouse gas CO2. CH4 was probably
present as well, and in this regard methanogenic bacteria, which belong to a
diverse group of anaerobic procaryotes that ferment CO 2 plus H2 to CH4, may
have contributed to modification of the early atmosphere. Molecular oxygen was
not present, as is indicated by the study of rocks from that era, which contain
iron carbonate rather than iron oxide. Multicellular organisms originated as
cells within colonies that became increasingly specialized. The development of
photosynthesis allowed the Sun's energy to be harvested directly by life forms.
The resultant oxygen accumulated in the atmosphere and formed the ozone layer
in the upper atmosphere. Aided by the absorption of harmful UV radiation in the
ozone layer, life colonized Earth's surface. Our own planet is a very good
example of how life forms modified the atmosphere over the planets' life time.
We show that these facts have to be taken into account when we discover and
characterize atmospheres of Earth-like exoplanets. If life has originated and
evolved on a planet, then it should be expected that a strong co-evolution
occurred between life and the atmosphere, the result of which is the planets'
climate.
|
1005.3589v1
|
2010-06-04
|
Spitzer spectral line mapping of the HH211 outflow
|
Aims: We employ archival Spitzer slit-scan observations of the HH211 outflow
in order to investigate its warm gas content, assess the jet mass flux in the
form of H2 and probe for the existence of an embedded atomic jet. Methods:
Detected molecular and atomic lines are interpreted by means of emission line
diagnostics and an existing grid of molecular shock models. The physical
properties of the warm gas are compared against other molecular jet tracers and
to the results of a similar study towards the L1448-C outflow. Results: We have
detected and mapped the v=0-0 S(0) - S(7) H2 lines and fine-structure lines of
S, Fe+, and Si+. H2 is detected down to 5" from the source and is characterized
by a "cool" T~300K and a "warm" T~1000 K component, with an extinction Av ~ 8
mag. The amount of cool H2 towards the jet agrees with that estimated from CO
assuming fully molecular gas. The warm component is well fitted by C-type
shocks with a low beam filling factor ~ 0.01-0.04 and a mass-flux similar to
the cool H2. The fine-structure line emission arises from dense gas with
ionization fraction ~0.5 - 5 x 10e-3, suggestive of dissociative shocks. Line
ratios to sulfur indicate that iron and silicon are depleted compared to solar
abundances by a factor ~10-50. Conclusions: Spitzer spectral mapping
observations reveal for the first time a cool H$_2$ component towards the CO
jet of HH211 consistent with the CO material being fully molecular and warm at
~ 300 K. The maps also reveal for the first time the existence of an embedded
atomic jet in the HH211 outflow that can be traced down to the central source
position. Its significant iron and silicon depletion excludes an origin from
within the dust sublimation zone around the protostar. The momentum-flux seems
insufficient to entrain the CO jet, although current uncertainties on jet speed
and shock conditions are too large for a definite conclusion.
|
1006.0821v1
|
2010-06-07
|
Evidence for a Truncated Accretion Disc in the Low Luminosity Seyfert Galaxy, NGC 7213?
|
We present the broad-band 0.6-150 keV Suzaku and Swift BAT spectra of the low
luminosity Seyfert galaxy, NGC 7213. The time-averaged continuum emission is
well fitted by a single powerlaw of photon index Gamma = 1.75 and from
consideration of the Fermi flux limit we constrain the high energy cutoff to be
350 keV < E < 25 MeV. Line emission from both near-neutral iron K_alpha at 6.39
keV and highly ionised iron, from Fe_(xxv) and Fe_(xxvi), is strongly detected
in the Suzaku spectrum, further confirming the results of previous observations
with Chandra and XMM-Newton. We find the centroid energies for the Fe_(xxv) and
Fe_(xxvi) emission to be 6.60 keV and 6.95 keV respectively, with the latter
appearing to be resolved in the Suzaku spectrum. We show that the Fe_(xxv) and
Fe_(xxvi) emission can result from a highly photo-ionised plasma of column
density N_(H) ~ 3 x 10^(23) cm^(-2). A Compton reflection component, e.g.,
originating from an optically-thick accretion disc or a Compton-thick torus,
appears either very weak or absent in this AGN, subtending < 1 sr to the X-ray
source, consistent with previous findings. Indeed the absence of either neutral
or ionised Compton reflection coupled with the lack of any relativistic Fe K
signatures in the spectrum suggests that an inner, optically-thick accretion
disc is absent in this source. Instead, the accretion disc could be truncated
with the inner regions perhaps replaced by a Compton-thin Radiatively
Inefficient Accretion Flow. Thus, the Fe_(xxv) and Fe_(xxvi) emission could
both originate in ionised material perhaps at the transition region between the
hot, inner flow and the cold, truncated accretion disc on the order of 10^(3) -
10^(4) gravitational radii from the black hole. The origin for the unresolved
neutral Fe K_alpha emission is then likely to be further out, perhaps
originating in the optical BLR or a Compton-thin pc-scale torus.
|
1006.1318v1
|
2010-06-10
|
Fe II Emission in AGN: The Role of Total and Gas-Phase Iron Abundance
|
Active galactic nuclei (AGN) have Fe II emission from the broad line region
(BLR) that differs greatly in strength from object to object. We examine the
role of the total and gas-phase iron abundance in determining Fe II strength.
Using AGN spectra from the Sloan Digital Sky Survey (SDSS) in the redshift
range of 0.2 < z < 0.35, we measure the Fe/Ne abundance of the narrow line
region (NLR) using the [Fe VII]/[Ne V] line intensity ratio. We find no
significant difference in the abundance of Fe relative to Ne in the NLR as a
function of Fe II/Hbeta. However, the N II/S II ratio increases a by a factor
of 2 with increasing Fe II strength. This indicates a trend in N/S abundance
ratio, and by implication in the overall metallicity of the NLR gas, with
increasing Fe II strength. We propose that the wide range of Fe II strength in
AGN largely results from the selective depletion of Fe into grains in the low
ionization portion of the BLR. Photoionization models show that the strength of
the optical Fe II lines varies almost linearly with gas-phase Fe abundance,
while the ultraviolet Fe II strength varies more weakly. Interstellar depletion
of Fe can be as large as two orders of magnitude, sufficient to explain the
wide range of optical Fe II strength in AGN. This picture is consistent with
the similarity of the BLR radius to the dust sublimation radius and with
indications of Fe II emitting gas flowing inwards from the dusty torus.
|
1006.2043v2
|
2010-07-22
|
Neutron-rich chromium isotope anomalies in supernova nanoparticles
|
Neutron-rich isotopes with masses near that of iron are produced in type Ia
and II supernovae. Traces of such nucleosynthesis are found in primitive
meteorites in the form of variations in the isotopic abundance of 54Cr, the
most neutron-rich stable isotope of chromium. The hosts of these isotopic
anomalies must be presolar grains that condensed in the outflows of supernovae,
offering the opportunity to study the nucleosynthesis of iron-peak nuclei in
ways that complement spectroscopic observations and can inform models of
stellar evolution. However, despite almost two decades of extensive search, the
carrier of 54Cr anomalies is still unknown, presumably because it is
fine-grained and is chemically labile. Here we identify in the primitive
meteorite Orgueil the carrier of 54Cr-anomalies as nanoparticles, most likely
spinels that show large enrichments in 54Cr relative to solar composition
(54Cr/52Cr ratio >3.6xsolar). Such large enrichments in 54Cr can only be
produced in supernovae. The mineralogy of the grains supports condensation in
the O/Ne-O/C zones of a type II supernova, although a type Ia origin cannot be
excluded. We suggest that planetary materials incorporated different amounts of
these nanoparticles, possibly due to late injection by a nearby supernova that
also delivered 26Al and 60Fe to the solar system. This idea explains why the
relative abundance of 54Cr and other neutron-rich isotopes vary between planets
and meteorites. We anticipate that future isotopic studies of the grains
identified here will shed new light on the birth of the solar system and the
conditions insupernovae.
|
1007.4016v1
|
2010-07-29
|
The Mouse that Roared: A Superflare from the dMe Flare Star EV Lac detected by Swift and Konus-Wind
|
We report on a large stellar flare from the nearby dMe flare star EV Lac
observed by the Swift and Konus-Wind satellites and the Liverpool Telescope. It
is the first large stellar flare from a dMe flare star to result in a Swift
trigger based on its hard X-ray intensity. Its peak f_X from 0.3--100 keV of
5.3x10^-8 erg/cm2/s is nearly 7000 times larger than the star's quiescent
coronal flux, and the change in magnitude in the white filter is >4.7. This
flare also caused a transient increase in EV Lac's bolometric luminosity
(L_bol) during the early stages of the flare, with a peak estimated L_X/L_bol
~3.1. We apply flare loop hydrodynamic modeling to the plasma parameter
temporal changes to derive a loop semi-length of l/Rstar =0.37 +/-0.07. The
soft X-ray spectrum of the flare reveals evidence of iron Kalpha emission at
6.4 keV. We model the Kalpha emission as fluorescence from the hot flare source
irradiating the photospheric iron, and derive loop heights of h/Rstar=0.1,
consistent within factors of a few with the heights inferred from hydrodynamic
modeling. The Kalpha emission feature shows variability on time scales of ~200
s which is difficult to interpret using the pure fluorescence hypothesis. We
examine Kalpha emission produced by collisional ionization from accelerated
particles, and find parameter values for the spectrum of accelerated particles
which can accommodate the increased amount of Kalpha flux and the lack of
observed nonthermal emission in the 20-50 keV spectral region.
|
1007.5300v1
|
2010-08-18
|
Optical properties of the iron-arsenic superconductor BaFe1.85Co0.15As2
|
The transport and complex optical properties of the electron-doped
iron-arsenic superconductor BaFe1.85Co0.15As2 with Tc = 25 K have been examined
in the Fe-As planes above and below Tc. A Bloch-Gruneisen analysis of the
resistivity yields a weak electron-phonon coupling constant lambda_ph ~ 0.2.
The low-frequency optical response in the normal state appears to be dominated
by the electron pocket and may be described by a weakly-interacting Fermi
liquid with a Drude plasma frequency of omega_p,D ~ 7840 cm-1 (~ 0.972 eV) and
scattering rate 1/tau_D ~ 125 cm-1 (~ 15 meV) just above Tc. The
frequency-dependent scattering rate 1/tau(omega) has kinks at ~ 12 and 55 meV
that appear to be related to bosonic excitations. Below Tc the majority of the
superconducting plasma frequency originates from the electron pocket and is
estimated to be omega_p,S ~ 5200 cm-1 (lambda0 ~ 3000 Angstroms) for T << Tc,
indicating that less than half the free carriers in the normal state have
collapsed into the condensate, suggesting that this material is not in the
clean limit. Supporting this finding is the observation that this material
falls close to the universal scaling line for a BCS dirty-limit superconductor
in the weak-coupling limit. There are two energy scales for the
superconductivity in the optical conductivity and photo-induced reflectivity at
Delta1 ~ 3.1 +/- 0.2 meV and Delta2 ~ 7.4 +/- 0.3 meV. This corresponds to
either the gaping of the electron and hole pockets, respectively, or an
anisotropic s-wave gap on the electron pocket; both views are consistent with
the s+/- model.
|
1008.3098v2
|
2010-08-18
|
Cold fronts and multi-temperature structures in the core of Abell 2052
|
The physics of the coolest phases in the hot Intra-Cluster Medium (ICM) of
clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the
central Active Galactic Nucleus (AGN) contain enough energy to heat the
surrounding gas and stop cooling, but locally blobs or filaments of gas appear
to be able to cool to low temperatures of 10^4 K. In X-rays, however, gas with
temperatures lower than 0.5 keV is not observed. Using a deep XMM-Newton
observation of the cluster of galaxies Abell 2052, we derive 2D maps of the
temperature, entropy, and iron abundance in the core region. About 130 kpc
South-West of the central galaxy, we discover a discontinuity in the surface
brightness of the hot gas which is consistent with a cold front. Interestingly,
the iron abundance jumps from ~0.75 to ~0.5 across the front. In a smaller
region to the North-West of the central galaxy we find a relatively high
contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below
that temperature. However, the region appears to be associated with much cooler
H-alpha filaments in the optical waveband. The elliptical shape of the cold
front in the SW of the cluster suggests that the front is caused by sloshing of
the hot gas in the clusters gravitational potential. This effect is probably an
important mechanism to transport metals from the core region to the outer parts
of the cluster. The smooth temperature profile across the sharp jump in the
metalicity indicates the presence of heat conduction and the lack of mixing
across the discontinuity. The cool blob of gas NW of the central galaxy was
probably pushed away from the core and squeezed by the adjacent bubble, where
it can cool efficiently and relatively undisturbed by the AGN. Shock induced
mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively
and explain our non-detection of gas below 0.5 keV.
|
1008.3109v2
|
2010-08-18
|
Open clusters towards the Galactic center: chemistry and dynamics. A VLT spectroscopic study of NGC6192, NGC6404, NGC6583
|
In the framework of the study of the Galactic metallicity gradient and its
time evolution, we present new high-resolution spectroscopic observations
obtained with FLAMES and the fiber link to UVES at VLT of three open clusters
(OCs) located within $\sim$7~kpc from the Galactic Center (GC): NGC~6192,
NGC~6404, NGC~6583. We also present new orbit determination for all OCs with
Galactocentric distances (R$_{\rm{GC}}) \leq$8~kpc and metallicity from
high-resolution spectroscopy. We aim to investigate the slope of the inner disk
metallicity gradient as traced by OCs and at discussing its implication on the
chemical evolution of our Galaxy. We have derived memberships of a group of
evolved stars for each clusters, obtaining a sample of 4, 4, and 2 member stars
in NGC~6192, NGC~6404, and NGC~6583, respectively. Using standard LTE analysis
we derived stellar parameters and abundance ratios for the iron-peak elements
Fe, Ni, Cr, and for the $\alpha$-elements Al, Mg, Si, Ti, Ca. We calculated the
orbits of the OCs currently located within 8~kpc from the GC, and discuss their
implication on the present-time radial location. {The average metallicities of
the three clusters are all oversolar: [Fe/H]= $+0.12\pm0.04$ (NGC~6192),
$+0.11\pm0.04$ (NGC 6404), $+0.37\pm0.03$ (NGC 6583). They are in qualitative
agreement with their Galactocentric distances, being all internal OCs, and thus
expected to be metal richer than the solar neighborhood. The abundance ratios
of the other elements over iron [X/Fe] are consistent with solar values. The
clusters we have analysed, together with other OC and Cepheid data, confirm a
steep gradient in the inner disk, a signature of an evolutionary rate different
than in the outer disk.
|
1008.3158v1
|
2010-10-01
|
Absorption Features in Spectra of Magnetized Neutron Stars
|
The X-ray spectra of some magnetized isolated neutron stars (NSs) show
absorption features with equivalent widths (EWs) of 50 - 200 eV, whose nature
is not yet well known. To explain the prominent absorption features in the soft
X-ray spectra of the highly magnetized (B ~ 10^{14} G) X-ray dim isolated NSs
(XDINSs), we theoretically investigate different NS local surface models,
including naked condensed iron surfaces and partially ionized hydrogen model
atmospheres, with semi-infinite and thin atmospheres above the condensed
surface. We also developed a code for computing light curves and integral
emergent spectra of magnetized neutron stars with various temperature and
magnetic field distributions over the NS surface. We compare the general
properties of the computed and observed light curves and integral spectra for
XDINS RBS\,1223 and conclude that the observations can be explained by a thin
hydrogen atmosphere above the condensed iron surface, while the presence of a
strong toroidal magnetic field component on the XDINS surface is unlikely. We
suggest that the harmonically spaced absorption features in the soft X-ray
spectrum of the central compact object (CCO) 1E 1207.4-5209 (hereafter 1E 1207)
correspond to peaks in the energy dependence of the free-free opacity in a
quantizing magnetic field, known as quantum oscillations. To explore observable
properties of these quantum oscillations, we calculate models of hydrogen NS
atmospheres with B ~ 10^{10} - 10^{11} G (i.e., electron cyclotron energy
E_{c,e} ~ 0.1 - 1 keV) and T_eff = 1 - 3 MK. Such conditions are thought to be
typical for 1E 1207. We show that observable features at the electron cyclotron
harmonics with EWs \approx 100 - 200 eV can arise due to these quantum
oscillations.
|
1010.0125v1
|
2010-10-21
|
The RAVE Survey: Rich in Very Metal-Poor Stars
|
Very metal-poor stars are of obvious importance for many problems in chemical
evolution, star formation, and galaxy evolution. Finding complete samples of
such stars which are also bright enough to allow high-precision individual
analyses is of considerable interest. We demonstrate here that stars with iron
abundances [Fe/H] < -2 dex, and down to below -4 dex, can be efficiently
identified within the Radial Velocity Experiment (RAVE) survey of bright stars,
without requiring additional confirmatory observations. We determine a
calibration of the equivalent width of the Calcium triplet lines measured from
the RAVE spectra onto true [Fe/H], using high spectral resolution data for a
subset of the stars. These RAVE iron abundances are accurate enough to obviate
the need for confirmatory higher-resolution spectroscopy. Our initial study has
identified 631 stars with [Fe/H] <= -2, from a RAVE database containing
approximately 200,000 stars. This RAVE-based sample is complete for stars with
[Fe/H] < -2.5, allowing statistical sample analysis. We identify three stars
with [Fe/H] <= -4. Of these, one was already known to be `ultra metal-poor',
one is a known carbon-enhanced metal-poor star, but we obtain [Fe/H]= -4.0,
rather than the published [Fe/H]=-3.3, and derive [C/Fe] = +0.9, and [N/Fe] =
+3.2, and the third is at the limit of our S/N. RAVE observations are on-going
and should prove to be a rich source of bright, easily studied, very metal-poor
stars.
|
1010.4491v1
|
2010-10-30
|
Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition
|
Nematicity, defined as broken rotational symmetry, has recently been observed
in competing phases proximate to the superconducting phase in the cuprate high
temperature superconductors. Similarly, the new iron-based high temperature
superconductors exhibit a tetragonal to orthorhombic structural transition
(i.e. a broken C4 symmetry) that either precedes or is coincident with a
collinear spin density wave (SDW) transition in undoped parent compounds, and
superconductivity arises when both transitions are suppressed via doping.
Evidence for strong in-plane anisotropy in the SDW state in this family of
compounds has been reported by neutron scattering, scanning tunneling
microscopy, and transport measurements. Here we present an angle resolved
photoemission spectroscopy study of detwinned single crystals of a
representative family of electron-doped iron-arsenide superconductors,
Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via
application of in-plane uniaxial stress, enabling measurements of single domain
electronic structure in the orthorhombic state. At low temperatures, our
results clearly demonstrate an in-plane electronic anisotropy characterized by
a large energy splitting of two orthogonal bands with dominant dxz and dyz
character, which is consistent with anisotropy observed by other probes. For
compositions x>0, for which the structural transition (TS) precedes the
magnetic transition (TSDW), an anisotropic splitting is observed to develop
above TSDW, indicating that it is specifically associated with TS. For
unstressed crystals, the band splitting is observed close to TS, whereas for
stressed crystals the splitting is observed to considerably higher
temperatures, revealing the presence of a surprisingly large in-plane nematic
susceptibility in the electronic structure.
|
1011.0050v2
|
2010-11-07
|
On the multi-orbital band structure and itinerant magnetism of iron-based superconductors
|
This paper explains the multi-orbital band structures and itinerant magnetism
of the iron-pnictide and chalcogenides. We first describe the generic band
structure of an isolated FeAs layer. Use of its Abelian glide-mirror group
allows us to reduce the primitive cell to one FeAs unit. From
density-functional theory, we generate the set of eight Fe $d$ and As $p$
localized Wannier functions for LaOFeAs and their tight-binding (TB)
Hamiltonian, $h(k)$. We discuss the topology of the bands, i.e. allowed and
avoided crossings, the origin of the d6 pseudogap, as well as the role of the
As $p$ orbitals and the elongation of the FeAs$_{4}$ tetrahedron. We then
couple the layers, mainly via interlayer hopping between As $p_{z}$ orbitals,
and give the formalism for simple and body-centered tetragonal stackings. This
allows us to explain the material-specific 3D band structures. Due to the high
symmetry, several level inversions take place as functions of $k_{z}$ or
pressure, resulting in linear band dispersions (Dirac cones). The underlying
symmetry elements are, however, easily broken, so that the Dirac points are not
protected, nor pinned to the Fermi level. From the paramagnetic TB Hamiltonian,
we form the band structures for spin spirals with wavevector $q$ by coupling
$h(k)$ and $h (k+q)$. The band structure for stripe order is studied as a
function of the exchange potential, $\Delta$, using Stoner theory. Gapping of
the Fermi surface (FS) for small $\Delta $ requires matching of FS dimensions
(nesting) and $d$-orbital characters. The origin of the propeller-shaped FS is
explained. Finally, we express the magnetic energy as the sum over
band-structure energies, which enables us to understand to what extent the
magnetic energies might be described by a Heisenberg Hamiltonian, and the
interplay between the magnetic moment and the elongation of the FeAs4
tetrahedron.
|
1011.1658v3
|
2010-12-02
|
Tracing the evolution of NGC6397 through the chemical composition of its stellar populations
|
With the aim to constrain multiple populations in the metal-poor globular
cluster NGC6397, we analyse and discuss the chemical compositions of a large
number of elements in 21 red giant branch stars. High-resolution spectra were
obtained with the FLAMES/UVES spectrograph on VLT. We have determined non-LTE
abundances of Na and LTE abundances for the remaining 21 elements, including O,
Mg, Al, alpha, iron-peak, and neutron-capture elements, many of which have not
previously been analysed for this cluster. We have also considered the
influence of possible He enrichment in the analysis of stellar spectra. We find
that the Na abundances of evolved, as well as unevolved, stars show a distinct
bimodality, which suggests the presence of two stellar populations; one
primordial stellar generation with composition similar to field stars, and a
second generation that is enriched in material processed through
hydrogen-burning (enriched in Na and Al and depleted in O and Mg). The cluster
is dominated (75%) by the second generation. The red giant branch show a
similar bimodal distribution in the Stroemgren colour index c_y=c_1-(b-y),
implying a large difference also in N abundance. The two populations have the
same composition of all analysed elements heavier than Al, within the
measurement uncertainty of the analysis, with the possible exception of [Y/Fe].
Using two stars with close to identical stellar parameters, one from each
generation, we estimate the difference in He content, Delta Y=0.01+-0.06, given
the assumption that the mass fraction of iron is the same for the stars.
Finally, we show that winds from fast rotating massive stars of the first
generation can be held responsible for the abundance patterns observed in
NGC6397 second generation long-lived stars and estimate that the initial mass
of the cluster were at least ten times higher than its present-day value.
|
1012.0477v2
|
2011-01-24
|
A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars
|
A comprehensive model atom for Fe with more than 3000 energy levels is
presented. As a test and first application of this model atom, Fe abundances
are determined for the Sun and five stars with well determined stellar
parameters and high-quality observed spectra. Non-LTE leads to systematically
depleted total absorption in the Fe I lines and to positive abundance
corrections in agreement with the previous studies, however, the magnitude of
non-LTE effect is smaller compared to the earlier results. Non-LTE corrections
do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient
stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars
HD 84937 and HD 122563, respectively, depending on the assumed efficiency of
collisions with hydrogen atoms. Based on the analysis of the Fe I/Fe II
ionization equilibrium in these two stars, we recommend to apply the Drawin
formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe II
lines, non-LTE corrections do not exceed 0.01 dex in absolute value. The solar
non-LTE abundance obtained from 54 Fe I lines is 7.56+-0.09 and the abundance
from 18 Fe II lines varies between 7.41+-0.11 and 7.56+-0.05 depending on the
source of the gf-values. Thus, gf-values available for the iron lines are not
accurate enough to pursue high-accuracy absolute abundance determinations.
Lines of Fe I give, on average, a 0.1 dex lower abundance compared to those of
Fe II lines for HD 61421 and HD 102870, even when applying a differential
analysis relative to the Sun. A disparity between Fe I and Fe II points to
problems of stellar atmosphere modelling or/and effective temperature
determination.
|
1101.4570v1
|
2011-01-25
|
On the Structure of Vacancy Ordered Superconducting Alkali Metal Iron Selenide
|
With single crystal X-ray diffraction studies, we compare the structures of
three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2,
K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost
identical ordered vacancy structure with a ({\sqrt}5 x {\sqrt}5 x 1) super
cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters
at 250K of a = b = 8.729(2) {\AA} and c = 14.120(3) {\AA}, a = b = 8.7186(12)
{\AA} and c = 14.0853(19) {\AA} and at 295 K, a = b = 8.8617(16) {\AA} and c =
15.304(3) {\AA} for the three crystals, respectively. The structure contains
two iron sites; one is almost completely empty, whilst the other is fully
occupied. There are similarly two alkali metal sites that are occupied in the
range of 72.2(2) % to 85.3(3) %. The inclusion of alkali metals and the
presence of vacancies within the structure allows for considerable relaxation
of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series,
and the resulting shift of the Se - F - Se bond angles to less distorted
geometry could be important in understanding the associated increase in the
superconducting transition temperature. The structure of these superconductors
distinguishes themselves from the structure of the non-superconducting phases
by an almost complete absence of Fe on the (0 0.5 0.25) site as well as lower
alkali metal occupancy that ensures an exact Fe2+ oxidation state, which are
clearly critical parameters in the promotion of superconductivity.
|
1101.4882v2
|
2011-01-25
|
A possible solution to the [alpha/Fe]-sigma problem in early type galaxies within a hierarchical galaxy formation model
|
The most massive elliptical galaxies apparently formed the fastest, because
the ratio of alpha elements (such as oxygen) to iron is the smallest. In fact,
iron is mainly produced from type Ia supernovae on a timescale of ~ 0.1-1
billion years, while the alpha elements come from massive stars on timescales
of a few tens of million years (Matteucci 1994). Reproducing such a alpha/Fe
correlation has long been a severe problem for cosmological theories of galaxy
formation, which envisage massive galaxies to assemble gradually from smaller
progenitors, and to be characterized by a star formation history too much
extended towards late cosmic times. While it has recently become clear that
feedback from Active Galactic Nuclei (AGN) activity play a role in the late
quenching of star formation (e.g. Cattaneo et al. 2009), and that early star
formation history in the galaxy progenitors affect the alpha/Fe ratio (Calura &
Menci 2009), major mergers alone cannot enhance the star formation in the
high-redshift progenitors to the levels required to match the steepness of the
observed alpha/Fe correlation (Spolaor et al. 2010). Here we report that the
inclusion of the effects of fly-by 'harassments', that trigger lower level
starbursts, combined with the AGN quenching of the starburst activity,
considerably enhances the capability to account for the observed alpha/Fe ratio
in ellipticals within cosmological galaxy formation models . The critical
difference between the earlier work and the present result is the effect of
starbursts driven by fly-by encounters that would have been very common amongst
the high-redshift progenitors of massive galaxies and which would have boosted
star formation in the first 2 billion years after the Big Bang, combined with
quenching of the burst activity within the first 3-4 Gyr.
|
1101.4938v1
|
2011-04-26
|
On the driver of relativistic effects strength in Seyfert galaxies
|
Spectroscopy of X-ray emission lines emitted in accretion discs around
supermassive black holes is one of the most powerful probes of the accretion
flow physics and geometry, while also providing in principle observational
constraints on the black hole spin.[...] We aim at determining the ultimate
physical driver of the strength of this relativistic reprocessing feature. We
first extend the hard X-ray flux-limited sample of Seyfert galaxies studied so
far (FERO, de la Calle Perez et al. 2010) to obscured objects up to a column
density N_H=6x10^23 atoms/cm/cm. We verify that none of the line properties
depends on the AGN optical classification, as expected from the Seyfert
unification scenarios. There is also no correlation between the accretion disc
inclination, as derived from formal fits of the line profiles, and the optical
type or host galaxy aspect angle, suggesting that the innermost regions of the
accretion disc and the host galaxy plane are not aligned. [...]. Data are not
sensitive enough to the detailed ionisation state of the line-emitting disc.
However, the lack of dependency of the line EW on either the luminosity or the
rest-frame centroid energy rules out that disc ionisation plays an important
role on the EW dynamical range in Seyferts. The dynamical range of the
relativistically broadened K-alpha iron line EW in nearby Seyferts appears to
be mainly determined by the properties of the innermost accretion flow. We
discuss several mechanisms (disc ionisation, disc truncation, aberration due to
a mildly relativistic outflowing corona) which can explain this. [...]
Observational data are still not in contradiction with scenarios invoking
different mechanisms for the spectral complexity around the iron line, most
notably the "partial covering" absorption scenario. (abridged).
|
1104.4918v1
|
2011-04-27
|
Spitzer spectra of evolved stars in omega Centauri and their low-metallicity dust production
|
Dust production is explored around 14 metal-poor ([Fe/H] = -1.91 to -0.98)
giant stars in the Galactic globular cluster omega Centauri using new Spitzer
IRS spectra. This sample includes the cluster's post-AGB and carbon stars and
is thus the first representative spectral study of dust production in a
metal-poor ([Fe/H] < -1) population. Only the more metal rich stars V6 and V17
([Fe/H] = -1.08, -1.06) exhibit silicate emission, while the five other stars
with mid-infrared excess show only a featureless continuum which we argue is
caused by metallic iron dust grains. We examine the metallicity of V42, and
find it is likely part of the metal-rich population ([Fe/H] ~ -0.8). Aside from
the post-AGB star V1, we find no star from the cluster's bulk, metal-poor
([Fe/H] < -1.5) population - including the carbon stars - to be producing
detectable amounts of dust. We compare the dust production to the stars'
H-alpha line profiles obtained at the Magellan/Clay telescope at Las Campanas
Observatory, finding pulsation shocking in the strongest pulsators (V6, V17 and
V42), but evidence of outflow in all other stars. We conclude that the onset of
dust production does not signify a fundamental change in the material leaving
the star. Our data add to a growing body of evidence that metallic iron
dominates dust production in metal-poor, oxygen-rich stars, but that dust is
probably not the primary accelerant of winds in this mass-metallicity regime.
|
1104.5155v1
|
2011-05-16
|
The influence of non-isotropic scattering of thermal radiation on spectra of brown dwarfs and hot exoplanets
|
(abridged) We calculate near-infrared thermal emission spectra using a
doubling-adding radiative transfer code, which includes scattering by clouds
and haze. Initial temperature profiles and cloud optical depths are taken from
the drift-phoenix brown dwarf model. As is well known, cloud particles change
the spectrum compared to when clouds are ignored. The clouds reduce fluxes in
the near-infrared spectrum and make it redder than for the clear sky case. We
also confirm that not including scattering in the spectral calculations can
result in errors on the spectra of many tens of percent, both in magnitude and
in variations with wavelength. This is especially apparent for particles that
are larger than the wavelength and only have little iron in them. Scattering
particles will show deeper absorption features than absorbing (e.g. iron)
particles and particle size will also affect the calculated infrared colours.
Large particles also tend to be strongly forward-scattering, and we show that
assuming isotropic scattering in this case also leads to very large errors in
the spectrum. Thus, care must be taken in the choice of radiative transfer
method for heat balance or spectral calculations when clouds are present in the
atmosphere. Besides the choice of radiative transfer method, the type of
particles that are predicted by models will change conclusions about e.g.
infrared colours and trace gas abundances. As a result, knowledge of the
scattering properties of the clouds is essential when deriving temperature
profiles or gas abundances from direct infrared observations of exoplanets or
brown dwarfs and from secondary eclipse measurements of transiting exoplanets,
since scattering clouds will change the depth of gas absorption features, among
other things. Thus, ignoring the presence of clouds can yield retrieved
properties that differ significantly from the real atmospheric properties.
|
1105.3062v2
|
2011-07-05
|
GRAVITAS : General Relativistic Astrophysics VIa Timing And Spectroscopy: An ESA M3 mission proposal
|
GRAVITAS is an X-ray observatory, designed and optimised to address the ESA
Cosmic Vision theme of "Matter under extreme conditions". It was submitted as a
response to the call for M3 mission proposals. The concept centres around an
X-ray telescope of unprecedented effective area, which will focus radiation
emitted from close to the event horizon of black holes or the surface of
neutron stars. To reveal the nature and behaviour of matter in the most extreme
astrophysical environments, GRAVITAS targets a key feature in the X-ray spectra
of compact objects: the iron Kalpha line at ~6.5 keV. The energy, profile, and
variability of this emission line, and the properties of the surrounding
continuum emission, shaped by General Relativity (GR) effects, provide a unique
probe of gravity in its strong field limit. Among its prime targets are
hundreds of supermassive black holes in bright Active Galactic Nuclei (AGN),
which form the perfect laboratory to help understand the physical processes
behind black hole growth. Accretion plays a fundamental role in the shaping of
galaxies throughout cosmic time, via the process of feedback. Modest
(~sub-arcmin) spatial resolution would deliver the necessary sensitivity to
extend high quality X-ray spectroscopy of AGN to cosmologically-relevant
distances. Closer to home, ultra-high count rate capabilities and
sub-millisecond time resolution enable the study of GR effects and the equation
of state of dense matter in the brightest X-ray binaries in our own Galaxy,
using multiple probes, such as the broad iron line, the shape of the disk
continuum emission, quasi-periodic oscillations, reverberation mapping, and
X-ray burst oscillations. Despite its breakthrough capabilities, all enabling
technologies for GRAVITAS are already in a high state of readiness. It is based
on ultra light-weight X-ray optics and a focal plane detector using silicon
technology.
|
1107.0955v1
|
2011-08-04
|
Identifying a new intermediate-polar using XMM-Newton and INTEGRAL
|
The bright X-ray source, 2XMMi J180438.7-145647 is fortunate to have long
baseline observations in INTEGRAL that compliment observations taken by other
missions. Optical spectroscopy of this object has suggested a distance of ~7
kpc and an identification with a low mass X-ray binary. We instead use the
X-ray data from 0.3-40 keV to identify the source as a bright intermediate
polar (IP) with an estimate for the white dwarf mass of~0.60 M_solar. This
identification is supported by the presence of an iron triplet, the component
lines of which are some of the strongest seen in IPs; and the signature of the
spin period of the white dwarf at ~24 mins. We note that the lack of broad-band
variability may suggest that this object is a stream-fed IP, similar in many
respects to the well studied IP, V2400 Oph. Phase-binning has allowed us to
create spectra corresponding to the peaks and troughs of the lightcurve from
which we determine that the spectra appear harder in the troughs, consistent
with the behaviour of other IPs binned on their spin periods. This work
strongly suggests a mis-identification in the optical due to the presence of
large columns of enshrouding material. We instead propose a distance to the
source of <2.5 kpc to be consistent with the luminosities of other IPs in the
dim, hard state. The relatively high flux of the source together with the
strength of the iron lines may, in future, allow the source to be used to
diagnose the properties of the shock heated plasma and the reflected component
of the emission.
|
1108.1105v1
|
2011-08-12
|
The High Energy view of the Broad Line Radio Galaxy 3C 111
|
We present the analysis of Suzaku and XMM-Newton observations of the
broad-line radio galaxy (BLRG) 3C 111. Its high energy emission shows
variability, a harder continuum with respect to the radio quiet AGN population,
and weak reflection features. Suzaku found the source in a minimum flux level;
a comparison with the XMM-Newton data implies an increase of a factor of 2.5 in
the 0.5-10 keV flux, in the 6 months separating the two observations. The iron
K complex is detected in both datasets, with rather low equivalent width(s).
The intensity of the iron K complex does not respond to the change in continuum
flux. An ultra-fast, high-ionization outflowing gas is clearly detected in the
XIS data; the absorber is most likely unstable. Indeed, during the XMM-Newton
observation, which was 6 months after, the absorber was not detected. No clear
roll-over in the hard X-ray emission is detected, probably due to the emergence
of the jet as a dominant component in the hard X-ray band, as suggested by the
detection above ~ 100 keV with the GSO on-board Suzaku, although the present
data do not allow us to firmly constrain the relative contribution of the
different components. The fluxes observed by the gamma-ray satellites CGRO and
Fermi would be compatible with the putative jet component if peaking at
energies E ~ 100 MeV. In the X-ray band, the jet contribution to the continuum
starts to be significant only above 10 keV. If the detection of the jet
component in 3C 111 is confirmed, then its relative importance in the X-ray
energy band could explain the different observed properties in the high-energy
emission of BLRGs, which are otherwise similar in their other multiwavelength
properties. Comparison between X-ray and gamma-ray data taken at different
epochs suggests that the strong variability observed for 3C 111 is probably
driven by a change in the primary continuum.
|
1108.2609v1
|
2011-08-30
|
The Bulge Radial Velocity/Abundance Assay
|
The Bulge Radial Velocity/Abundance Assay (BRAVA) has accomplished a survey
of 10,000 red giants in the Southern Galactic bulge, approximately spanning -8
deg. < l < +8 deg. and -3 deg. <b < -8 deg., a region within roughly 1 kpc from
the nucleus. We find that the Galactic bulge at b=-4 deg. displays a clear
departure from solid body rotation, and that the rotation field along the major
axis at b=-6 deg. and b=-8 deg is identical to that at lower latitude; this is
"cylindrical" rotation, a hallmark observed in edge-on bars. Comparison of the
BRAVA dataset with an N-body bar shows that >90% of the bulge population is in
the bar, leaving little room for a "classical" bulge component. We also report
on the first iron abundance and composition measurements in the outer bulge, at
b=-8 deg. The iron abundance in this field falls on the trend of a suspected
gradient measured from high resolution spectroscopy of bulge clump stars.
Further, we find that the trends of [\alpha/Fe] vs [Fe/H] that characterize the
bulge at lower latitude are present 1 kpc from the nucleus, consistent with a
rapid (<1 Gyr) timescale for the formation of the bulge, even near its
boundary. Although the dynamics of the bulge are consistent with those of a
dynamically buckled N-body bar, the presence of an abundance gradient is not
compatible with purely dynamical processes; we propose that missing baryonic
physics is needed. We also report on the remarkable massive bulge globular
cluster Terzan 5, which has a bimodal abundance and composition distribution,
and is proposed as the remnant of a population of primordial building block
stellar systems that formed the bulge. Terzan 5 is presently a unique case, and
it is important to test whether the dissolution of systems similar to it
populated the bulge.
|
1108.6044v1
|
2011-09-01
|
Suzaku Observations of Iron K-lines from the Intracluster Medium of the Coma Cluster
|
The Coma cluster was observed with an X-ray Imaging Spectrometer (XIS)
onboard Suzaku in six pointings, including the central X-ray peak region, the
14'west offset region, 30'and 34'north-west offset regions, and 44'and
60'south-west offset regions. Owing to its lower background level, Suzaku has
better sensitivity to Fe K-alpha lines than other satellites. Using precise Fe
line measurements, we studied the temperature structure, possible bulk motions,
and iron abundance distributions in the intracluster medium (ICM). The observed
spectra were well-represented by a single-temperature model, and two- or three-
temperature model did not improve chi-square substantially. The temperature,
derived from K-alpha line ratios of H-like and He-like Fe, agree with those
derived from the single-temperature model. Because the line ratio is a steep
function of temperature, the consistency supports the accuracy of temperature
measurements conducted with Suzaku. Within the 34'region, the redshift derived
from the central energy of the He-like Fe line is consistent with that from
optical observations, within a calibration error of 18 eV or 818 km/s in the
line of sight. This value is smaller than the sound velocity of ICM, which is
1500 km/s. The central energy of Fe lines at the 44'offset region around the
NGC 4839 subcluster is also consistent with those within the 34'region. These
results on the temperature and velocity structure suggest that the core of the
cluster is in a relaxed state, and non-thermal electrons relevant to the radio
halo are accelerated by intracluster turbulence rather than large-scale shocks.
Fe abundance is almost constant at 0.4 solar within the 34'region, and
decreases with radius. This value is slightly lower than those of other
clusters, which means the gas have been mixed well during a past merger
associated with the growth of the cluster.
|
1109.0154v2
|
2011-09-09
|
The Chemical Evolution of Globular Clusters - II. Metals and Fluorine
|
In the first paper in this series, we proposed a new framework in which to
model the chemical evolution of globular clusters. This model, is predicated
upon the assumption that clusters form within an interstellar medium enriched
locally by the ejecta of a single Type Ia supernova and varying numbers of
asymptotic giant branch stars, superimposed on an ambient medium pre-enriched
by low-metallicity Type II supernovae. Paper I was concerned with the
application of this model to the observed abundances of several reactive
elements and so-called non-metals for three classical intermediate-metallicity
clusters, with the hallmark of the work being the successful recovery of many
of their well-known elemental and isotopic abundance anomalies. Here, we expand
upon our initial analysis by (a) applying the model to a much broader range of
metallicities (from the factor of three explored in Paper I, to now, a factor
of ~50; i.e., essentially, the full range of Galactic globular cluster
abundances, and (b) incorporating a broader suite of chemical species,
including a number of iron-peak isotopes, heavier alpha-elements, and fluorine.
While most empirical globular cluster abundance trends are reproduced, our
model would suggest the need for a higher production of Ca, Si, and Cu in
low-metallicity (or so-called "prompt") Type Ia supernovae than predicted in
current stellar models in order to reproduce the observed trends in NGC 6752,
and a factor of two reduction in carbon production from asymptotic giant branch
stars to explain the observed trends between carbon and nitrogen. Observations
of heavy-element isotopes produced primarily by Type Ia supernovae, including
those of titanium, iron, and nickel, could support/refute unequivocally our
proposed framework. Hydrodynamical simulations would be necessary to study its
viability from a dynamical point of view.
|
1109.1938v1
|
2011-09-21
|
NGC 5548: Lack of a Broad Fe K Line and Constraints on the Location of the Hard X-ray Source
|
We present an analysis of the co-added and individual 0.7-40 keV spectra from
seven Suzaku observations of the Sy 1.5 galaxy NGC 5548 taken over a period of
eight weeks. We conclude that the source has a moderately ionized, three-zone
warm absorber, a power-law continuum, and exhibits contributions from cold,
distant reflection. Relativistic reflection signatures are not significantly
detected in the co-added data, and we place an upper limit on the equivalent
width of a relativistically broad Fe K line at EW \leq 26 eV at 90% confidence.
Thus NGC 5548 can be labeled an "weak" type-1 AGN in terms of its observed
inner disk reflection signatures, in contrast to sources with very broad,
strong iron lines such as MCG-6-30-15, which are likely much fewer in number.
We compare physical properties of NGC 5548 and MCG-6-30-15 that might explain
this difference in their reflection properties. Though there is some evidence
that NGC 5548 may harbor a truncated inner accretion disk, this evidence is
inconclusive, so we also consider light bending of the hard X-ray continuum
emission in order to explain the lack of relativistic reflection in our
observation. If the absence of a broad Fe K line is interpreted in the
light-bending context, we conclude that the source of the hard X-ray continuum
lies at <100 gravitational radii. We note, however, that light-bending models
must be expanded to include a broader range of physical parameter space in
order to adequately explain the spectral and timing properties of average AGN,
rather than just those with strong, broad iron lines.
|
1109.4651v1
|
2011-10-20
|
High Resolution XMM-Newton Spectroscopy of the Cooling Flow Cluster A3112
|
We examine high signal to noise XMM-Newton European Photon Imaging Camera
(EPIC) and Reflection Grating Spectrometer (RGS) observations to determine the
physical characteristics of the gas in the cool core and outskirts of the
nearby rich cluster A3112. The XMM-Newton Extended Source Analysis Software
data reduction and background modeling methods were used to analyze the XMM-
Newton EPIC data. From the EPIC data we find that the iron and silicon
abundance gradients show significant increase towards the center of the cluster
while the oxygen abundance profile is centrally peaked but has a shallower
distribution than that of iron. The X-ray mass modeling is based on the
temperature and deprojected density distributions of the intra-cluster medium
determined from EPIC observations. The total mass of A3112 obeys the M-T
scaling relations found using XMM-Newton and Chandra observations of massive
clusters at R500. The gas mass fraction f_gas= 0.149^{+0.036}_{-0.032} at R500,
is consistent with the seven-year WMAP results. The comparisons of line fluxes
and flux limits on the Fe XVII and Fe XVIII lines obtained from high resolution
RGS spectra indicate that there is no spectral evidence for cooler gas
associated with the cluster with temperature below 1.0 keV in the central <38"
(\sim 52 kpc) region of A3112. High resolution RGS spectra also yield an upper
limit to the turbulent motions in compact core of A3112 (206 km/s). We find
that the energy contribution of turbulence to total energy is less than 6 per
cent. This upper limit is consistent with the amount of energy contribution
measured in recent high resolution simulations of relaxed galaxy clusters.
|
1110.4422v2
|
2011-11-11
|
Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Vir
|
The spectral energy distribution (SED) in chemically peculiar stars may be
significantly affected by their abundance anomalies. The observed SED
variations are usually assumed to be a result of inhomogeneous surface
distribution of chemical elements, flux redistribution and stellar rotation.
However, the direct evidence for this is still only scarce. We aim to identify
the processes that determine the SED and its variability in the UV and visual
spectral domains of the helium-weak star CU Vir. We used the model atmospheres
to obtain the emergent flux and predict the rotationally modulated flux
variability of the star. We show that most of the light variations in the vby
filters of the Stromgren photometric system are a result of the uneven surface
distribution of silicon, chromium, and iron. Our models are only able to
explain a part of the variability in the u filter, however. The observed UV
flux distribution is very well reproduced, and the models are able to explain
most of the observed features in the UV light curve. The variability observed
in the visible is merely a faint gleam of that in the UV. While the amplitude
of the light curves reaches only several hundredths of magnitude in the visual
domain, it reaches about 1 mag in the UV. The visual and UV light variability
of CU Vir is caused by the flux redistribution from the far UV to near UV and
visible regions, inhomogeneous distribution of the elements and stellar
rotation. Bound-free transitions of silicon and bound-bound transitions of iron
and chromium contribute the most to the flux redistribution. This mechanism can
explain most of the rotationally modulated light variations in the filters
centred on the Paschen continuum and on the UV continuum of the star CU Vir.
However, another mechanism(s) has to be invoked to fully explain the observed
light variations in the u filter and in the region 2000-2500 A.
|
1111.2746v1
|
2011-11-28
|
Dust in Historical Galactic Type Ia Supernova Remnants with Herschel
|
The origin of interstellar dust in galaxies is poorly understood,
particularly the relative contributions from supernovae and the cool stellar
winds of low-intermediate mass stars. Here, we present Herschel PACS and SPIRE
photometry at 70-500um of the historical young supernova remnants: Kepler and
Tycho; both thought to be the remnants of Type Ia explosion events. We detect a
warm dust component in Kepler's remnant with T = 82K and mass 0.0031Msun; this
is spatially coincident with thermal X-ray emission optical knots and
filaments, consistent with the warm dust originating in the circumstellar
material swept up by the primary blast wave of the remnant. Similarly for
Tycho's remnant, we detect warm dust at 90K with mass 0.0086Msun. Comparing the
spatial distribution of the warm dust with X-rays from the ejecta and swept-up
medium, and Ha emission arising from the post-shock edge, we show that the warm
dust is swept up interstellar material. We find no evidence of a cool (25-50 K)
component of dust with mass >0.07Msun as observed in core-collapse remnants of
massive stars. Neither the warm or cold dust components detected here are
spatially coincident with supernova ejecta material. We compare the lack of
observed supernova dust with a theoretical model of dust formation in Type Ia
remnants which predicts dust masses of 0.088(0.017)Msun for ejecta expanding
into surrounding densities of 1(5)cm-3. The model predicts that silicon- and
carbon-rich dust grains will encounter the interior edge of the observed dust
emission at 400 years confirming that the majority of the warm dust originates
from swept up circumstellar or interstellar grains (for Kepler and Tycho
respectively). The lack of cold dust grains in the ejecta suggests that Type Ia
remnants do not produce substantial quantities of iron-rich dust grains and has
important consequences for the 'missing' iron mass observed in ejecta.
|
1111.6627v2
|
2011-12-23
|
Non-standard s process in low metallicity massive rotating stars
|
Context. Rotation is known to affect the nucleosynthesis of light elements in
massive stars, mainly by rotation-induced mixing. In particular, rotation
boosts the primary nitrogen production. Models of rotating stars are able to
reproduce the nitrogen observed in low-Z halo stars. Aims. Here we present the
first grid of stellar models for rotating massive stars at low Z, where a full
s-process network is used to study the impact of rotation-induced mixing on the
nucleosynthesis of heavy elements. Methods. We used the Geneva stellar
evolution code that includes an enlarged reaction network with nuclear species
up to bismuth to calculate 25 M$_\odot$ models at three different Z and with
different initial rotation rates. Results. First, we confirm that
rotation-induced mixing leads to a production of primary $^{22}$Ne, which is
the main neutron source for the s process in massive stars. Therefore rotation
boosts the s process in massive stars at all Z. Second, the neutron-to-seed
ratio increases with decreasing Z in models including rotation, which leads to
the complete consumption of all iron seeds at Z < 1e-3 by the end of core
He-burning. Thus at low Z, the iron seeds are the main limitation for this
boosted s process. Third, as Z decreases, the production of elements up to the
Ba peak increases at the expense of the elements of the Sr peak. We studied the
impact of the initial rotation rate and of the uncertain
$^{17}$O$(\alpha,\gamma)$ rate (which strongly affects the neutron poison
strength of $^{16}$O) on our results. This study shows that rotating models can
produce significant amounts of elements up to Ba over a wide range of Z.
Fourth, compared to the He-core, the primary $^{22}$Ne production in the
He-shell is even higher (> 1% in mass fraction at all Z), which could open the
door for an explosive neutron capture nucleosynthesis in the He-shell, with a
primary neutron source.
|
1112.5548v1
|
2012-01-24
|
Effective temperatures, rotational velocities, microturbulent velocities and abundances in the atmospheres of the Sun,. HD1835 and HD10700
|
We describe our procedure to determine effective temperatures, rotational
velocities, microturbulent velocities, and chemical abundances in the
atmospheres of Sun-like stars. We use independent determinations of iron
abundances using the fits to the observed Fe I and Fe II atomic absorption
lines. We choose the best solution from the fits to these spectral features for
the model atmosphere that provides the best confidence in the determined log
N(Fe), Vt, and vsini. First, we compute the abundance of iron for a set of
adopted microturbulent velocities. To determine the most self-consistent
effective temperature and microturbulent velocity in any star's atmosphere, we
used an additional constraint where we minimise the dependence of the derived
abundances of Fe I and Fe II on the excitation potential of the corresponding
lines. We analyse the spectra of the Sun and two well known solar type stars,
HD1835 and HD10700 to determine their abundances, microturbulent velocity and
rotational velocity. For the Sun abundances of elements obtained from the fits
of their absorption features agree well enough (+/- 0.1 dex) with the known
values for the Sun. We determined a rotational velocity of vsini = 1.6 +/- 0.3
km/s for the spectrum of the Sun as a star. For HD1835 the self-consistent
solution for Fe I and Fe II lines log N(Fe)=+0.2 was obtained with a model
atmosphere of 5807/4.47/+0.2 andmicroturbulent velocity Vt = 0.75 km/s, and
leads to vsini = 7.2 $\pm$ 0.5 km/s. For HD10700 the self-consistent solution
log N(Fe) = -4.93 was obtained using a model atmosphere of 5383/4.59/-0.6and
microturbulent velocity Vt = 0.5 km/s. The Fe I and Fe II lines give rise to a
vsini = 2.4 +/- 0.4 km/s. Using the Teff found from the ionisation equilibrium
parameters for all three stars, we found abundances of a number of other
elements: Ti, Ni, Ca, Si, Cr. ... Abriged.
|
1201.5099v2
|
2012-02-14
|
On The Effect of Giant Planets on the Scattering of Parent Bodies of Iron Meteorite from the Terrestrial Planet Region into the Asteroid Belt: A Concept Study
|
In their model for the origin of the parent bodies of iron meteorites, Bottke
et al proposed differentiated planetesimals that were formed in the region of
1-2 AU during the first 1.5 Myr, as the parent bodies, and suggested that these
objects and their fragments were scattered into the asteroid belt as a result
of interactions with planetary embryos. Although viable, this model does not
include the effect of a giant planet that might have existed or been growing in
the outer regions. We present the results of a concept study where we have
examined the effect of a planetary body in the orbit of Jupiter on the early
scattering of planetesimals from terrestrial region into the asteroid belt. We
integrated the orbits of a large battery of planetesimals in a disk of
planetary embryos, and studied their evolutions for different values of the
mass of the planet. Results indicate that when the mass of the planet is
smaller than 10 Earth-masses, its effects on the interactions among
planetesimals and planetary embryos is negligible. However, when the planet
mass is between 10 and 50 Earth-masses, simulations point to a transitional
regime with ~50 Earth-mass being the value for which the perturbing effect of
the planet can no longer be ignored. Simulations also show that further
increase of the mass of the planet strongly reduces the efficiency of the
scattering of planetesimals from the terrestrial planet region into the
asteroid belt. We present the results of our simulations and discuss their
possible implications for the time of giant planet formation.
|
1202.2975v1
|
2012-03-08
|
Suzaku observations of the Hydra A cluster out to the virial radius
|
We report Suzaku observations of the northern half of the Hydra A cluster out
to ~1.4 Mpc, reaching the virial radius. This is the first Suzaku observations
of a medium-size (kT ~3 keV) cluster out to the virial radius. Two observations
were conducted, north-west and north-east offsets, which continue in a filament
direction and a void direction of the large-scale structure of the Universe,
respectively. The X-ray emission and distribution of galaxies elongate in the
filament direction. The temperature profiles in the two directions are mostly
consistent with each other within the error bars and drop to 1.5 keV at 1.5
r_500. As observed by Suzaku in hot clusters, the entropy profile becomes
flatter beyond r_500, in disagreement with the r^1.1 relationship that is
expected from accretion shock heating models. When scaled with the average
intracluster medium (ICM) temperature, the entropy profiles of clusters
observed with Suzaku are universal and do not depend on system mass. The
hydrostatic mass values in the void and filament directions are in good
agreement, and the Navarro, Frenk, and White universal mass profile represents
the hydrostatic mass distribution up to ~ 2 r_500. Beyond r_500, the ratio of
gas mass to hydrostatic mass exceeds the result of the Wilkinson microwave
anisotropy probe, and at r_100, these ratios in the filament and void
directions reach 0.4 and 0.3, respectively. We discuss possible deviations from
hydrostatic equilibrium at cluster outskirts. We derived radial profiles of the
gasmass- to-light ratio and iron-mass-to-light ratio out to the virial radius.
Within r_500, the iron-mass-to-light ratio of the Hydra A cluster was compared
with those in other clusters observed with Suzaku.
|
1203.1700v4
|
2012-03-14
|
Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects
|
(Abridged) Aims: We study the effects related to departures from non-local
thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red
giant stars in Galactic globular cluster NGC 6752, to assess their influence on
the formation of Ba II lines. Methods: One-dimensional (1D) local thermodynamic
equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D
ATLAS stellar model atmospheres. The three-dimensional (3D) LTE abundances were
obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE
abundances using 3D-1D abundance corrections, i.e., the differences between the
abundances obtained from the same spectral line using the 3D hydrodynamical
(CO5BOLD) and classical 1D (LHD) stellar model atmospheres. Results: The mean
1D barium-to-iron abundance ratios derived for 20 giants are <[Ba/Fe]>_{1D
NLTE} = 0.05 \pm0.06 (stat.) \pm0.08 (sys.). The 3D-1D abundance correction
obtained for 8 giants is small (~+0.05 dex), thus leads to only minor
adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for
the 20 giants, <[Ba/Fe]>_{3D+NLTE} = 0.10 \pm0.06(stat.) \pm0.10(sys.). The
intrinsic abundance spread between the individual cluster stars is small and
can be explained in terms of uncertainties in the abundance determinations.
Conclusions: Deviations from LTE play an important role in the formation of
barium lines in the atmospheres of red giants studied here. The role of 3D
hydrodynamical effects should not be dismissed either, even if the obtained
3D-1D abundance corrections are small. This result is a consequence of subtle
fine-tuning of individual contributions from horizontal temperature
fluctuations and differences between the average temperature profiles in the 3D
and 1D model atmospheres: owing to the comparable size and opposite sign, their
contributions nearly cancel each other.
|
1203.3124v2
|
2012-03-20
|
Manganese in dwarf spheroidal galaxies
|
We provide manganese abundances (corrected for the effect of the hyperfine
structure) for a large number of stars in the dwarf spheroidal galaxies
Sculptor and Fornax, and for a smaller number in the Carina and Sextans dSph
galaxies. Abundances had already been determined for a number of other elements
in these galaxies, including alpha and iron-peak ones, which allowed us to
build [Mn/Fe] and [Mn/alpha] versus [Fe/H] diagrams. The Mn abundances imply
sub-solar [Mn/Fe] ratios for the stars in all four galaxies examined. In
Sculptor, [Mn/Fe] stays roughly constant between [Fe/H]\sim -1.8 and -1.4 and
decreases at higher iron abundance. In Fornax, [Mn/Fe] does not vary in any
significant way with [Fe/H]. The relation between [Mn/alpha] and [Fe/H] for the
dSph galaxies is clearly systematically offset from that for the Milky Way,
which reflects the different star formation histories of the respective
galaxies. The [Mn/alpha] behavior can be interpreted as a result of the
metal-dependent Mn yields of type II and type Ia supernovae. We also computed
chemical evolution models for star formation histories matching those
determined empirically for Sculptor, Fornax, and Carina, and for the Mn yields
of SNe Ia, which were assumed to be either constant or variable with
metallicity. The observed [Mn/Fe] versus [Fe/H] relation in Sculptor, Fornax,
and Carina can be reproduced only by the chemical evolution models that include
a metallicity-dependent Mn yield from the SNe Ia.
|
1203.4491v2
|
2012-04-02
|
Correlated Gravitational Wave and Neutrino Signals from General-Relativistic Rapidly Rotating Iron Core Collapse
|
We present results from a new set of 3D general-relativistic hydrodynamic
simulations of rotating iron core collapse. We assume octant symmetry and focus
on axisymmetric collapse, bounce, the early postbounce evolution, and the
associated gravitational wave (GW) and neutrino signals. We employ a
finite-temperature nuclear equation of state, parameterized electron capture in
the collapse phase, and a multi-species neutrino leakage scheme after bounce.
The latter captures the important effects of deleptonization, neutrino cooling
and heating and enables approximate predictions for the neutrino luminosities
in the early evolution after core bounce. We consider 12-solar-mass and
40-solar-mass presupernova models and systematically study the effects of (i)
rotation, (ii) progenitor structure, and (iii) postbounce neutrino leakage on
dynamics, GW, and, neutrino signals. We demonstrate, that the GW signal of
rapidly rotating core collapse is practically independent of progenitor mass
and precollapse structure. Moreover, we show that the effects of neutrino
leakage on the GW signal are strong only in nonrotating or slowly rotating
models in which GW emission is not dominated by inner core dynamics. In rapidly
rotating cores, core bounce of the centrifugally-deformed inner core excites
the fundamental quadrupole pulsation mode of the nascent protoneutron star. The
ensuing global oscillations (f~700-800 Hz) lead to pronounced oscillations in
the GW signal and correlated strong variations in the rising luminosities of
antineutrino and heavy-lepton neutrinos. We find these features in cores that
collapse to protoneutron stars with spin periods <~ 2.5 ms and rotational
energies sufficient to drive hyper-energetic core-collapse supernova
explosions. Hence, joint GW + neutrino observations of a core collapse event
could deliver strong evidence for or against rapid core rotation. [abridged]
|
1204.0512v2
|
2012-04-04
|
The Suzaku X-ray spectrum of NGC 3147. Further insights on the best "true" Seyfert 2 galaxy candidate
|
NGC 3147 is so far the most convincing case of a "true" Seyfert 2 galaxy,
i.e. a source genuinely lacking the Broad Line Regions. We obtained a Suzaku
observation with the double aim to study in more detail the iron line complex,
and to check the Compton-thick hypothesis for the lack of observed optical
broad lines.
The Suzaku XIS and HXD/PIN spectra of the source were analysed in detail. The
line complex is composed of at least two unresolved lines, one at about 6.45
keV and the other one at about 7 keV, most likely identified with Fe XVII/XIX,
the former, and Fe XXVI, the latter. The high-ionization line can originate
either in a photoionized matter or in an optically thin thermal plasma. In the
latter case, an unusually high temperature is implied. In the photoionized
model case, the large equivalent width can be explained either by an extreme
iron overabundance or by assuming that the source is Compton-thick. In the
Compton-thick hypothesis, however, the emission above 2 keV is mostly due to a
highly ionized reflector, contrary to what is usually found in Compton-thick
Seyfert 2s, where reflection from low ionized matter dominates. Moreover, the
source flux varied between the XMM-Newton and the Suzaku observations, taken
3.5 years apart, confirming previous findings and indicating that the size of
the emitting region must be smaller than a parsec.
The hard X-ray spectrum is also inconclusive on the Compton-thick hypothesis.
Weighting the various arguments, a "true" Seyfert 2 nature of NGC 3147 seems to
be still the most likely explanation, even if the "highly ionized reflector"
Compton-thick hypothesis cannot at present be formally rejected.
|
1204.0946v1
|
2012-04-24
|
Intrinsic crystal phase separation in antiferromagnetic superconductor RbyFe2-xSe2: a diffraction study
|
The crystal and magnetic structures of the superconducting iron based
chalcogenides RbyFe2-xSe2 have been studied by means of single crystal
synchrotron x-ray and high resolution neutron powder diffraction in the
temperature range 2-570 K. The ground state of the crystal is an intrinsically
phase separated state with two distinct by symmetry phases. The main phase has
the iron vacancy ordered \surd5x\surd5 superstructure (I4/m space group) with
AFM ordered Fe spins. The minority phase does not have \surd5x\surd5-type of
ordering and has smaller in plane lattice constant a and larger tetragonal
c-axis and can be well described assuming the parent average vacancy disordered
structure (I4/mmm space group) with the refined stoichiometry
Rb0.60(5)(Fe1.10(5))Se)2. The minority phase amounts to 8-10% mass fraction.
The unit cell volume of the minority phase is 3.2% smaller than the one of the
main phase at T = 2 K and has quite different temperature dependence. The
minority phase transforms to the main vacancy ordered phase on heating above
the phase separation temperature T_P = 475 K. The spatial dimensions of the
phase domains strongly increase above TP from 1000 \AA\ to > 2500 \AA\ due to
the merging of the regions of the main phase that were separated by the second
phase at low temperatures. Additional annealing of the crystals at the
temperature T=488 K close to T_P for the long time drastically reduces the
amount of the minority phase.
|
1204.5449v3
|
2012-04-26
|
XMM-Newton observations of GX 13+1: correlation between photoionised absorption and broad line emission
|
We analysed data from five XMM-Newton observations of GX 13+1 to investigate
the variability of the photo-ionised absorber present in this source. We fitted
EPIC and RGS spectra obtained from the "least-variable" intervals with a model
consisting of disc-blackbody and blackbody components together with a Gaussian
emission feature at ~6.55-6.7 keV modified by absorption due to cold and
photo-ionised material. We found a significant correlation between the hard,
~6-10 keV, flux, the ionisation and column density of the absorber and the
equivalent width of the broad iron line. We interpret the correlation in a
scenario in which a disc wind is thermally driven at large, ~10^{10} cm, radii
and the broad line results from reprocessed emission in the wind and/or hot
atmosphere. The breadth of the emission line is naturally explained by a
combination of scattering, recombination and fluorescence processes. We
attribute the variations in the absorption and emission along the orbital
period to the view of different parts of the wind, possibly located at slightly
different inclination angles. We constrain the inclination of GX 13+1 to be
between 60 and 80 degrees from the presence of strong absorption in the line of
sight, that obscures up to 80% of the total emission in one observation, and
the absence of eclipses. We conclude that the presence of a disc wind and/or a
hot atmosphere can explain the current observations of narrow absorption and
broad iron emission features in neutron star low mass X-ray binaries as a
class.
|
1204.5904v1
|
2012-05-30
|
Overabundance of alpha-elements in exoplanet host stars
|
We present the results for a chemical abundance analysis between
planet-hosting and stars without planets for 12 refractory elements for a total
of 1111 nearby FGK dwarf stars observed within the context of the HARPS GTO
programs. Of these stars, 109 are known to harbour high-mass planetary
companions and 26 stars are hosting exclusively Neptunians and super-Earths. We
found that the [X/Fe] ratios for Mg, Al, Si, Sc, and Ti both for giant and
low-mass planet hosts are systematically higher than those of comparison stars
at low metallicities ([Fe/H] < from -0.2 to 0.1 dex depending on the element).
The most evident discrepancy between planet-hosting and stars without planets
is observed for Mg. Our data suggest that the planet incidence is greater among
the thick disk population than among the thin disk for mettallicities bellow
-0.3 dex. After examining the [alpha/Fe] trends of the planet host and non-host
samples we conclude that a certain chemical composition, and not the Galactic
birth place of the stars, is the determinating factor for that. The inspection
of the Galactic orbital parameters and kinematics of the planet-hosting stars
shows that Neptunian hosts tend to belong to the "thicker" disk compared to
their high-mass planet-hosting counterparts.We also found that Neptunian hosts
follow the distribution of high-alpha stars in the UW vs V velocities space,
but they are more enhanced in Mg than high-alpha stars without planetary
companions. Our results indicate that some metals other than iron may also have
an important contribution to planet formation if the amount of iron is low.
These results may provide strong constraints for the models of planet
formation, especially for planets with low mass.
|
1205.6670v2
|
2012-07-07
|
An upper limit on the sulphur abundance in HE 1327-2326
|
Context: Star HE 1327-2326 is a unique object, with the lowest measured iron
abundance ([Fe/H] ~ -6) and a peculiar chemical composition that includes large
overabundances of C, N, and O with respect to iron. One important question is
whether the chemical abundances in this star reflect the chemical composition
of the gas cloud from which it was formed or if they have been severely
affected by other processes, such as dust-gas winnowing. Aims: We measure or
provide an upper limit to the abundance of the volatile element sulphur, which
can help to discriminate between the two scenarios. Methods: We observed HE
1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to
observe the S I lines of Multiplet 3 at 1045 nm. Results: We do not detect the
S I line. A 3sigma$upper limit on the equivalent width (EW) of any line in our
spectrum is EW<0.66 pm. Using either one-dimensional static or
three-dimensional hydrodynamical model-atmospheres, this translates into a
robust upper limit of [S/H]<-2.6. Conclusions: This upper limit does not
provide conclusive evidence for or against dust-gas winnowing, and the evidence
coming from other elements (e.g., Na and Ti) is also inconclusive or
contradictory. The formation of dust in the atmosphere versus an origin of the
metals in a metal-poor supernova with extensive "fall-back" are not mutually
exclusive. It is possible that dust formation distorts the peculiar abundance
pattern created by a supernova with fall-back, thus the abundance ratios in HE
1327-2326 may be used to constrain the properties of the supernova(e) that
produced its metals, but with some caution.
|
1207.1806v3
|
2012-07-25
|
Rapid-response mode VLT/UVES spectroscopy of super iron-rich gas exposed to GRB 080310. Evidence of ionization in action and episodic star formation in the host
|
We analyse high-resolution near-UV and optical spectra of the afterglow of
GRB 080310, obtained with the Very Large Telescope Ultraviolet and Visual
Echelle Spectrograph (VLT/UVES), to investigate the circumburst environment and
the interstellar medium of the gamma-ray burst (GRB) host galaxy. The VLT
rapid-response mode (RRM) enabled the observations to start only 13 minutes
after the Swift trigger and a series of four exposures to be collected before
dawn. A low neutral-hydrogen column-density (log N (HI) = 18.7) is measured at
the host-galaxy redshift of z = 2.42743. At this redshift, we also detect a
large number of resonance ground-state absorption lines (e.g., CII, MgII, AlII,
SiII, CrII, CIV, SiIV), as well as time-varying absorption from the
fine-structure levels of FeII. Time-varying absorption from a highly excited
FeIII energy level (7S3), giving rise to the so-called UV34 line triplet, is
also detected, for the first time in a GRB afterglow. The CrII ground-state and
all observed FeII energy levels are found to depopulate with time, whilst the
FeIII 7S3 level is increasingly populated. This absorption-line variability is
clear evidence of ionization by the GRB, which is for the first time
conclusively observed in a GRB afterglow spectrum. We derive ionic column
densities at each epoch of observations by fitting absorption lines with a
four-component Voigt-profile model. We perform CLOUDY photo-ionization
modelling of the expected pre-burst ionic column densities, to estimate that,
before the onset of the burst, [C/H] = -1.3 \pm 0.2, [O/H] < -0.8, [Si/H] =
-1.2 \pm 0.2, [Cr/H] = +0.7 \pm 0.2, and [Fe/H] = +0.2 \pm 0.2 for the
integrated line profile, indicating strong overabundances of iron and chromium.
For one of the components, we observe even more extreme ratios of [Si/Fe] \leq
-1.47 and [C/Fe] \leq -1.74. [abridged]
|
1207.6102v1
|
2012-08-14
|
The Golden Channel at a Neutrino Factory revisited: improved sensitivities from a Magnetised Iron Neutrino Detector
|
This paper describes the performance and sensitivity to neutrino mixing
parameters of a Magnetised Iron Neutrino Detector (MIND) at a Neutrino Factory
with a neutrino beam created from the decay of 10 GeV muons. Specifically, it
is concerned with the ability of such a detector to detect muons of the
opposite sign to those stored (wrong-sign muons) while suppressing
contamination of the signal from the interactions of other neutrino species in
the beam. A new more realistic simulation and analysis, which improves the
efficiency of this detector at low energies, has been developed using the GENIE
neutrino event generator and the GEANT4 simulation toolkit. Low energy neutrino
events down to 1 GeV were selected, while reducing backgrounds to the $10^{-4}$
level. Signal efficiency plateaus of ~60% for $\nu_\mu$ and ~70% for
$\bar{\nu}_\mu$ events were achieved starting at ~5 GeV. Contamination from the
$\nu_\mu\rightarrow \nu_\tau$ oscillation channel was studied for the first
time and was found to be at the level between 1% and 4%. Full response matrices
are supplied for all the signal and background channels from 1 GeV to 10 GeV.
The sensitivity of an experiment involving a MIND detector of 100 ktonnes at
2000 km from the Neutrino Factory is calculated for the case of $\sin^2
2\theta_{13}\sim 10^{-1}$. For this value of $\theta_{13}$, the accuracy in the
measurement of the CP violating phase is estimated to be $\Delta
\delta_{CP}\sim 3^\circ - 5^\circ$, depending on the value of $\delta_{CP}$,
the CP coverage at $5\sigma$ is 85% and the mass hierarchy would be determined
with better than $5\sigma$ level for all values of $\delta_{CP}$.
|
1208.2735v2
|
2012-09-13
|
Solar Fe abundance and magnetic fields - Towards a consistent reference metallicity
|
We investigate the impact on Fe abundance determination of including magnetic
flux in series of 3D radiation-MHD simulations of solar convection which we
used to synthesize spectral intensity profiles corresponding to disc centre. A
differential approach is used to quantify the changes in theoretical equivalent
width of a set of 28 iron spectral lines spanning a wide range in lambda,
excitation potential, oscillator strength, Land\'e factor, and formation
height. The lines were computed in LTE using the spectral synthesis code LILIA.
We used input magnetoconvection snapshots covering 50 minutes of solar
evolution and belonging to series having an average vertical magnetic flux
density of 0, 50, 100 and 200 G. For the relevant calculations we used the
Copenhagen Stagger code. The presence of magnetic fields causes both a direct
(Zeeman-broadening) effect on spectral lines with non-zero Land\'e factor and
an indirect effect on temperature-sensitive lines via a change in the
photospheric T-tau stratification. The corresponding correction in the
estimated atomic abundance ranges from a few hundredths of a dex up to |Delta
log(Fe)| ~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the IR. The
largest modification to previous solar abundance determinations based on
visible spectral lines is instead due to the indirect effect, i.e., the
line-weakening caused by a warmer stratification on an optical depth scale. Our
results indicate that the average solar iron abundance obtained when using
magnetoconvection models can be 0.03-0.11 dex higher than when using the
simpler HD convection approach. We demonstrate that accounting for magnetic
flux is important in state-of-the-art solar photospheric abundance
determinations based on 3D simulations.
|
1209.2771v1
|
2012-10-02
|
X-ray spectroscopy of the ADC source X1822-371 with Chandra and XMM-Newton
|
The eclipsing low-mass X-ray binary X1822-371 is the prototype of the
accretion disc corona (ADC) sources. We analyse two Chandra observations and
one XMM-Newton observation to study the discrete features and their variation
as a function of the orbital phase, deriving constraints on the temperature,
density, and location of the plasma responsible for emission lines. The HETGS
and XMM/Epic-pn observed X1822-371 for 140 and 50 ks, respectively. We
extracted an averaged spectrum and five spectra from five selected
orbital-phase intervals that are 0.04-0.25, 0.25-0.50, 0.50-0.75, 0.75-0.95,
and, finally, 0.95-1.04; the orbital phase zero corresponds to the eclipse
time. All spectra cover the energy band between 0.35 and 12 keV. We confirm the
presence of local neutral matter that partially covers the X-ray emitting
region; the equivalent hydrogen column is $5 \times 10^{22}$ cm$ ^{-2}$ and the
covered fraction is about 60-65%. We identify emission lines from highly
ionised elements, and a prominent fluorescence iron line associated with a
blending of FeI-FeXV resonant transitions. The transitions of He-like ions show
that the intercombination dominates over the forbidden and resonance lines. The
line fluxes are the highest during the orbital phases between 0.04 and 0.75. We
discuss the presence of an extended, optically thin corona with optical depth
of about 0.01 that scatters the X-ray photons from the innermost region into
the line of sight. The photoionised plasma producing most of the observed lines
is placed in the bulge at the outer radius of the disc distant from the central
source of $6 \times 10^{10}$ cm. The OVII and the fluorescence iron line are
probably produced in the photoionised surface of the disc at inner radii.
(Abridged)
|
1210.0874v1
|
2012-10-25
|
XMM-Newton observations of the Galactic Centre Region - I: The distribution of low-luminosity X-ray sources
|
We exploit XMM-Newton archival data in a study of the extended X-ray emission
emanating from the Galactic Centre (GC) region. EPIC-pn and EPIC-MOS
observations, with a total exposure approaching 0.5 and 1 Ms respectively, were
used to create mosaiced images of a 100 pc x 100 pc region centred on Sgr A* in
four bands covering the 2-10 keV energy range. We have also constructed a set
of narrow-band images corresponding to the neutral iron fluorescence line at
6.4 keV and the K-shell lines at 6.7 keV and 6.9 keV from helium-like and
hydrogenic iron. We use a combination of spatial and spectral information to
decompose the GC emission into three distinct components. These comprise: the
emission from hard X-ray emitting unresolved point sources; the reflected
continuum and fluorescent line emission from dense molecular material; and the
soft diffuse emission from thermal plasma in the temperature range, kT ~
0.8-1.5 keV.
We show that the unresolved-source component accounts for the bulk of the
6.7-keV and 6.9-keV line emission. We fit the observed X-ray surface brightness
distribution with an empirical 2-d model, which we then compare with a 3-d
mass-model prediction for the old stellar population in the GC. The X-ray
surface brightness falls-off more rapidly with angular offset from Sgr A* than
predicted. One interpretation is that the 2-10 keV X-ray emissivity increases
from 5 x 10^27 erg s^-1 Msun^-1 at 20' up to almost twice this value at 2'.
Alternatively, some refinement of the mass model may be required.
The unresolved hard X-ray emitting source population, on the basis of
spectral comparisons, is most likely dominated by magnetic CVs. We use the
X-ray observations to set constraints on the number density of such sources.
Our analysis does not support the conjecture that a significant fraction of the
hard X-ray emission from the GC originates in very-hot diffuse thermal plasma.
|
1210.6808v1
|
2012-11-12
|
Theory of Quantum Phase Transition in Iron-based Superconductors with Half-Dirac Nodal Electron Fermi Surface
|
The quantum phase transition in iron-based superconductors with 'half-Dirac'
node at the electron Fermi surface as a $T=0$ structural phase transition
described in terms of nematic order is discussed. An effective low energy
theory that describes half-Dirac nodal Fermions and their coupling to Ising
nematic order that describes the phase transition is derived and analyzed using
renormalization group (RG) study of the large-$N_f$ version of the theory. The
inherent absence of Lorentz invariance of the theory leads to RG flow structure
where the velocities $v_F$ and $v_\Delta$ at the paired half-Dirac nodes
($1\overline{1}$ and $2\overline{2}$) in general flow differently under RG,
implying that the nodal electron gap is deformed and the $C_4$ symmetry is
broken, explaining the structural (orthogonal to orthorhombic) phase transition
at the quantum critical point (QCP). The theory is found to have Gaussian fixed
point $\lambda^*=0, (v_{\Delta}/v_F)^*=0$ with stable flow lines toward it,
suggesting a second order nematic phase transition. Interpreting the
fermion-Ising nematic boson interaction as a decay process of nematic Ising
order parameter scalar field fluctuations into half-Dirac nodal fermions, I
find that the theory surprisingly behaves as systems with dynamical critical
exponent $z = 1$, reflecting undamped quantum critical dynamics and emergent
fully relativistic field theory arising from the non(fully)-relativistic field
theory and is direct consequence of $(v_{\Delta}/v_F)^*=0$ fixed point. The
nematic critical fluctuations lead to remarkable change to the spectral
function peak where at a critical point $\lambda_c$, directly related to
nematic QCP, the central spectral peak collapses and splits into satellite
spectral peaks around nodal point. The vanishing of the zero modes density of
states leads to the undamped $z=1$ quantum critical dynamics.
|
1211.2790v4
|
2012-12-04
|
An Examination of the Spectral Variability in NGC 1365 with Suzaku
|
We present jointly analyzed data from three deep Suzaku observations of NGC
1365. These high signal-to-noise spectra enable us to examine the nature of
this variable, obscured AGN in unprecedented detail on timescales ranging from
hours to years. We find that, in addition to the power-law continuum and
absorption from ionized gas seen in most AGN, inner disk reflection and
variable absorption from neutral gas within the Broad Emission Line Region are
both necessary components in all three observations. We confirm the clumpy
nature of the cold absorbing gas, though we note that occultations of the inner
disk and corona are much more pronounced in the high-flux state (2008) than in
the low-flux state (2010) of the source. The onset and duration of the "dips"
in the X-ray light curve in 2010 are both significantly longer than in 2008,
however, indicating that either the distance to the gas from the black hole is
larger, or that the nature of the gas has changed between epochs. We also note
significant variations in the power-law flux over timescales similar to the
cold absorber, both within and between the three observations. The warm
absorber does not vary significantly within observations, but does show
variations in column density of a factor of more than 10 on timescales less
than 2 weeks that seem unrelated to the changes in the continuum, reflection or
cold absorber. By assuming a uniform iron abundance for the reflection and
absorption, we have also established that an iron abundance of roughly 3.5
times the solar value is sufficient to model the broad-band spectrum without
invoking an additional partial-covering absorber. Such a measurement is
consistent with previous published constraints from the 2008 Suzaku observation
alone, and with results from other Seyfert AGN in the literature.
|
1212.0772v1
|
2012-12-06
|
Radiative cooling in collisionally and photo ionized plasmas
|
We discuss recent improvements in the calculation of the radiative cooling in
both collisionally and photo ionized plasmas. We are extending the spectral
simulation code Cloudy so that as much as possible of the underlying atomic
data is taken from external databases, some created by others, some developed
by the Cloudy team. This paper focuses on recent changes in the treatment of
many stages of ionization of iron, and discusses its extensions to other
elements. The H-like and He-like ions are treated in the iso-electronic
approach described previously. Fe II is a special case treated with a large
model atom. Here we focus on Fe III through Fe XXIV, ions which are important
contributors to the radiative cooling of hot, 1e5 to 1e7 K, plasmas and for
X-ray spectroscopy. We use the Chianti atomic database to greatly expand the
number of transitions in the cooling function. Chianti only includes lines that
have atomic data computed by sophisticated methods. This limits the line list
to lower excitation, longer wavelength, transitions. We had previously included
lines from the Opacity Project database, which tends to include higher energy,
shorter wavelength, transitions. These were combined with various forms of the
g-bar approximation, a highly approximate method of estimating collision rates.
For several iron ions the two databases are almost entirely complementary. We
adopt a hybrid approach in which we use Chianti where possible, supplemented by
lines from the Opacity Project for shorter wavelength transitions. The total
cooling including the lightest thirty elements differs significantly from some
previous calculations.
|
1212.1233v1
|
2013-01-04
|
Optical properties of iron-based superconductor LiFeAs single crystal
|
We have measured the reflectivity spectra of the iron based superconductor
LiFeAs (Tc = 17.6 K) in the temperature range from 4 to 300 K. In the
superconducting state (T < Tc), the clear opening of the optical absorption gap
was observed below 25 cm-1, indicating an isotropic full gap formation. In the
normal state (T > Tc), the optical conductivity spectra display a typical
metallic behavior with the Drude type spectra at low frequencies, but we found
that the introduction of the two Drude components best fits the data,
indicating the multiband nature of this superconductor. A theoretical analysis
of the low temperature data (T=4K < Tc) also suggests that two superconducting
gaps best fit the data and their values were estimated as {\Delta}1 = 1.59 meV
and {\Delta}2 = 3.15 meV, respectively. Using the Ferrell-Glover-Tinkham (FGT)
sum rule and dielectric function {\epsilon}1({\omega}), the superconducting
plasma frequency ({\omega}ps) is consistently estimated to be 6,665 cm-1,
implying that about 59 % of the free carriers in the normal state condenses
into the SC condensate. To investigate the various interband transition
processes (for {\omega} > 200 cm-1), we have also performed the local-density
approximation (LDA) band calculation and calculated the optical spectra of the
interband transitions. The theoretical results provided a qualitative agreement
with the experimental data below 4000 cm-1
|
1301.0694v2
|
2013-02-06
|
Octant sensitivity for large theta(13) in atmospheric and long baseline neutrino experiments
|
One of the unknown parameters in neutrino oscillations is the octant of the
mixing angle theta_{23}. In this paper, we discuss the possibility of
determining the octant of theta_{23} in the long baseline experiments T2K and
NOvA in conjunction with future atmospheric neutrino detectors, in light of
non-zero value of theta_{13} measured by reactor experiments. We consider two
detector technologies for atmospheric neutrinos - magnetized iron calorimeter
and non-magnetized Liquid Argon Time Projection Chamber. We present the octant
sensitivity for T2K/NOvA and atmospheric neutrino experiments separately as
well as combined. For the long baseline experiments, a precise measurement of
theta_{13}, which can exclude degenerate solutions in the wrong octant,
increases the sensitivity drastically. For theta_{23} = 39^o and sin^2 2
theta_{13} = 0.1, at least ~2 sigma sensitivity can be achieved by T2K+NOvA for
all values of delta_{CP} for both normal and inverted hierarchy. For
atmospheric neutrinos, the moderately large value of theta_{13} measured in the
reactor experiments is conducive to octant sensitivity because of enhanced
matter effects. A magnetized iron detector can give a 2 sigma octant
sensitivity for 500 kT yr exposure for theta_{23} = 39^o, delta_{CP} = 0 and
normal hierarchy. This increases to 3 sigma for both hierarchies by combining
with T2K+NOvA. This is due to a preference of different theta_{23} values at
the minimum chi^2 by T2K/NOvA and atmospheric neutrino experiments. A Liquid
Argon detector for atmospheric neutrinos with the same exposure can give higher
octant sensitivity, due to the interplay of muon and electron contributions and
superior resolutions. We obtain a ~3 sigma sensitivity for theta_{23} = 39^o
for normal hierarchy. This increases to > ~4 sigma for all values of delta_{CP}
if combined with T2K+NOvA. For inverted hierarchy the combined sensitivity is
~3 sigma.
|
1302.1370v2
|
2013-02-13
|
Measuring Black Hole Spin using X-ray Reflection Spectroscopy
|
I review the current status of X-ray reflection (a.k.a. broad iron line)
based black hole spin measurements. This is a powerful technique that allows us
to measure robust black hole spins across the mass range, from the stellar-mass
black holes in X-ray binaries to the supermassive black holes in active
galactic nuclei. After describing the basic assumptions of this approach, I lay
out the detailed methodology focusing on "best practices" that have been found
necessary to obtain robust results. Reflecting my own biases, this review is
slanted towards a discussion of supermassive black hole (SMBH) spin in active
galactic nuclei (AGN). Pulling together all of the available XMM-Newton and
Suzaku results from the literature that satisfy objective quality control
criteria, it is clear that a large fraction of SMBHs are rapidly-spinning,
although there are tentative hints of a more slowly spinning population at high
(M>5*10^7Msun) and low (M<2*10^6Msun) mass. I also engage in a brief review of
the spins of stellar-mass black holes in X-ray binaries. In general,
reflection-based and continuum-fitting based spin measures are in agreement,
although there remain two objects (GROJ1655-40 and 4U1543-475) for which that
is not true. I end this review by discussing the exciting frontier of
relativistic reverberation, particularly the discovery of broad iron line
reverberation in XMM-Newton data for the Seyfert galaxies NGC4151, NGC7314 and
MCG-5-23-16. As well as confirming the basic paradigm of relativistic disk
reflection, this detection of reverberation demonstrates that future large-area
X-ray observatories such as LOFT will make tremendous progress in studies of
strong gravity using relativistic reverberation in AGN.
|
1302.3260v2
|
2013-03-12
|
Fast and Furious: Shock Heated Gas as the Origin of Spatially Resolved Hard X-ray Emission in the Central 5 kpc of the Galaxy Merger NGC 6240
|
We have obtained a deep, sub-arcsecond resolution X-ray image of the nuclear
region of the luminous galaxy merger NGC 6240 with Chandra, which resolves the
X-ray emission from the pair of active nuclei and the diffuse hot gas in great
detail. We detect extended hard X-ray emission from kT~6 keV (~70 million K)
hot gas over a spatial scale of 5 kpc, indicating the presence of fast shocks
with velocity of ~2200 km/s. For the first time we obtain the spatial
distribution of this highly ionized gas emitting FeXXV, which shows a
remarkable correspondence to the large scale morphology of H_2(1-0) S(1) line
emission and H\alpha filaments. Propagation of fast shocks originated in the
starburst driven wind into the ambient dense gas can account for this
morphological correspondence. With an observed L(0.5-8 keV)=5.3E+41 erg/s, the
diffuse hard X-ray emission is 100 times more luminous than that observed in
the classic starburst galaxy M82. Assuming a filling factor of 1% for the 70 MK
temperature gas, we estimate its total mass (M_{hot}=1.8E+8 Msun) and thermal
energy (E_{th}=6.5E+57 ergs). The total iron mass in the highly ionized plasma
is M_{Fe}=4.6E+5 Msun. Both the energetics and the iron mass in the hot gas are
consistent with the expected injection from the supernovae explosion during the
starburst that is commensurate with its high star formation rate. No evidence
for fluorescent Fe I emission is found in the CO filament connecting the two
nuclei.
|
1303.2980v2
|
2013-03-12
|
Lithium abundance in the metal-poor open cluster NGC 2243
|
Lithium is a fundamental element for studying the mixing mechanisms acting in
the stellar interiors, for understanding the chemical evolution of the Galaxy
and the Big Bang nucleosynthesis. The study of Li in stars of open clusters
(hereafter OC) allows a detailed comparison with stellar evolutionary models
and permits us to trace its galactic evolution. The OC NGC 2243 is particularly
interesting because of its low metallicity ([Fe/H]=$-0.54 \pm0.10$ dex). We
measure the iron and lithium abundance in stars of the metal-poor OC NGC 2243.
The first aim is to determine whether the Li dip extends to such low
metallicities, the second is to compare the results of our Li analysis in this
OC with those present in 47 Tuc, a globular cluster of similar metallicity. We
performed a detailed analysis of high-resolution spectra obtained with the
multi-object facility FLAMES at the ESO VLT 8.2m telescope. Lithium abundance
was derived through line equivalent widths and the OSMARCS atmosphere models.
We determine a Li dip center of 1.06 $M_\odot$, which is much smaller than that
observed in solar metallicity and metal-rich clusters. This finding confirms
and strengthens the conclusion that the mass of the stars in the Li dip
strongly depends on stellar metallicity. The mean Li abundance of the cluster
is $\log n{\rm (Li)}=2.70$ dex, which is substantially higher than that
observed in 47 Tuc. We estimated an iron abundance of [Fe/H]=$-0.54 \pm0.10$
dex for NGC 2243, which is similar (within the errors) to previous findings.
The [$ \alpha$/Fe] content ranges from $0.00\pm0.14$ for Ca to $0.20\pm0.22$
for Ti, which is low when compared to thick disk stars and to Pop II stars, but
compatible with thin disk objects. We found a mean radial velocity of 61.9
$\pm$ 0.8 \kms for the cluster.
|
1303.3027v1
|
2013-04-27
|
Environments of Strong / Ultrastrong, Ultraviolet Fe II Emitting Quasars
|
We have investigated the strength of ultraviolet Fe II emission from quasars
within the environments of Large Quasar Groups (LQGs) in comparison with
quasars elsewhere, for 1.1 <= <z_LQG> <= 1.7, using the DR7QSO catalogue of the
Sloan Digital Sky Survey. We use the Weymann et al. W2400 equivalent width,
defined between the rest-frame continuum-windows 2240-2255 and 2665-2695 Ang.,
as the measure of the UV Fe II emission. We find a significant shift of the
W2400 distribution to higher values for quasars within LQGs, predominantly for
those LQGs with 1.1 <= <z_LQG> <= 1.5. There is a tentative indication that the
shift to higher values increases with the quasar i magnitude. We find evidence
that within LQGs the ultrastrong emitters with W2400 >= 45 Ang. (more
precisely, ultrastrong-plus with W2400 >= 44 Ang.) have preferred
nearest-neighbour separations of ~ 30-50 Mpc to the adjacent quasar of any
W2400 strength. No such effect is seen for the ultrastrong emitters that are
not in LQGs. The possibilities for increasing the strength of the Fe II
emission appear to be iron abundance, Ly-alpha fluorescence, and
microturbulence, and probably all of these operate. The dense environment of
the LQGs may have led to an increased rate of star formation and an enhanced
abundance of iron in the nuclei of galaxies. Similarly the dense environment
may have led to more active blackholes and increased Ly-alpha fluorescence. The
preferred nearest-neighbour separation for the stronger emitters would appear
to suggest a dynamical component, such as microturbulence. In one particular
LQG, the Huge-LQG (the largest structure known in the early universe), six of
the seven strongest emitters very obviously form three pairings within the
total of 73 members.
|
1304.7396v2
|
2013-05-15
|
Dust input from AGB stars in the Large Magellanic Cloud
|
The dust-forming population of AGB stars and their input to the interstellar
dust budget of the Large Magellanic Cloud (LMC) are studied with evolutionary
dust models with the main goals (1) to investigate how the amount and
composition of dust from AGB stars vary over galactic history; (2) to
characterise the mass and metallicity distribution of the present population of
AGB stars; (3) to quantify the contribution of AGB stars of different mass and
metallicity to the present stardust population in the interstellar medium
(ISM). We use models of the stardust lifecycle in the ISM developed and tested
for the Solar neighbourhood. The first global spatially resolved reconstruction
of the star formation history of the LMC from the Magellanic Clouds Photometric
Survey is employed to calculate the stellar populations in the LMC. The dust
input from AGB stars is dominated by carbon grains from stars with masses < 4
Msun almost over the entire history of the LMC. The production of silicate,
silicon carbide and iron dust is delayed until the ISM is enriched to about
half the present metallicity in the LMC. For the first time, theoretically
calculated dust production rates of AGB stars are compared to those derived
from IR observations of AGB stars for the entire galaxy. We find good agreement
within scatter of various observational estimates. We show that the majority of
silicate and iron grains in the present stardust population originate from a
small population of intermediate-mass stars consisting of only about 4% of the
total number of stars, whereas in the Solar neighbourhood they originate from
low-mass stars. With models of the lifecycle of stardust grains in the ISM we
confirm a large discrepancy between dust input from stars and the existing
interstellar dust mass in the LMC reported in Matsuura et al. 2009.
|
1305.3521v1
|
2013-06-05
|
Chemical abundances of the Milky Way thick disk and stellar halo II.: sodium, iron-peak and neutron-capture elements
|
We present chemical abundance analyses of sodium, iron-peak and
neutron-capture elements for 97 kinematically selected thick disk, inner halo
and outer halo stars with metallicities -3.3<[Fe/H]<-0.5. The main aim of this
study is to examine chemical similarities and differences among metal-poor
stars belonging to these old Galactic components as a clue to determine their
early chemodynamical evolution. In our previous paper, we obtained abundances
of alpha elements by performing a one-dimensional LTE abundance analysis based
on the high-resolution (R~50000) spectra obtained with the Subaru/HDS. In this
paper, a similar analysis is performed to determine abundances of an additional
17 elements. We show that, in metallicities below [Fe/H]~-2, the abundance
ratios of many elements in the thick disk, inner halo, and outer halo
subsamples are largely similar. In contrast, in higher metallicities
([Fe/H]>-1.5), differences in some of the abundance ratios among the three
subsamples are identified. Specifically, the [Na/Fe], [Ni/Fe], [Cu/Fe], and
[Zn/Fe] ratios in the inner and outer halo subsamples are found to be lower
than those in the thick disk subsample. In contrast to what was observed for
[Mg/Fe] in our previous paper, [Eu/Fe] ratios are more enhanced in the two halo
subsamples rather than in the thick disk subsample. The observed distinct
chemical abundances of some elements between the thick disk and inner/outer
halo subsamples with [Fe/H]>-1.5 support the hypothesis that these components
formed through different mechanisms. In particular, our results favor the
scenario that the inner and outer halo components formed through an assembly of
multiple progenitor systems that experienced various degrees of chemical
enrichments, while the thick disk formed through rapid star formation with an
efficient mixing of chemical elements.
|
1306.0954v1
|
2013-06-09
|
Advanced burning stages and fate of 8-10 Mo stars
|
The stellar mass range 8<M/Mo<12 corresponds to the most massive AGB stars
and the most numerous massive stars. It is host to a variety of supernova
progenitors and is therefore very important for galactic chemical evolution and
stellar population studies. In this paper, we study the transition from
super-AGB star to massive star and find that a propagating neon-oxygen burning
shell is common to both the most massive electron capture supernova (EC-SN)
progenitors and the lowest mass iron-core collapse supernova (FeCCSN)
progenitors. Of the models that ignite neon burning off-center, the 9.5Mo model
would evolve to an FeCCSN after the neon-burning shell propagates to the
center, as in previous studies. The neon-burning shell in the 8.8Mo model,
however, fails to reach the center as the URCA process and an extended (0.6 Mo)
region of low Ye (0.48) in the outer part of the core begin to dominate the
late evolution; the model evolves to an EC-SN. This is the first study to
follow the most massive EC-SN progenitors to collapse, representing an
evolutionary path to EC-SN in addition to that from SAGB stars undergoing
thermal pulses. We also present models of an 8.75Mo super-AGB star through its
entire thermal pulse phase until electron captures on 20Ne begin at its center
and of a 12Mo star up to the iron core collapse. We discuss key uncertainties
and how the different pathways to collapse affect the pre-supernova structure.
Finally, we compare our results to the observed neutron star mass distribution.
|
1306.2030v1
|
2013-06-10
|
X-ray Outflows and Super-Eddington Accretion in the Ultraluminous X-ray Source Holmberg IX X-1
|
Studies of X-ray continuum emission and flux variability have not
conclusively revealed the nature of ultra-luminous X-ray sources (ULXs) at the
high-luminosity end of the distribution (those with Lx > 1e40 erg/s). These are
of particular interest because the luminosity requires either super-Eddington
accretion onto a black hole of mass ~10 Msun, or more standard accretion onto
an intermediate-mass black hole. Super-Eddington accretion models predict
strong outflowing winds, making atomic absorption lines a key diagnostic of the
nature of extreme ULXs. To search for such features, we have undertaken a long,
500 ks observing campaign on Holmberg IX X-1 with Suzaku. This is the most
sensitive dataset in the iron K bandpass for a bright, isolated ULX to date,
yet we find no statistically significant atomic features in either emission or
absorption; any undetected narrow features must have equivalent widths less
than 15-20 eV at 99% confidence. These limits are far below the >150 eV lines
expected if observed trends between mass inflow and outflow rates extend into
the super-Eddington regime, and in fact rule out the line strengths observed
from disk winds in a variety of sub-Eddington black holes. We therefore cannot
be viewing the central regions of Holmberg IX X-1 through any substantial
column of material, ruling out models of spherical super-Eddington accretion.
If Holmberg IX X-1 is a super-Eddington source, any associated outflow must
have an anisotropic geometry. Finally, the lack of iron emission suggests that
the stellar companion cannot be launching a strong wind, and that Holmberg IX
X-1 must primarily accrete via roche-lobe overflow.
|
1306.2317v1
|
2013-06-11
|
Thermal convection in Earth's inner core with phase change at its boundary
|
Inner core translation, with solidification on one hemisphere and melting on
the other, provides a promising basis for understanding the hemispherical
dichotomy of the inner core, as well as the anomalous stable layer observed at
the base of the outer core - the F-layer - which might be sustained by
continuous melting of inner core material. In this paper, we study in details
the dynamics of inner core thermal convection when dynamically induced melting
and freezing of the inner core boundary (ICB) are taken into account. If the
inner core is unstably stratified, linear stability analysis and numerical
simulations consistently show that the translation mode dominates only if the
viscosity $\eta$ is large enough, with a critical viscosity value, of order $3
10^{18}$ Pas, depending on the ability of outer core convection to supply or
remove the latent heat of melting or solidification. If $\eta$ is smaller, the
dynamical effect of melting and freezing is small. Convection takes a more
classical form, with a one-cell axisymmetric mode at the onset and chaotic
plume convection at large Rayleigh number. [...] Thermal convection requires
that a superadiabatic temperature profile is maintained in the inner core,
which depends on a competition between extraction of the inner core internal
heat by conduction and cooling at the ICB. Inner core thermal convection
appears very likely with the low thermal conductivity value proposed by Stacey
& Davis (2007), but nearly impossible with the much higher thermal conductivity
recently put forward. We argue however that the formation of an iron-rich layer
above the ICB may have a positive feedback on inner core convection: it implies
that the inner core crystallized from an increasingly iron-rich liquid,
resulting in an unstable compositional stratification which could drive inner
core convection, perhaps even if the inner core is subadiabatic.
|
1306.2482v1
|
2013-06-25
|
Superorbital Phase-Resolved Analysis of SMC X-1
|
The high-mass X-ray binary SMC X-1 is an eclipsing binary with an orbital
period of 3.89 d. This system exhibits a superorbital modulation with a period
varying between ~40 d and ~65 d. The instantaneous frequency and the
corresponding phase of the superorbital modulation can be obtained by a
recently developed time-frequency analysis technique, the Hilbert-Huang
transform (HHT). We present a phase-resolved analysis of both the spectra and
the orbital profiles with the superorbital phase derived from the HHT. The
X-ray spectra observed by the Proportional Counter Array onboard the Rossi
X-ray Timing Explorer are fitted well by a blackbody plus a Comptonized
component. The plasma optical depth, which is a good indicator of the
distribution of material along the line of sight, is significantly
anti-correlated with the flux detected at 2.5-25 keV. However, the relationship
between the plasma optical depth and the equivalent width of the iron line is
not monotonic: there is no significant correlation for fluxes higher than ~35
mCrab but clear positive correlation when the intensity is lower than ~20
mCrab. This indicates that the iron line production is dominated by different
regions of this binary system in different superorbital phases. To study the
dependence of the orbital profile on the superorbital phase, we obtained the
eclipse profiles by folding the All Sky Monitor light curve with the orbital
period for different superorbital states. A dip feature, similar to the
pre-eclipse dip in Her X-1, lying at orbital phase ~0.6-0.85, was discovered
during the superorbital transition state. This indicates that the accretion
disk has a bulge that absorbs considerable X-ray emission in the stream-disk
interaction region. The dip width is anti-correlated with the flux, and this
relation can be interpreted by the precessing tilted accretion disk scenario.
|
1306.5819v1
|
2013-07-30
|
New and updated stellar parameters for 71 evolved planet hosts. On the metallicity - giant planet connection
|
It is still being debated whether the well-known metallicity - giant planet
correlation for dwarf stars is also valid for giant stars. For this reason,
having precise metallicities is very important. Different methods can provide
different results that lead to discrepancies in the analysis of planet hosts.
To study the impact of different analyses on the metallicity scale for evolved
stars, we compare different iron line lists to use in the atmospheric parameter
derivation of evolved stars. Therefore, we use a sample of 71 evolved stars
with planets. With these new homogeneous parameters, we revisit the metallicity
- giant planet connection for evolved stars. A spectroscopic analysis based on
Kurucz models in local thermodynamic equilibrium (LTE) was performed through
the MOOG code to derive the atmospheric parameters. Two different iron line
list sets were used, one built for cool FGK stars in general, and the other for
giant FGK stars. Masses were calculated through isochrone fitting, using the
Padova models. Kolmogorov-Smirnov tests (K-S tests) were then performed on the
metallicity distributions of various different samples of evolved stars and red
giants. All parameters compare well using a line list set, designed
specifically for cool and solar-like stars to provide more accurate
temperatures. All parameters derived with this line list set are preferred and
are thus adopted for future analysis. We find that evolved planet hosts are
more metal-poor than dwarf stars with giant planets. However, a bias in giant
stellar samples that are searched for planets is present. Because of a colour
cut-off, metal-rich low-gravity stars are left out of the samples, making it
hard to compare dwarf stars with giant stars. Furthermore, no metallicity
enhancement is found for red giants with planets ($\log g < 3.0$\,dex) with
respect to red giants without planets.
|
1307.7870v1
|
2013-08-07
|
Abundance gradients in spiral disks: is the gradient inversion at high redshift real?
|
We compute the abundance gradients along the disk of the Milky Way by means
of the two-infall model: in particular, the gradients of oxygen and iron and
their temporal evolution. First, we explore the effects of several physical
processes which influence the formation and evolution of abundance gradients.
They are: i) the inside-out formation of the disk, ii) a threshold in the gas
density for star formation, iii) a variable star formation efficiency along the
disk, iv) radial flows and their speed, and v) different total surface mass
density (gas plus stars) distributions for the halo. We are able to reproduce
at best the present day gradients of oxygen and iron if we assume an inside-out
formation, no threshold gas density, a constant efficiency of star formation
along the disk and radial gas flows. It is particularly important the choice of
the velocity pattern for radial flows and the combination of this velocity
pattern with the surface mass density distribution in the halo. Having selected
the best model, we then explore the evolution of abundance gradients in time
and find that the gradients in general steepen in time and that at redshift z~3
there is a gradient inversion in the inner regions of the disk, in the sense
that at early epochs the oxygen abundance decreases toward the Galactic center.
This effect, which has been observed, is naturally produced by our models if an
inside-out formation of the disk and and a constant star formation efficiency
are assumed. The inversion is due to the fact that in the inside-out formation
a strong infall of primordial gas, contrasting chemical enrichment, is present
in the innermost disk regions at early times. The gradient inversion remains
also in the presence of radial flows, either with constant or variable speed in
time, and this is a new result.
|
1308.1549v1
|
2013-08-09
|
KASCADE-Grande measurements of energy spectra for elemental groups of cosmic rays
|
The KASCADE-Grande experiment, located at KIT-Karlsruhe, Germany, consists of
a large scintillator array for measurements of charged particles, N_ch, and of
an array of shielded scintillation counters used for muon counting, N_mu.
KASCADE-Grande is optimized for cosmic ray measurements in the energy range 10
PeV to 1000 PeV, thereby enabling the verification of a knee in the iron
spectrum expected at approximately 100 PeV. Exploring the composition in this
energy range is of fundamental importance for understanding the transition from
galactic to extragalactic cosmic rays. Following earlier studies of elemental
spectra reconstructed in the knee energy range from KASCADE data, we have now
extended these measurements to beyond 100 PeV. By analysing the two-dimensional
shower size spectrum N_ch vs. N_mu, we reconstruct the energy spectra of
different mass groups by means of unfolding methods. The procedure and its
results, giving evidence for a knee-like structure in the spectrum of iron
nuclei, will be presented.
|
1308.2098v1
|
2013-08-09
|
S-process in low-mass extremely metal-poor stars
|
Extremely metal-poor low-mass stars experience an ingestion of protons into
the helium-rich layer during the core He-flash, resulting in the production of
neutrons through the reactions
^{12}C(p,\gamma)^{13}N(\beta)^{13}C(\alpha,n)^{16}O. This is a potential site
for the production of s-process elements in extremely metal-poor stars not
occurring in more metal-rich counterparts. Observationally, the signatures of
s-process elements in the two most iron deficient stars observed to date,
HE1327-2326 & HE0107-5240, still await for an explanation. We investigate the
possibility that low-mass EMP stars could be the source of s-process elements
observed in extremely iron deficient stars, either as a result of
self-enrichment or in a binary scenario as the consequence of a mass transfer
episode. We present evolutionary and post-processing s-process calculations of
a 1Msun stellar model with metallicities Z=0, 10^{-8} and 10^{-7}. We assess
the sensitivity of nucleosynthesis results to uncertainties in the input
physics of the stellar models, particularly regarding the details of convective
mixing during the core He-flash. Our models provide the possibility of
explaining the C, O, Sr, and Ba abundance for the star HE0107-5240 as the
result of mass-transfer from a low-mass EMP star. The drawback of our model is
that if mass would be transferred before the primary star enters the AGB phase,
nitrogen would be overproduced and the ^{12}C/^{13}C abundance ratio would be
underproduced in comparison to the observed values. Our results show that
low-mass EMP stars cannot be ruled out as the companion stars that might have
polluted HE1327-2326 & HE0107-5240 and produced the observed s-process pattern.
However, more detailed studies of the core He-flash and the proton ingestion
episode are needed to determine the robustness of our predictions.
|
1308.2224v1
|
2013-08-12
|
Distinguishing $s^{\pm}$ and $s^{++}$ electron pairing symmetries by neutron spin resonance in superconducting NaFe$_{0.935}$Co$_{0.045}$As
|
A determination of the superconducting (SC) electron pairing symmetry forms
the basis for establishing a microscopic mechansim for superconductivity. For
iron pnictide superconductors, the $s^\pm$-pairing symmetry theory predicts the
presence of a sharp neutron spin resonance at an energy below the sum of hole
and electron SC gap energies ($E\leq 2\Delta$) below $T_c$. On the other hand,
the $s^{++}$-pairing symmetry expects a broad spin excitation enhancement at an
energy above $2\Delta$ below $T_c$. Although the resonance has been observed in
iron pnictide superconductors at an energy below $2\Delta$ consistent with the
$s^\pm$-pairing symmetry, the mode has also be interpreted as arising from the
$s^{++}$-pairing symmetry with $E\ge 2\Delta$ due to its broad energy width and
the large uncertainty in determining the SC gaps. Here we use inelastic neutron
scattering to reveal a sharp resonance at E=7 meV in SC
NaFe$_{0.935}$Co$_{0.045}$As ($T_c = 18$ K). On warming towards $T_c$, the mode
energy hardly softens while its energy width increases rapidly. By comparing
with calculated spin-excitations spectra within the $s^{\pm}$ and
$s^{++}$-pairing symmetries, we conclude that the ground-state resonance in
NaFe$_{0.935}$Co$_{0.045}$As is only consistent with the $s^{\pm}$-pairing, and
is inconsistent with the $s^{++}$-pairing symmetry.
|
1308.2453v1
|
2013-08-12
|
3-D non-LTE radiative transfer effects in Fe I lines: II. Line formation in 3-D radiation hydrodynamic simulations
|
We investigate the effects of horizontal radiative transfer (RT) a NLTE on
important diagnostic iron lines in a realistic 3-D HD simulation. Using a
multi-level atom we compute and compare widely used Fe I line profiles at 3
different levels of approximation (3-D NLTE, 1-D NLTE, LTE). We find that the
influence of horizontal RT is of the same order of magnitude as that of NLTE,
although spatially more localized. Also, depending on the temperature of the
surroundings, horizontal RT is found to weaken or strengthen spectral lines.
Line depths and equivalent width may differ by up to 20% against the
corresponding LTE value if 3-D RT is applied. Residual intensity contrasts in
LTE are found to be larger than those in 3-D NLTE by up to a factor of two.
When compared to 1-D NLTE, we find that horizontal RT weakens the contrast by
up to 30% almost independently of the angle of line of sight. While the CLV of
the 1-D and 3-D NLTE contrasts are of similar form, the LTE contrast CLV shows
a different run. The determination of temperatures by 1-D NLTE inversions of
spatially resolved observations may produce errors of up to 200 K if one
neglects 3-D RT. We find a linear correlation between the intensity difference
of 1-D and 3-D NLTE and a simple estimate of the temperature in the horizontal
environment of the line formation region. This correlation could be used to
coarsely correct for the effects of horizontal RT in inversions done in 1-D
NLTE. Horizontal RT is less important if one considers spatially averaged line
profiles because local line strengthening and weakening occur with similar
frequency in our HD atmosphere. Thus, the iron abundance is underestimated by
0.012 dex if calculated using 1-D NLTE RT. Since effects of horizontal RT are
largest for spatially resolved quantities, the use of 3-D RT is particularly
important for the interpretation of high spatial resolution observations.
|
1308.2490v1
|
2013-08-21
|
A High Resolution View of the Warm Absorber in the Quasar MR2251-178
|
High resolution X-ray spectroscopy of the warm absorber in the nearby quasar,
MR2251-178 (z = 0.06398) is presented. The observations were carried out in
2011 using the Chandra High Energy Transmission Grating and the XMM-Newton
Reflection Grating Spectrometer, with net exposure times of approximately 400
ks each. A multitude of absorption lines from C to Fe are detected, revealing
at least 3 warm absorbing components ranging in ionization parameter from
log(\xi/erg cm s^-1) = 1-3 and with outflow velocities < 500 km/s. The lowest
ionization absorber appears to vary between the Chandra and XMM-Newton
observations, which implies a radial distance of between 9-17 pc from the black
hole. Several broad soft X-ray emission lines are strongly detected, most
notably from He-like Oxygen, with FWHM velocity widths of up to 10000 km/s,
consistent with an origin from Broad Line Region (BLR) clouds. In addition to
the warm absorber, gas partially covering the line of sight to the quasar
appears to be present, of typical column density N_H = 10^23 cm^-2. We suggest
that the partial covering absorber may arise from the same BLR clouds
responsible for the broad soft X-ray emission lines. Finally the presence of a
highly ionised outflow in the iron K band from both 2002 and 2011 Chandra HETG
observations appears to be confirmed, which has an outflow velocity of -15600
\pm 2400 km/s. However a partial covering origin for the iron K absorption
cannot be excluded, resulting from low ionization material with little or no
outflow velocity.
|
1308.4550v1
|
2013-09-13
|
The Stellar Abundances for Galactic Archaeology (SAGA) Database III - Analysis of Enrichment Histories for Elements and Two Modes of Star Formation during the Early Evolution of Milky Way
|
We study the enrichment histories for nine elements, C, four alpha-elements
of Mg, Si, Ca, and Ti, Sc, and three iron-peak elements of Co, Ni, and Zn, by
using a large number of stellar data, collected by the Stellar Abundances for
Galactic Archaeology (SAGA) database. We find statistically significant
changes, or breaks, of the mean abundance ratios to iron at three metallicities
of [Fe/H]-1.8, -2.2, and -3.3. Across the first one, the mean abundance ratios
decrease with the metallicity by similar extents for all the elements with the
sufficient data. Across the latter two, downward trends with the metallicity
are also detected but for limited elements, C, Co, Zn, and possibly Sc, and for
two of Co and Zn, respectively. The breaks define four stellar populations with
the different abundance patters which are dominant in each metallicity range
divided by the breaks, Pop IIa, IIb, IIc, and IId in order of increasing
metallicity. We also explore their spatial distributions with the spectroscopic
distances to demonstrate that Pops IIa and IIb spread over the Galactic halo
while Pops IIc and IId are observed near the Galactic plane. In particular, Pop
IIc stars emerge around [Fe/H] -2.6 and coexist with Pop IIb stars, segregated
by the spatial distributions. Our results reveal two distinct modes of star
formation during the early stages of Galaxy formation, which are associated
with the variations of IMF and the spatial distribution of remnant low-mass
stars. For the two lower-metallicity populations, the enhancements of Zn and Co
indicate a high-mass and top-heavy IMF together with the statistics on the
carbon-enhanced stars. We discuss the relevance to the kinematically resolved
structures of the Galactic halo and the possible sites of these populations
within the framework of hierarchical structure formation scenario.
|
1309.3430v2
|
2013-09-17
|
Chemical gradients in the Milky Way from the RAVE data. I. Dwarf stars
|
Aim: We aim at measuring the chemical gradients of the elements Mg, Al, Si,
and Fe along the Galactic radius to provide new constraints on the chemical
evolution models of the Galaxy and Galaxy models such as the Besancon model.
Methods: We analysed three different samples selected from three independent
datasets: a sample of 19,962 dwarf stars selected from the RAVE database, a
sample of 10,616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS)
dataset, and a mock sample (equivalent to the RAVE sample) created by using the
GALAXIA code, which is based on the Besancon model. We measured the chemical
gradients as functions of the guiding radius (Rg) at different distances from
the Galactic plane reached by the stars along their orbit (Zmax).
Results: The chemical gradients of the RAVE and GCS samples are negative and
show consistent trends, although they are not equal: at Zmax<0.4 kpc and
4.5<Rg(kpc)<9.5, the iron gradient for the RAVE sample is d[Fe/H]/dRg=-0.065
dex kpc^{-1}, whereas for the GCS sample it is d[Fe/H]/dRg=-0.043 dex kpc^{-1}
with internal errors +-0.002 and +-0.004 dex kpc^{-1}, respectively. The
gradients of the RAVE and GCS samples become flatter at larger Zmax.
Conversely, the mock sample has a positive iron gradient of
d[Fe/H]/dRg=+0.053+-0.003 dex kpc^{-1} at Zmax<0.4 kpc and remains positive at
any Zmax. These positive and unrealistic values originate from the lack of
correlation between metallicity and tangential velocity in the Besancon model.
The discrepancies between the observational samples and the mock sample can be
reduced by i) decreasing the density, ii) decreasing the vertical velocity, and
iii) increasing the metallicity of the thick disc in the Besancon model.
|
1309.4279v1
|
2013-09-24
|
The magnetic and structural properties near the Lifshitz point in Fe1+xTe
|
We construct a phase diagram of the parent compound Fe1+xTe as a function of
interstitial iron x in terms of the electronic, structural, and magnetic
properties. For a concentration of x < 10%, Fe1+xTe undergoes a "semimetal" to
metal transition at approximately 70 K that is also first-order and coincident
with a structural transition from a tetragonal to a monoclinic unit cell. For x
~ 14%, Fe1+xTe undergoes a second-order phase transition at approximately 58 K
corresponding to a "semimetal" to "semimetal" transition along with a
structural orthorhombic distortion. At a critical concentration of x ~ 11%,
Fe1+xTe undergoes two transitions: the higher temperature one is a second-order
transition to an orthorhombic phase with incommensurate magnetic ordering and
temperature-dependent propagation vector, while the lower temperature one
corresponds to nucleation of a monoclinic phase with a nearly commensurate
magnetic wavevector. While both structural and magnetic transitions display
similar critical behavior for x < 10% and near the critical concentration of x
~ 11%, samples with large interstitial iron concentrations show a marked
deviation between the critical response indicating a decoupling of the order
parameters. Analysis of temperature dependent inelastic neutron data reveals
incommensurate magnetic fluctuations throughout the Fe1+xTe phase diagram are
directly connected to the "semiconductor"-like resistivity above T_N and
implicates scattering from spin fluctuations as the primary reason for the
semiconducting or poor metallic properties. The results suggest that doping
driven Fermi surface nesting maybe the origin of the gapless and incommensurate
spin response at large interstitial concentrations.
|
1309.6310v1
|
2013-10-09
|
Four new X-ray-selected supernova remnants in the Large Magellanic Cloud
|
Aims: We present a detailed multi-wavelength study of four new supernova
remnants (SNRs) in the Large Magellanic Cloud (LMC). The objects were
identified as SNR candidates in X-ray observations performed during the survey
of the LMC with XMM-Newton.
Methods: Data obained with XMM-Newton are used to investigate the
morphological and spectral features of the remnants in X-rays. We measure the
plasma conditions, look for supernova (SN) ejecta emission, and constrain some
of the SNR properties (e.g. age and ambient density). We supplement the X-ray
data with optical, infrared, and radio-continuum archival observations, which
allow us to understand the conditions resulting in the current appearance of
the remnants. Based on the spatially-resolved star formation history (SFH) of
the LMC together with the X-ray spectra, we attempt to type the supernovae that
created the remnants.
Results: We confirm all four objects as SNRs, to which we assign the names
MCSNR J0508-6830, MCSNR J0511-6759, MCSNR J0514-6840, and MCSNR J0517-6759. In
the first two remnants, an X-ray bright plasma is surrounded by very faint [S
II] emission. The emission from the central plasma is dominated by Fe L-shell
lines, and the derived iron abundance is greatly in excess of solar. This
establishes their type Ia (i.e. thermonuclear) SN origin. They appear to be
more evolved versions of other Magellanic Cloud iron-rich SNRs which are
centrally-peaked in X-rays. From the two other remnants (MCSNR J0514-6840 and
MCSNR J0517-6759), we do not see ejecta emission. At all wavelengths at which
they are detected, the local environment plays a key role in their
observational appearance. We present evidence that MCSNR J0517-6759 is close to
and interacting with a molecular cloud, suggesting a massive progenitor.
|
1310.2569v1
|
2013-10-14
|
The Reflection Component from Cygnus X-1 in the Soft State Measured by NuSTAR and Suzaku
|
The black hole binary Cygnus X-1 was observed in late-2012 with the Nuclear
Spectroscopic Telescope Array (NuSTAR) and Suzaku, providing spectral coverage
over the ~1-300 keV range. The source was in the soft state with a
multi-temperature blackbody, power-law, and reflection components along with
absorption from highly ionized material in the system. The high throughput of
NuSTAR allows for a very high quality measurement of the complex iron line
region as well as the rest of the reflection component. The iron line is
clearly broadened and is well-described by a relativistic blurring model,
providing an opportunity to constrain the black hole spin. Although the spin
constraint depends somewhat on which continuum model is used, we obtain a*>0.83
for all models that provide a good description of the spectrum. However, none
of our spectral fits give a disk inclination that is consistent with the most
recently reported binary values for Cyg X-1. This may indicate that there is a
>13 degree misalignment between the orbital plane and the inner accretion disk
(i.e., a warped accretion disk) or that there is missing physics in the
spectral models.
|
1310.3830v1
|
2013-10-16
|
Identification of a Jet-Driven Supernova Remnant in the Small Magellanic Cloud: Possible Evidence for the Enhancement of Bipolar Explosions at Low Metallicity
|
Recent evidence has suggested that the supernova remnant (SNR) 0104-72.3 in
the Small Magellanic Cloud (SMC) may be the result of a prompt Type Ia SN based
on enhanced iron abundances and its association with a star-forming region. In
this paper, we present evidence that SNR 0104-72.3 arose from a jet-driven
bipolar core-collapse SN. Specifically, we use serendipitous Chandra X-ray
Observatory data of SNR 0104-72.3 taken due to its proximity to the calibration
source SNR E0102-72.3. We analyze 56 Advanced CCD Imaging Spectrometer (ACIS)
observations of SNR 0104-72.3 to produce imaging and spectra with an effective
exposure of 528.6 ks. We demonstrate that SNR 0104-72.3 is highly elliptical
relative to other nearby young SNRs, suggesting a core-collapse SN origin.
Furthermore, we compare ejecta abundances derived from spectral fits to
nucleosynthetic yields of Type Ia and core-collapse (CC) SNe, and we find that
the iron, neon, and silicon abundances are consistent with either a spherical
CC SN of a 18-20 solar mass progenitor or an aspherical CC SN of a 25 solar
mass progenitor. We show that the star-formation history at the site of SNR
0104-72.3 is also consistent with a CC origin. Given the bipolar morphology of
the SNR, we favor the aspherical CC SN scenario. This result may suggest
jet-driven SNe occur frequently in the low-metallicity environment of the SMC,
consistent with the observational and theoretical work on broad-line Type Ic
SNe and long-duration gamma-ray bursts.
|
1310.4498v2
|
2013-10-23
|
Functional Renormalization Group for multi-orbital Fermi Surface Instabilities
|
Technological progress in material synthesis, as well as artificial
realization of condensed matter scenarios via ultra-cold atomic gases in
optical lattices or epitaxial growth of thin films, is opening the gate to
investigate a plethora of unprecedented strongly correlated electron systems.
In a large subclass thereof, a metallic state of layered electrons undergoes an
ordering transition below some temperature into unconventional states of matter
driven by electronic correlations, such as magnetism, superconductivity, or
other Fermi surface instabilities. While this type of phenomena has been a
well-established direction of research in condensed matter for decades, the
variety of today's accessible scenarios pose fundamental new challenges to
describe them. A core complication is the multi-orbital nature of the
low-energy electronic structure of these systems, such as the multi-d orbital
nature of electrons in iron pnictides and transition-metal oxides in general,
but also electronic states of matter on lattices with multiple sites per unit
cell such as the honeycomb or kagome lattice. In this review, we propagate the
functional renormalization group (FRG) as a suited approach to investigate
multi-orbital Fermi surface instabilities. The primary goal of the review is to
describe the FRG in explicit detail and render it accessible to everyone both
at a technical and intuitive level. Summarizing recent progress in the field of
multi-orbital Fermi surface instabilities, we illustrate how the unbiased
fashion by which the FRG treats all kinds of ordering tendencies guarantees an
adequate description of electronic phase diagrams and often allows to obtain
parameter trends of sufficient accuracy to make qualitative predictions for
experiments. This review includes detailed and illustrative illustrations of
magnetism and, in particular, superconductivity for the iron pnictides from the
viewpoint of FRG. Furthermore, it discusses candidate scenarios for topological
bulk singlet superconductivity and exotic particle-hole condensates on
hexagonal lattices such as sodium-doped cobaltates, graphene doped to van Hove
Filling, and the kagome Hubbard model. In total, the FRG promises to be one of
the most versatile and revealing numerical approaches to address unconventional
Fermi surface instabilities in future fields of condensed matter research.
|
1310.6191v1
|
2013-11-20
|
The curious time lags of PG 1244+026: Discovery of the iron K reverberation lag
|
High-frequency iron K reverberation lags, where the red wing of the line
responds before the line centroid, are a robust signature of relativistic
reflection off the inner accretion disc. In this letter, we report the
discovery of the Fe K lag in PG 1244+026 from ~120 ks of data (1 orbit of the
XMM-Newton telescope). The amplitude of the lag with respect to the continuum
is 1000 s at a frequency of ~1e-4 Hz. We also find a possible
frequency-dependence of the line: as we probe higher frequencies (i.e. shorter
timescales from a smaller emitting region) the Fe K lag peaks at the red wing
of the line, while at lower frequencies (from a larger emitting region) we see
the dominant reflection lag from the rest frame line centroid. The mean energy
spectrum shows a strong soft excess, though interestingly, there is no
indication of a soft lag. Given that this source has radio emission and it has
little reported correlated variability between the soft excess and the hard
band, we explore one possible explanation in which the soft excess in this
source is dominated by the steep power-law like emission from a jet, and that a
corona (or base of the jet) irradiates the inner accretion disc, creating the
blurred reflection features evident in the spectrum and the lag. General
Relativistic ray-tracing models fit the Fe K lag well, with the best-fit giving
a compact X-ray source at a height of 5 gravitational radii and a black hole
mass of 1.3e7 Msun.
|
1311.5164v1
|
2013-12-08
|
On the kinematic separation of field and cluster stars across the Bulge globular NGC 6528
|
We present deep and precise multi-band photometry of the Galactic Bulge
globular cluster NGC6528. The current dataset includes optical and
near-infrared images collected with ACS/WFC, WFC3/UVIS, and WFC3/IR on board
the Hubble Space Telescope. The images cover a time interval of almost ten
years and we have been able to carry out a proper-motion separation between
cluster and field stars. We performed a detailed comparison in the m_F814W,
m_F606W - m_F814W Color-Magnitude Diagram with two empirical calibrators
observed in the same bands. We found that NGC6528 is coeval with and more
metal-rich than 47Tuc. Moreover, it appears older and more metal-poor than the
super-metal-rich open cluster NGC6791. The current evidence is supported by
several diagnostics (red horizontal branch, red giant branch bump, shape of the
sub-giant branch, slope of the main sequence) that are minimally affected by
uncertainties in reddening and distance. We fit the optical observations with
theoretical isochrones based on a scaled-solar chemical mixture and found an
age of 11 +- 1 Gyr and an iron abundance slightly above solar ([Fe/H = +0.20).
The iron abundance and the old cluster age further support the recent
spectroscopic findings suggesting a rapid chemical enrichment of the Galactic
Bulge.
|
1312.2272v2
|
2013-12-16
|
Magnetic structure map for face-centered tetragonal iron: appearance of a new collinear spin structure
|
For face-centered cubic (fcc) and tetragonal (fct) iron a large number of
magnetic configurations as a function of crystal structural parameters were
studied by means of density functional theory. The stability of magnetic
structures was defined by the magnetic re-orientation energy $\Delta
{E}^i_\text{reor}$ as the difference of the total energy of configuration $i$
and that of the fcc ferromagnetic state. The Cluster Expansion technique was
applied to several volumes deriving $\Delta {E}_\text{reor}$ for more than
90.000 collinear spin structures for each volume. A variety of structures with
promisingly low $\Delta {E}_\text{reor}$ were tetragonally distorted according
to a two-dimensional mesh defined by volume per atom and $c/a$ ratio of the
distortion. At each of the points on this mesh $\Delta E_\text{reor}$ of all
collinear structure were compared to results for non-collinear spin spirals
(SS) which were calculated for a grid of propagation directions. The lowest
$\Delta {E}_\text{reor}$ of all investigated spin structures then defined the
magnetic structure map spanned by volume per atom and $c/a$ ratio. Three local
minima were identified and for each of the minima SS were calculated on a fine
grid of propagation vectors. For the volume per atom of 10.6 \AA$^3$ and the
distortion range $0.94 \le c/a \le 1.01$ we found a new, surprisingly simple
collinear spin structure with four atoms per fct unit cell to be the most
stable one. This structure was called AFM/NM because it consists of two atoms
with anti-ferromagnetically ordered local moments of $\pm 1.8 \mu_\text{B}$ and
of two nonmagnetic atoms with zero local moment. It seems that the newly
detected AFM/NM structure explains a variety of puzzling experimental results.
|
1312.4313v2
|
2014-01-07
|
Superparamagnetic iron oxide polyacrylic acid coated γ-Fe2O3 nanoparticles does not affect kidney function but causes acute effect on the cardiovascular function in healthy mice
|
This study describes the distribution of intravenously injected polyacrylic
acid (PAA) coated {\gamma}-Fe2O3 NPs (10 mg kg-1) at the organ, cellular and
subcellular levels in healthy BALB/cJ mice and in parallel addresses the
effects of NP injection on kidney function, blood pressure and vascular
contractility. Magnetic resonance imaging (MRI) and transmission electron
microscopy (TEM) showed accumulation of NPs in the liver within 1h after
intravenous infusion, accommodated by intracellular uptake in endothelial and
Kupffer cells with subsequent intracellular uptake in renal cells, particularly
the cytoplasm of the proximal tubule, in podocytes and mesangial cells. The
renofunctional effects of NPs were evaluated by arterial acid-base status and
measurements of glomerular filtration rate (GFR) after instrumentation with
chronically indwelling catheters. Arterial pH was 7.46 and 7.41 in mice 0.5 h
after injections of saline or NP, and did not change over the next 12h. In
addition, the injections of NP did not affect arterial PCO2 or [HCO3-] either.
Twenty-four and 96h after NP injections, the GFR averaged 11.0 and 13.0 ml
min-1 g-1, respectively, values which were statistically comparable with
controls (14.0 and 14.0 ml min-1 g-1). Mean arterial blood pressure (MAP)
decreased 12-24h after NP injections (111 vs 123 min-1) associated with a
decreased contractility of small mesenteric arteries revealed by myography to
characterise endothelial function. In conclusion, our study demonstrates that
accumulation of superparamagnetic iron oxide nanoparticles does not affect
kidney function in healthy mice but temporarily decreases blood pressure.
|
1401.1847v1
|
2014-02-12
|
Stokes $IQUV$ Magnetic Doppler Imaging of Ap stars II: Next Generation Magnetic Doppler Imaging of $α^2$ CVn
|
We present updated magnetic field maps of the chemically peculiar B9p star
$\alpha^2$ CVn created using a series of time resolved observations obtained
using the high resolution spectropolarimeters ESPaDOnS and Narval. We compare
these new magnetic field maps with the original magnetic Doppler imaging maps
based on spectra recorded with the MuSiCoS spectropolarimeter and taken a
decade earlier. These new maps are inferred from line profiles in all four
Stokes parameters using the magnetic Doppler imaging code INVERS10. With the
addition of new lines exhibiting Stokes $IQUV$ signatures we have a unique
insight into how the derived magnetic surface structure may be affected by the
atomic lines chosen for inversion. We report new magnetic maps of $\alpha^2$
CVn created using strong iron lines (directly comparable to the published
MuSiCoS maps), weak iron lines and chromium lines, all of which yield a
magnetic field structure roughly consistent with that obtained previously.
We then derive an updated magnetic structure map for $\alpha^2$ CVn based on
the complete sample of Fe and Cr lines, which we believe to produce a more
representative model of the magnetic topology of $\alpha^2$ CVn. In agreement
with the previous mapping, this new updated magnetic map shows a dipolar-like
field which has complex sub-structure which cannot be explained by a simple low
order multipolar geometry. Our new maps show that regardless of the atomic line
or species choice, the reconstructed magnetic field is consistent with that
published previously, suggesting that the reconstructed field is a realistic
representation of the magnetic field of $\alpha^2$ CVn. $\alpha^2$ CVn is the
first Ap star for which multiple, high resolution magnetic maps have been
derived, providing important observational evidence for the stability of both
the large and small-scale magnetic field.
|
1402.2938v1
|
2014-02-15
|
Revealing the location and structure of the accretion disk-wind in PDS456
|
We present evidence for the rapid variability of the high velocity iron
K-shell absorption in the nearby ($z=0.184$) quasar PDS456. From a recent long
Suzaku observation in 2013 ($\sim1$Ms effective duration) we find that the the
equivalent width of iron K absorption increases by a factor of $\sim5$ during
the observation, increasing from $<105$eV within the first 100ks of the
observation, towards a maximum depth of $\sim500$eV near the end. The implied
outflow velocity of $\sim0.25$c is consistent with that claimed from earlier
(2007, 2011) Suzaku observations. The absorption varies on time-scales as short
as $\sim1$ week. We show that this variability can be equally well attributed
to either (i) an increase in column density, plausibly associated with a clumpy
time-variable outflow, or (ii) the decreasing ionization of a smooth
homogeneous outflow which is in photo-ionization equilibrium with the local
photon field. The variability allows a direct measure of absorber location,
which is constrained to within $r=200-3500$$\rm{r_{g}}$ of the black hole. Even
in the most conservative case the kinetic power of the outflow is $\gtrsim6\%$
of the Eddington luminosity, with a mass outflow rate in excess of $\sim40\%$
of the Eddington accretion rate. The wind momentum rate is directly equivalent
to the Eddington momentum rate which suggests that the flow may have been
accelerated by continuum-scattering during an episode of Eddington-limited
accretion.
|
1402.3700v2
|
2014-03-11
|
Broad iron emission line and kilohertz quasi-periodic oscillations in the neutron star system 4U 1636-53
|
Both the broad iron (Fe) line and the frequency of the kilohertz
quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries
(LMXBs) can potentially provide independent measures of the inner radius of the
accretion disc. We use XMM-Newton and simultaneous Rossi X-ray Timing Explorer
observations of the LMXB 4U 1636-53 to test this hypothesis. We study the
properties of the Fe-K emission line as a function of the spectral state of the
source and the frequency of the kHz QPOs. We find that the inner radius of the
accretion disc deduced from the frequency of the upper kHz QPO varies as a
function of the position of the source in the colour-colour diagram, in
accordance with previous work and with the standard scenario of accretion disc
geometry. On the contrary, the inner disc radius deduced from the profile of
the Fe line is not correlated with the spectral state of the source. The values
of the inner radius inferred from kHz QPOs and Fe lines, in four observations,
do not lead to a consistent value of the neutron star mass, regardless of the
model used to fit the Fe line. Our results suggest that either the kHz QPO or
the standard relativistic Fe line interpretation does not apply for this
system. Furthermore, the simultaneous detection of kHz QPOs and broad Fe lines
is difficult to reconcile with models in which the broadening of the Fe line is
due to the reprocessing of photons in an outflowing wind.
|
1403.2700v3
|
2014-04-14
|
Interplane resistivity of underdoped single crystals (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, $0 \leq x<0.34$
|
Temperature-dependent inter-plane resistivity, $\rho _c(T)$, was measured in
hole-doped iron-arsenide superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ over a
doping range from parent compound to optimal doping $T_c\approx 38~K$, $0\leq x
\leq 0.34$. Measurements were undertaken on high-quality single crystals grown
from FeAs flux. The coupled magnetic/structural transition at $T_{SM}$ leads to
clear accelerated decrease of $\rho_c(T)$ on cooling in samples with $T_c
<$26~K ($x <0.25$). This decrease in hole-doped material is in notable contrast
to an increase in $\rho_c(T)$ in the electron-doped Ba(Fe$_{1-x}$Co$_x$)Fe
$_2$As$_2$ and iso-electron substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$. The
$T_{SM}$ decreases very sharply with doping, dropping from $T_s$=71~K to zero
on increase of $T_c$ from approximately 25 to 27~K. The $\rho_c(T)$ becomes
$T$-linear close to optimal doping. The broad crossover maximum in $\rho_c(T)$,
found in the parent BaFe$_2$As$_2$ at around $T_{max} \sim$200~K, shifts to
higher temperature $\sim$250~K with doping $x$=0.34. The maximum shows clear
correlation with the broad crossover feature found in the temperature-dependent
in-plane resistivity $\rho_a(T)$. The doping evolution of $T_{max}$ in
(Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ is in notable contrast with both rapid
suppression of $T_{max}$ found in Ba(Fe$_{1-x}TM_x$)$_2$As$_2$
($TM$=Co,Rh,Ni,Pd) and its rapid increase BaFe$_2$(As$_{1-x}$P$_x$)$_2$. This
observation suggest that pseudogap features are much stronger in hole-doped
than in electron-doped iron-based superconductors, revealing significant
electron-hole doping asymmetry similar to the cuprates. This paper replaces:
cond-mat:1106.0533.
|
1404.3664v1
|
2014-06-04
|
New Fe I level energies and line identifications from stellar spectra
|
The spectrum of the Fe I atom is critical to many areas of astrophysics and
beyond. Measurements of the energies of its high-lying levels remain woefully
incomplete, however, despite extensive analysis of ultraviolet laboratory iron
absorption spectra, optical laboratory iron emission spectra, and the solar
infrared spectrum. In this work we use as sources the high-resolution archival
absorption-line ultraviolet and optical spectra of stars, whose warm
temperatures favor moderate Fe I excitation. We derive the energy for a
particular upper level in Kurucz's semiempirical calculations by adopting a
trial value that yields the same wavelength for a given line predicted to be
about as strong as that of a strong unidentified spectral line observed in the
stellar spectra, then checking the new wavelengths of other strong predicted
transitions that share the same upper level for coincidence with other strong
observed unidentified lines. To date this analysis has provided the upper
energies of 66 Fe I levels. Many new level energies are higher than those
accessible to laboratory experiments; several exceed the Fe I ionization
energy. These levels provide new identifications for over two thousand
potentially detectable lines. Almost all of the new levels of odd parity
include UV lines that were detected but unclassified in laboratory Fe I
absorption spectra, providing an external check on the energy values. We
motivate and present the procedure, provide the resulting new level energies
and their uncertainties, list all the potentially detectable UV and optical new
Fe I line identifications and their gf-values, point out new lines of
astrophysical interest, and discuss the prospects for additional Fe I
energy-level determinations in the near future.
|
1406.0933v3
|
2014-06-16
|
Location and sizes of forsterite grains in protoplanetary disks: interpretation from the Herschel DIGIT programme
|
The spectra of protoplanetary disks contain mid- and far- infrared emission
features produced by forsterite dust grains. The spectral features contain
information about the forsterite temperature, chemical composition and grain
size. We aim to characterize how the 23 and 69 micron features can be used to
constrain the physical locations of forsterite in disks. We check for
consistency between two independent forsterite temperature measurements: the
23/69 feature strength ratio and the shape of the 69 micron band. We performed
radiative transfer modeling to study the effect of disk properties to the
forsterite spectral features. Temperature-dependent forsterite opacities were
considered in self-consistent models to compute forsterite emission from
protoplanetary disks. Modelling grids are presented to study the effects of
grain size, disk gaps, radial mixing and optical depth to the forsterite
features. Independent temperature estimates derived from the 23/69 feature
strength ratio and the 69 micron band shape are most inconsistent for HD141569
and Oph IRS 48. A case study of the disk of HD141569 shows two solutions to fit
the forsterite spectrum. A model with T ~ 40 K, iron-rich (~0-1 % Fe) and 1
micron forsterite grains, and a model with warmer (T ~ 100 K), iron-free, and
larger (10 micron) grains. We find that for disks with low upper limits of the
69 micron feature (most notably in flat, self-shadowed disks), the forsterite
must be hot, and thus close to the star. We find no correlation between disk
gaps and the presence or absence of forsterite features. We argue that the 69
micron feature of the evolved transitional disks HD141569 and Oph IRS 48 is
most likely a tracer of larger (i.e. ~10 micron) forsterite grains.
|
1406.3951v1
|
2014-06-17
|
New Detections of Arsenic, Selenium, and Other Heavy Elements in Two Metal-Poor Stars
|
We use the Space Telescope Imaging Spectrograph on board the Hubble Space
Telescope to obtain new high-quality spectra covering the 1900 to 2360 Angstrom
wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive
abundances of Cu II, Zn II, As I, Se I, Mo II, and Cd II, which have not been
detected previously in either star. Abundances derived for Ge I, Te I, Os II,
and Pt I confirm those derived from lines at longer wavelengths. We also derive
upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean
[As/Fe] ratio derived from these two stars and five others in the literature is
unchanged over the metallicity range -2.8 < [Fe/H] < -0.6, <[As/Fe]> = +0.28
+/- 0.14 (std. dev. = 0.36 dex). The mean [Se/Fe] ratio derived from these two
stars and six others in the literature is also constant, <[Se/Fe]> = +0.16 +/-
0.09 (std. dev. = 0.26 dex). The As and Se abundances are enhanced relative to
a simple extrapolation of the iron-peak abundances to higher masses, suggesting
that this mass region (75 < A < 82) may be the point at which a different
nucleosynthetic mechanism begins to dominate the quasi-equilibrium alpha-rich
freezeout of the iron peak. <[CuII/CuI]> = +0.56 +/- 0.23 in HD 108317 and HD
128279, and we infer that lines of Cu I may not be formed in local
thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and
[Os/Fe] ratios are also derived from neutral and ionized species, and each
ratio pair agrees within the mutual uncertainties, which range from 0.15 to
0.52 dex.
|
1406.4554v1
|
2014-06-20
|
Suzaku Observation of the Black Hole Binary 4U 1630--47 in the Very High State
|
We report the results from an X-ray and near-infrared observation of the
Galactic black hole binary 4U 1630--47 in the very high state, performed with
{\it Suzaku} and IRSF around the peak of the 2012 September-October outburst.
The X-ray spectrum is approximated by a steep power law, with photon index of
3.2, identifying the source as being in the very high state. A more detailed
fit shows that the X-ray continuum is well described by a multi-color disc,
together with thermal and non-thermal Comptonization. The inner disc appears
slightly truncated by comparison with a previous high/soft state of this
source, even taking into account energetic coupling between the disc and
corona, although there are uncertainties due to the dust scattering correction.
The near-infrared fluxes are higher than the extrapolated disc model, showing
that there is a contribution from irradiation in the outer disk and/or the
companion star at these wavelengths. Our X-ray spectra do not show the Doppler
shifted iron emission lines indicating a baryonic jet which were seen four days
previously in an XMM-Newton observation, despite the source being in a similar
state. There are also no significant absorption lines from highly ionized irons
as are seen in the previous high/soft state data. We show that the increased
source luminosity is not enough on its own to make the wind so highly ionized
as to be undetectable. This shows that the disc wind has changed in terms of
its launch radius and/or density compared to the high/soft state.
|
1406.5512v1
|
2014-06-27
|
Chemical gradients in the Milky Way from the RAVE data. II. Giant stars
|
We provide new constraints on the chemo-dynamical models of the Milky Way by
measuring the radial and vertical chemical gradients for the elements Mg, Al,
Si, Ti, and Fe in the Galactic disc and the gradient variations as a function
of the distance from the Galactic plane ($Z$). We selected a sample of giant
stars from the RAVE database using the gravity criterium 1.7$<$log g$<$2.8. We
created a RAVE mock sample with the Galaxia code based on the Besan\c con model
and selected a corresponding mock sample to compare the model with the observed
data. We measured the radial gradients and the vertical gradients as a function
of the distance from the Galactic plane $Z$ to study their variation across the
Galactic disc. The RAVE sample exhibits a negative radial gradient of
$d[Fe/H]/dR=-0.054$ dex kpc$^{-1}$ close to the Galactic plane ($|Z|<0.4$ kpc)
that becomes flatter for larger $|Z|$. Other elements follow the same trend
although with some variations from element to element. The mock sample has
radial gradients in fair agreement with the observed data. The variation of the
gradients with $Z$ shows that the Fe radial gradient of the RAVE sample has
little change in the range $|Z|\lesssim0.6$ kpc and then flattens. The iron
vertical gradient of the RAVE sample is slightly negative close to the Galactic
plane and steepens with $|Z|$. The mock sample exhibits an iron vertical
gradient that is always steeper than the RAVE sample. The mock sample also
shows an excess of metal-poor stars in the [Fe/H] distributions with respect to
the observed data. These discrepancies can be reduced by decreasing the number
of thick disc stars and increasing their average metallicity in the Besan\c con
model.
|
1406.7244v1
|
2014-07-21
|
Constraints on Core Collapse from the Black Hole Mass Function
|
We model the observed black hole mass function under the assumption that
black hole formation is controlled by the compactness of the stellar core at
the time of collapse. Low compactness stars are more likely to explode as
supernovae and produce neutron stars, while high compactness stars are more
likely to be failed supernovae that produce black holes with the mass of the
helium core of the star. Using three sequences of stellar models and
marginalizing over a model for the completeness of the black hole mass
function, we find that the compactness xi(2.5) above which 50% of core
collapses produce black holes is xi(2.5)=0.24 (0.15 < xi(2.5) < 0.37) at 90%
confidence). While models with a sharp transition between successful and failed
explosions are always the most likely, the width of the transition between the
minimum compactness for black hole formation and the compactness above which
all core collapses produce black holes is not well constrained. The models also
predict that f=0.18 (0.09 < f < 0.39) of core collapses fail assuming a minimum
mass for core collapse of 8Msun. We tested four other criteria for black hole
formation based on xi(2.0) and xi(3.0), the compactnesses at enclosed masses of
2.0 or 3.0 rather than 2.5Msun, the mass of the iron core, and the mass inside
the oxygen burning shell. We found that xi(2.0) works as well as xi(2.5), while
the compactness xi(3.0) works significantly worse, as does using the iron core
mass or the mass enclosed by the oxygen burning shell. As expected from the
high compactness of 20-25Msun stars, black hole formation in this mass range
provides a natural explanation of the red supergiant problem.
|
1407.5622v1
|
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