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2004-04-30
|
A look with BeppoSAX at the low-luminosity Galactic X-ray source 4U 2206+54
|
A pointed observation of the low-luminosity galactic source 4U 2206+54 was
carried out in November 1998 with BeppoSAX. The light curve of 4U 2206+54 shows
erratic variability on a timescale of about 1 hour; neither hardness variations
nor time periodicities are detected throughout this 67 ks long observation.
Thanks to the wide spectral coverage capabilities of BeppoSAX we could observe
the source X-ray continuum over three energy decades, from 0.6 to 60 keV. The
spectrum could be equally well fitted either with a blackbody plus
Comptonization or with a high energy cutoff power law. No iron emission around
6.5 keV was detected, while a tentative detection of a cyclotron resonant
feature in absorption is presented. Comparison of the present BeppoSAX data
with the information available in the literature for this source suggests that
4U 2206+54 is a close binary system in which a (possibly magnetized) NS is
accreting from the companion star wind.
|
0404603v2
|
2004-05-04
|
Galactic Evolution of Nitrogen
|
We present detailed spectroscopic analysis of nitrogen abundances in 31
unevolved metal-poor stars analysed by spectral synthesis of the near-UV NH
band at 3360 A observed at high resolution with various telescopes. We found
that [N/Fe] scales with that of iron in the metallicity range -3.1 < [Fe/H] <0
with the slope 0.01+-0.02. Furthermore, we derive uniform and accurate (N/O)
ratios using oxygen abundances from near-UV OH lines obtained in our previous
studies. We find that a primary component of nitrogen is required to explain
the observations. The NH lines are discovered in the VLT/UVES spectra of the
very metal-poor subdwarfs G64-12 and LP815-43 indicating that these stars are N
rich. The results are compared with theoretical models and observations of
extragalactic HII regions and Damped Ly$\alpha$ systems. This is the first
direct comparison of the (N/O) ratios in these objects with those in Galactic
stars.
|
0405049v1
|
2004-05-07
|
Partial Covering Interpretation of the X-Ray Spectrum of the NLS1 1H 0707-495
|
The X-ray spectrum of 1H 0707-495 obtained with XMM-Newton showing a deep
flux drop at ~7 keV (Boller et al. 2002) is studied based on the partial
covering concept. The previously inferred extreme iron overabundance can be
reduced down to ~3x solar if the hard component gradually steepens at high
energies. The spectral shape supports that 1H 0707-495 is an AGN analogue of
the galactic black-hole binaries in the soft state. Interpreting the soft
excess as the emission from an optically-thick disk, the minimum black hole
mass M is estimated to be 2 x 10^6 solar masses from the intrinsic luminosity
corrected for partial covering. Based on the slim disk model, the observed disk
temperature implies that the luminosity is close to the Eddington limit. The
rapid and large flux variations with little change in the spectral shape can
also be explained, if not all, as due to changes in the partial covering
fraction. Partial covering may account for the large variability
characteristics of NLS1.
|
0405158v1
|
2004-05-13
|
Outflows in Regions of Star Formation
|
The high spatial and spectral resolution offered by the new generation of
infrared spectrometers at ESO is optimally suited for the observational study
of outflows from young stellar objects. Models of interstellar shock waves
would benefit from observations of spectrally resolved line profiles. This
applies also to attempts of measuring the rotation rates of jets very close to
their driving source, which in general suffer considerable extinction.
Observations of forbidden lines of ionised iron, [Fe II], could be used to
accomplish this. The possibility of using rotational lines of molecular
hydrogen, H2, to study the temporal evolution of outflow and disk gas is
discussed. Similarly, high resolution IR observations of fluorescent water
lines, H2O, open up the possibility to access outflow and disk water.
|
0405248v1
|
2004-05-13
|
Isothermal Shock Formation in Non-Equatorial Accretion Flows around Kerr Black Holes
|
We explore isothermal shock formation in non-equatorial, adiabatic accretion
flows onto a rotating black hole, with possible application to some active
galactic nuclei (AGNs). The isothermal shock jump conditions as well as the
regularity condition, previously developed for one-dimensional (1D) flows in
the equatorial plane, are extended to two-dimensional (2D), non-equatorial
flows, to explore possible geometrical effects. The basic hydrodynamic
equations with these conditions are self-consistently solved in the context of
general relativity to explore the formation of stable isothermal shocks. We
find that strong shocks are formed in various locations above the equatorial
plane, especially around a rapidly-rotating black hole with the prograde flows
(rather than a Schwarzschild black hole). The retrograde flows are generally
found to develop weaker shocks. The energy dissipation across the shock in the
hot non-equatorial flows above the cooler accretion disk may offer an
attractive illuminating source for the reprocessed features, such as the iron
fluorescence lines, which are often observed in some AGNs.
|
0405269v1
|
2004-05-26
|
Nucleosynthesis inside Gamma-Ray Burst Accretion Disks
|
We investigate nucleosynthesis inside both a gamma-ray burst accretion disk
and a wind launched from an inner region of the disk using one-dimensional
models of the disk and wind and a nuclear reaction network. Far from a central
black hole, the composition of accreting gas is taken to be that of an O-rich
layer of a massive star before core collapse. We find that the disk consists of
five layers characterized by dominant elements: O16, Si28, Fe54 (and Ni56),
He4, and nucleons, and the individual layers shift inward with keeping the
overall profiles of compositions as the accretion rate decreases. Ni56 are
abundantly ejected through the wind from the inner region of the disk with the
electron fraction \simeq 0.5. In addition to iron group, elements heavier than
Cu, in particular Cu63 and Zn64, are massively produced through the wind.
Various neutron-rich nuclei can be also produced in the wind from neutron-rich
regions of the disk, though the estimated yields have large uncertainties.
|
0405510v1
|
2004-05-28
|
Stars as thermonuclear reactors: their fuels and ashes
|
Atomic nuclei are transformed into each other in the cosmos by nuclear
reactions inside stars: -- the process of nucleosynthesis. The basic concepts
of determining nuclear reaction rates inside stars and how they manage to burn
their fuel so slowly most of the time are discussed. Thermonuclear reactions
involving protons in the hydrostatic burning of hydrogen in stars are discussed
first. This is followed by triple alpha reactions in the helium burning stage
and the issues of survival of carbon and oxygen in red giant stars connected
with nuclear structure of oxygen and neon. Advanced stages of nuclear burning
in quiescent reactions involving carbon, neon, oxygen and silicon are
discussed. The role of neutron induced reactions in nucleosynthesis beyond iron
is discussed briefly, as also the experimental detection of neutrinos from SN
1987A which confirmed broadly the ideas concerning gravitational collapse
leading to a supernova.
|
0405568v1
|
2004-06-05
|
Fe II and Mg II in Luminous, Intermediate-Redshift Narrow-line Seyfert 1 Galaxies from the Sloan Digital Sky Survey
|
We present results from analysis of spectra from a sample of ~900 quasars
from the Sloan Digital Sky Survey. These objects were selected for their
intermediate redshift (1.2<z<1.8), placing MgII and UV FeII in the optical band
pass, relatively narrow MgII lines, and moderately good signal-to-noise-ratio
spectra. Using a maximum likelihood analysis, we discovered that there is a
significant dispersion in the FeII/MgII ratios in the sample. Using
simulations, we demonstrate that this range, and corresponding correlation
between FeII equivalent width and FeII/MgII ratio, are primarily a consequence
of a larger dispersion of FeII equivalent width (EW) relative to MgII EW. This
larger dispersion in FeII EW could be a consequence of a range in iron
abundance, or in a range of FeII excitation. The latter possibility is
supported by evidence that objects with weak (zero) CII]\lambda 2325 equivalent
width are likely to have large FeII/MgII ratios. We discuss physical effects
that could produce a range of FeII/MgII ratio.
|
0406137v2
|
2004-06-09
|
An Observation of the Intermediate Polar XY Arietis with Chandra
|
Chandra serendipitously observed the eclipsing (80 deg < i < 84 deg)
intermediate polar, XY Arietis (=H0253+193), in two separate but continuous
observations five weeks apart. XY Ari was in a quiescent state during both
observations. We pursue the study of phase-resolved spectra for this system
focusing on the Fe K lines. From the combined and separate data sets, we
readily detect emission lines of iron near 6.4, 6.7, and 6.9 keV at better than
99% significance in contrast to previous results. We confirm the orbit-phased
sinusoidal absorption column behavior first observed with Ginga as well as a
sinusoid-like behavior as a function of spin phase. The presence of the 6.4,
6.7, and 6.9 keV lines requires different ionization states with xi <2 (6.4
keV) and xi \sim$3.5-4 (6.7 and 6.9 keV) that must vary with phase. We also
detect emission lines at 3.25, 4.8, and 5.4 keV that are not instrumental in
origin. The 4.8 keV line may be identified as Ca XIX (4.832 keV) and the 3.25
keV line may be Ar I K, but the 5.4 keV line has no obvious identification.
|
0406245v1
|
2004-06-29
|
A New Contributor to Chemical Evolution in High-Redshift Galaxies
|
The recent discovery of a new population of stars exhibiting unusual
elemental abundance patterns characterized by enhanced Ti to Ga elements and
low alpha and n-capture elements suggests the contribution of a new class of
supernovae, probably a kind of Type Ia supernovae associated with close binary
evolution. The role of these supernovae in chemical evolution is negligible in
normal galaxies that undergo moderate star formation such as our own. Thus,
while the frequency of occurrence would be too low to detect in low-redshift
galaxies, it may represent a prominent population in high-redshift objects such
as early epoch massive elliptical galaxies and QSOs. The chemical contributor
of this proposed type of supernovae in combination with recognized supernovae
is shown to be compatible with the recent observational features in the distant
universe, successfully reproducing the Type II supernovae-like abundance
pattern with enhancement of Ga and Ge in the gas of newborn massive galaxies
and high iron abundances in QSOs even at redshifts of around 6.
|
0406636v1
|
2004-07-03
|
Beyond the iron group: heavy metals in hot subdwarfs
|
We report the discovery of strong photospheric resonance lines of Ga III, Ge
IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdB and sdOB stars
at temperatures ranging from 22000 K to 40000 K. Lines of other heavy elements
are also detected, however in these cases more atomic data are needed. Based on
these discoveries, we present a hypothesis to explain the apparent lack of
silicon in sdB stars hotter than ~32000 K. The existence of triply ionised Ge,
Sn, and Pb suggests that rather than silicon sinking deep into the photosphere,
it is removed from the star in a fractionated stellar wind. This hypothesis
provides a challenge to diffusion models of sdB stars.
|
0407067v1
|
2004-07-07
|
A possible feature of thermal matter in relativistic jets of radio-loud quasars
|
It has been suggested that relativistic jets in quasars may contain a
considerable amount of thermal matter. In this paper, we explore the
possibility that the K-alpha line from the thermal matter may appear at tens of
keV due to a high Doppler blue-shift. In the jet comoving frame, the energy
density of photons originally emitted by the accretion disk and reflected off
the broad line region clouds dominates over that of photons of other origin. We
discuss the photoionization states of the thermal matter and find that the
irons elements are neutral. The high metallicity in quasars enhances the
possibility to detect the thermal matter in the relativistic jet in some
radio-loud quasars. A highly Doppler blue-shifted K-alpha line may be detected.
We make a rediction for 3C 273, in which the K-alpha line luminosity might be
of the order $3.0\times 10^{44}$ erg/s with an equivalent width of 2.4 keV.
Such a line could be detected in a future mission.
|
0407123v1
|
2004-07-08
|
Optical and near infrared observations of SN 1998bu
|
Infrared and optical spectra of SN 1998bu at an age of one year after
explosion are presented. The data show evidence for the radioactive decay of
56Co to 56Fe, long assumed to be the powering source for the supernova light
curve past maximum light. The spectra provide direct evidence for at least 0.4
solar masses of iron being present in the ejecta of the supernova. The fits to
the data also show that the widths of the emission lines increase with time.
Photometric measurements in the H-band show that the supernova is not fading
during the observation period. This is consistent with theoretical
expectations.
|
0407177v1
|
2004-07-21
|
X-ray binaries and their descendants: binary radio pulsars; evidence for three classes of neutron stars?
|
An important recent discovery by Pfahl et al. (2002) is that there are two
classes of Be X-ray binaries: one with orbits of small eccentricity (<0.25), in
which the neutron stars received hardly any kick velocity at birth and a class
with substantial orbital eccentricities, in which the neutron stars received a
kick velocity of order 100 km/s at birth. Also many of the double neutron stars
(5 out of 7) have a low orbital eccentricity (0.09 to 0.27), which indicates
that their second-born neutron stars received hardly any velocity kick at
birth. These second-born neutron stars tend to have low masses (1.25 to 1.35
Msun). It is proposed that the low-mass, low-kick neutron stars are formed by
the electron capture of degenerate O-Ne-Mg cores of stars with initial masses
below about 12-14 Msun, while the high-kick neutron stars originated from the
photo-disintegration collapse of the iron cores of stars which started out with
masses larger than this limit. The latter group may be further subdivided into
two classes of different mass.
|
0407451v1
|
2004-07-23
|
Neutrinos from microquasars
|
The jets of microquasars with high-mass stellar companions are exposed to the
dense matter field of the stellar wind as well as to the photon densities found
in the surrounding medium. Photopion and proton-proton interactions could then
lead to copious production of neutrinos. In this work, we analyze the hadronic
microquasar model, particularly in what concerns to the neutrino production.
Limits to this kind of models using data from AMANDA-II are established. New
constraints are also imposed upon specific microquasar models based on
photopion processes. These are very restrictive particularly for the case of
SS433, a microquasar for which the presence of accelerated hadrons has been
already inferred from iron X-ray line observations.
|
0407494v1
|
2004-07-27
|
The isolated elliptical NGC 4555 observed with Chandra
|
We present analysis of a Chandra observation of the elliptical galaxy NGC
4555. The galaxy lies in a very low density environment, either isolated from
all galaxies of similar mass or on the outskirts of a group. Despite this, NGC
4555 has a large gaseous halo, extending to ~60 kpc. We find the mean gas
temperature to be ~0.95 keV and the Iron abundance to be ~0.5 solar. We model
the surface brightness, temperature and abundance distribution of the halo and
use these results to estimate parameters such as the entropy and cooling time
of the gas, and the total gravitational mass of the galaxy. In contrast to
recent results showing that moderate luminosity ellipticals contain relatively
small quantities of dark matter, our results show that NGC 4555 has a massive
dark halo and large mass-to-light ratio (56.8 [+34.2,-35.8] solar at 50 kpc,
42.7 [+14.6,-21.2] solar at 5 effective radii, 1 sigma errors). We discuss this
disparity and consider possible mechanisms by which galaxies might reduce their
dark matter content.
|
0407552v1
|
2004-08-03
|
The Influence of Reaction Rates on the Final p-Abundances
|
The astrophysical p-process is responsible for the origin of the proton rich
nuclei,which are heavier than iron. A huge network involving thousands of
reaction rates is necessary to calculate the final p-abundances. But not all
rates included in the network have a strong influence on the p-nuclei
abundances. The p-process was investigated using a full nuclear reaction
network for a type II supernovae explosion when the shock front passes through
the O/Ne layer. Calculations were done with a multi-layer model adopting the
seed of a pre-explosion evolution of a 25 mass star. In extensive simulations
we investigated the impact of single reaction rates on the final p-abundances.
The results are important for the strategy of future experiments in this field.
|
0408060v1
|
2004-08-26
|
The narrow-line quasar NAB 0205+024 observed with XMM-Newton
|
The XMM-Newton observation of the narrow-line quasar NAB 0205+024 reveals
three striking differences since it was last observed in the X-rays with ASCA.
Firstly, the 2-10 keV power-law is notably steeper. Secondly, a hard X-ray
flare is detected, very similar to that seen in I Zw 1. Thirdly, a strong and
broad emission feature is detected with the bulk of its emission redward of 6.4
keV, and extending down to ~5 keV in the rest frame. The most likely
explanation for the broad feature is neutral iron emission emitted from a
narrow annulus of an accretion disc close to the black hole. The hard X-ray
flare could be the mechanism which illuminates this region of the disc,
allowing for the emission line to be detected. The combination of effects can
be understood in terms of the `thundercloud' model proposed by Merloni &
Fabian.
|
0408507v1
|
2004-09-13
|
Flux and energy modulation of redshifted iron emission in NGC3516: implications for the black hole mass
|
We report the tentative detection of the modulation of a transient,
redshifted Fe Kalpha emission feature in the X-ray spectrum of the Seyfert
galaxy NGC3516. The detection of the spectral feature at 6.1 keV, in addition
to a stable 6.4 keV line, has been reported previously. We find on re-analysing
the XMM-Newton data that the feature varies systematically in flux at intervals
of 25 ks. The peak moves in energy between 5.7 keV and 6.5 keV. The spectral
evolution of the feature agrees with Fe K emission arising from a spot on the
accretion disc, illuminated by a corotating flare located at a radius of (7-16)
rg, modulated by Doppler and gravitational effects as the flare orbits around
the black hole. Combining the orbital timescale and the location of the
orbiting flare, the mass of the black hole is estimated to be (1-5)e+7 Msun,
which is in good agreement with values obtained from other techniques.
|
0409293v1
|
2004-09-14
|
Cosmogenic Neutrinos from Ultra-High Energy Nuclei
|
We calculate the flux of neutrinos generated by the propagation of ultra-high
energy iron over cosmological distances and show that even if ultra-high energy
cosmic rays are composed of heavy nuclei, a significant flux of high-energy
neutrinos should be present throughout the universe. The resulting neutrino
flux has a new peak at $\sim 10^{14} eV$ generated by neutron decay and
reproduces the double peak structure due to photopion production at higher
energies ($\sim 10^{18}$ eV). Depending on the maximum energy and cosmological
evolution of extremely high energy cosmic accelerators the generated neutrino
flux can be detected by future experiments.
|
0409316v2
|
2004-09-22
|
Thorium-rich halo star HD221170: further evidence against the universality of the r-process
|
We report the abundance determination in the atmosphere of the bright halo
star HD221170. The spectra were taken with the Terskol Observatory's 2.0-m
telescope with a resolution R=45000 and signal-to-noise ratio up to 250 in the
wavelength region 3638-10275 \AA. The adopted atmospheric parameters correspond
to an effective temperature \Tef=4475 K, a surface gravity \lgg=1.0, a
microturbulent velocity \vmi=1.7 \kms, and a macroturbulent velocity \vma=4
\kms. The abundances of 43 chemical elements were determined with the method of
spectrum synthesis. The large overabundances (by 1 dex relative to iron) of
elements with Z$>38$ are shown to follow the same pattern as the solar
r-abundances. The present HD221170 analysis confirms the non-universality of
the r-process, or more exactly the observation that the astrophysical sites
hosting the r-process do not always lead to a unique relative abundance
distribution for the bulk Ba to Hg elements, the Pb-peak elements, and the
actinides.
|
0409547v1
|
2004-09-24
|
Three-Dimensional Delayed-Detonation Model of Type Ia Supernova
|
We study a Type Ia supernova explosion using large-scale three-dimensional
numerical simulations based on reactive fluid dynamics with a simplified
mechanism for nuclear reactions and energy release. The initial deflagration
stage of the explosion involves a subsonic turbulent thermonuclear flame
propagating in the gravitational field of an expanding white dwarf. The
deflagration produces an inhomogeneous mixture of unburned carbon and oxygen
with intermediate-mass and iron-group elements in central parts of the star.
During the subsequent detonation stage, a supersonic detonation wave propagates
through the material unburned by the deflagration. The total energy released in
this delayed-detonation process, (1.3-1.6)x10^51 ergs, is consistent with a
typical range of kinetic energies obtained from observations. In contrast to
the deflagration model that releases only about 0.6x10^51 ergs, the
delayed-detonation model does not leave carbon, oxygen, and intermediate-mass
elements in central parts of a white dwarf. This removes the key disagreement
between three-dimensional simulations and observations, and makes a delayed
detonation the mostly likely mechanism for Type Ia supernova explosions.
|
0409598v1
|
2004-09-26
|
The Low X-Ray State of LS 5039 / RX J1826.2-1450
|
Recent XMM-Newton and Chandra observations of the high mass X-ray binary LS
5039 / RX J1826.2-1450 caught the source in a faint X-ray state. In contrast
with previous RXTE observations, we fail to detect any evidence of iron line
emission. We also fail to detect X-ray pulsations. The X-ray spectrum can be
well fitted by a simple powerlaw, slightly harder than in previous
observations, and does not require the presence of any additional disk or
blackbody component. XMM-Newton data imply an X-ray photoelectric absorption
($N_{\rm H} \sim 7 \times 10^{21}$ cm$^{-2}$) consistent with optical
reddening, indicating that no strong local absorption occurs at the time of
these observations. We discuss possible source emission mechanisms and
hypotheses on the nature of the compact object, giving particular emphasis to
the young pulsar scenario.
|
0409608v1
|
2004-09-27
|
A puzzling event during the X-ray emission of the binary system GX 1+4
|
We report on a long X-ray observation of the slow-rotating binary pulsar GX
1+4. BeppoSAX observed, in the 0.1-200 keV energy range, an event in which the
source flux dropped for almost a day, and then recovered. During this event
only the high-energy emission was found to be pulsed and the pulsations were
shifted in phase of ~0.2 . The spectrum during the event was well fitted by a
Compton-reflection model. A broad iron line at ~6.55 keV was present outside of
the event, where instead two narrow emission lines at ~6.47 keV and ~7.05 keV
were detected. The pulse profile was highly variable as a function of both
energy and time. We interpret this low-flux event as an occultation of the
direct X-ray emission, due to the increase of a torus-like accretion disk; we
then discuss similarities between this source and the recently discovered
highly absorbed INTEGRAL sources.
|
0409626v3
|
2004-09-29
|
Fluorescence photons produced in air by extensive air showers
|
The air fluorescence technique has long been used to detect extensive air
showers and to reconstruct its geometry and energy. The fluorescence photon
yield of an electron in air is of main importance in the reconstruction
procedure. Historically, the fluorescence yield used in the reconstruction of
the showers is approximated at all energies by that of an electron with kinetic
energy of 80 MeV, because these are the most abundant in a shower.
In this paper, we calculate the fluorescence yield taking into account the
energy spectrum of electrons in showers initiated by proton and iron nuclei as
a function of height. We compare our results with previous calculations based
on mono energetic electrons (80 MeV) and a difference in excess of 8% is found.
Finally, the influence of a more realistic fluorescence yield in the shower
energy reconstruction is also discussed.
|
0409727v1
|
2004-10-05
|
Black Hole Spin in AGN and GBHCs
|
We discuss constraints on black hole spin and spin-related astrophysics as
derived from X-ray spectroscopy. After a brief discussion about the robustness
with which X-ray spectroscopy can be used to probe strong gravity, we summarize
how these techniques can constrain black hole spin. In particular, we highlight
XMM-Newton studies of the Seyfert galaxy MCG-6-30-15 and the stellar-mass black
hole GX339-4. The broad X-ray iron line profile, together with reasonable and
general astrophysical assumptions, allow a non-rotating black hole to be
rejected in both of these sources. If we make the stronger assertion of no
emission from within the innermost stable circular orbit, the MCG-6-30-15 data
constrain the dimensionless spin parameter to be a>0.93. Furthermore, these
XMM-Newton data are already providing evidence for exotic spin-related
astrophysics in the central regions of this object. We conclude with a
discussion of the impact that Constellation-X will have on the study of strong
gravity and black hole spin.
|
0410116v1
|
2004-10-08
|
Magnetorotational supernovae
|
We present the results of 2D simulations of the magnetorotational model of a
supernova explosion. After the core collapse the core consists of rapidly a
rotating proto-neutron star and a differentially rotating envelope. The
toroidal part of the magnetic energy generated by the differential rotation
grows linearly with time at the initial stage of the evolution of the magnetic
field. The linear growth of the toroidal magnetic field is terminated by the
development of magnetohydrodynamic instability, leading to drastic acceleration
in the growth of magnetic energy. At the moment when the magnetic pressure
becomes comparable with the gas pressure at the periphery of the proto-neutron
star $\sim 10-15$km from the star centre the MHD compression wave appears and
goes through the envelope of the collapsed iron core. It transforms soon to the
fast MHD shock and produces a supernova explosion. Our simulations give the
energy of the explosion $0.6\cdot 10^{51}$ ergs. The amount of the mass ejected
by the explosion is $\sim 0.14M_\odot$. The implicit numerical method, based on
the Lagrangian triangular grid of variable structure, was used for the
simulations.
|
0410234v2
|
2004-10-19
|
Metallicity measurements using atomic lines in M and K dwarf stars
|
We report the first survey of chemical abundances in M and K dwarf stars
using atomic absorption lines in high resolution spectra. We have measured Fe
and Ti abundances in 35 M and K dwarf stars using equivalent widths measured
from (lambda / Delta lambda) = 33,000 spectra. Our analysis takes advantage of
recent improvements in model atmospheres of low-temperature dwarf stars. The
stars have temperatures between 3300 and 4700 K, with most cooler than 4100 K.
They cover an iron abundance range of -2.44 < [Fe/H] < +0.16. Our measurements
show [Ti/Fe] decreasing with increasing [Fe/H], a trend similar to that
measured for warmer stars where abundance analysis techniques have been tested
more thoroughly. This study is a step toward the observational calibration of
procedures to estimate the metallicity of low-mass dwarf stars using
photometric and low-resolution spectral indices.
|
0410452v1
|
2004-10-22
|
Is the interstellar gas of starburst galaxies well mixed?
|
The extent to which the ISM in galaxies is well mixed is not yet settled.
Measured metal abundances in the diffuse neutral gas of star--forming gas--rich
dwarf galaxies are deficient with respect to that of the ionized gas. The
reasons, if real, are not clear and need to be based on firm grounds. Far-UV
spectroscopy of giant HII regions such as NGC604 in the spiral galaxy M33 using
FUSE allows us to investigate possible systematic errors in the metallicity
derivation. We still find underabundances of nitrogen, oxygen, argon, and iron
in the neutral phase by a factor of~6. This could either be explained by the
presence of less chemically evolved gas pockets in the sightlines or by dense
clouds out of which HIIregions form. Those could be more metallic than the
diffuse medium.
|
0410528v1
|
2004-10-29
|
On the primary particle type of the most energetic Fly's Eye event
|
The longitudinal profile of the 320 EeV event observed by the Fly's Eye
experiment is analysed. A method of testing the hypothesis of a specific
primary particle type is described. Results for different particle types are
summarized. For hadronic primaries between proton and iron nuclei, the
discrepancy between observed and simulated profiles is in the range of 0.6-1.0
sigma for two different hadronic interaction models investigated. For primary
photons, the discrepancy is 1.5sigma assuming a standard extrapolation of the
photonuclear cross-section with energy. Larger values of the cross-section at
highest energies make primary photon showers more similar to hadron-initiated
events. The influence of varying the extrapolation of the photonuclear
cross-section is studied.
|
0410739v1
|
2004-11-08
|
The Metallicity dependence of the Cepheid Period-Luminosity relation: methodology and results
|
We present the results of an observational campaign undertaken to assess the
influence of the iron content on the Cepheid Period-Luminosity relation. Our
data indicate that this dependence is not well represented by a simple linear
relation. Rather, the behaviour is markedly non monotonic, with the correction
peaking at about solar metallicity and declining for higher and lower values of
[Fe/H].
|
0411190v1
|
2004-11-10
|
High Resolution Spectroscopy of 4U 1728-34 from a Simultaneous Chandra-RXTE Observation
|
We report on a simultaneous Chandra and RossiXTE observation of the LMXB
atoll bursting source 4U 1728-34 performed on 2002 March 3-5. We fitted the
1.2-35 keV continuum spectrum with a blackbody plus a Comptonized component. An
overabundance of Si by a factor of ~2 with respect to Solar abundance is
required for a satisfactory fit. Large residuals at 6-10 keV can be fitted by a
broad (FWHM ~ 1.6 keV) Gaussian emission line, or, alternatively, by absorption
edges associated with Fe I and Fe XXV at ~7.1 keV and ~9 keV, respectively. In
this interpretation, we find no evidence of a broad, or narrow Fe Kalpha line,
between 6 and 7 keV. We tested our alternative modeling of the iron Kalpha
region by reanalyzing a previous BeppoSAX observation of 4U 1728-34, finding a
general agreement with our new spectral model.
|
0411270v2
|
2004-11-15
|
XMM-Newton observations of the Lockman Hole: III. A relativistic Fe line in the mean X-ray spectra of type-1 and type-2 AGN
|
Using the 770 ksec XMM-Newton survey of the Lockman Hole field in combination
with extensive optical identifications of the AGN population, we derive an
average rest-frame spectrum of AGN types-1 and 2. The most prominent feature in
the averaged spectrum is a strong fluorescent Fe line. In both type-1 and
type-2 AGN, a clear relativistic line profile is rev ealed. A laor line profile
with an inner disk radius smaller than the last stabl e orbit of a
Schwarzschild black hole is most consistent with the data, indicati ng that the
average supermassive black hole has significant spin. Equivalent wid ths of the
broad relativistic lines range between 400--600 eV. We used the disk reflection
model to compare the observed strength of the line with the amplitude of the
reflection component, concluding that to consistently describe the observations
the average iron abundance should be about three times the solar value.
|
0411340v2
|
2004-11-25
|
Muons with E_th >= 1 Gev and Mass Composition in the Energy Range 10^{18}-10^{20} ev Observed by Yakutsk Eas Array
|
The ratio of the muon flux density to charged particle flux density at
distances of 300 and 600 m from the shower axis ($\rhom(300)/\rhos(300)$ and
$\rhom(600)/\rhos(600)$) is measured. In addition, the energy dependence of
$\rhom(1000)$ is analysed for showers with energies above $10^{18}$ eV. A
comparison between the experimental data and calculations performed with the
QGSJET model is given for the cases of primary proton, iron nucleus and gamma-
ray. We conclude that the showers with $\E\ge3\times10^{18}$ eV can be formed
by light nuclei with a pronounced fraction of protons and helium nuclei. It is
not excluded however that a small part of showers with energies above $10^{19}$
eV could be initiated by primary gamma-rays.
|
0411683v2
|
2004-12-03
|
Stellar Evolution with Enriched Surface Convection Zones I. General Effects of Planet Consumption
|
Abundance analyses of stars with planets have revealed that their
metallicities are enhanced relative to field stars. Such a trend was originally
suggested to be due to accretion of iron-rich planetary material. Based on this
assumption, we have developed a stellar evolution code to model stars with
non-uniform metallicity distributions. We have calculated ``polluted'' stellar
evolution tracks for stars with M=0.9-1.2 M_sun. Our models encompass a range
of initial metal content from Z=0.01 to 0.03, and include metallicity
enhancements within the stellar convection zone corresponding to
Delta-Z=0.005-0.03. We find that the primary effects of metal enhancement on
stellar structure and evolution are expansion of the convection zone and
downward shift of effective temperature. In addition, we have computed the
surface metallicities expected for stars of different mass for fixed quantities
of pollution; there appears to be no correlation with present observational
data on the metallicities of stars known to harbor planets.
|
0412104v2
|
2004-12-09
|
XMM-Newton observation of 4U 1543-475: the X-ray spectrum of a stellar-mass black-hole at low luminosity
|
We report the results of an observation of the galactic black-hole binary 4U
1543-475 performed by XMM-Newton on 2002 August 18, about two months after the
start of an outburst detected by Rossi-XTE. Despite the relatively low flux of
the source, corresponding to a luminosity Lx ~ 4x10^34 erg/s (i.e. about
10^(-5) times the Eddington luminosity), we could obtain a good quality
spectrum thanks to the high throughput of the XMM-Newton EPIC instrument. The
spectrum is well fit by a power law with photon index 1.9-2 without any
evidence for iron emission lines or for thermal emission from an accretion
disk. We could estimate an upper-limit on the disk bolometric luminosity as a
function of the colour temperature: it is always lower than ~10^33 erg/s, i.e.
less than 10 % of the source total luminosity. Finally, we evaluated that the
disk colour temperature must satisy the condition kTcol<0.25 keV in order to
obtain an acceptable value for the disk inner radius.
|
0412209v1
|
2004-12-15
|
The XMM-Newton spectrum of the high-z optically-obscured QSO RX J1343.4+0001: a classic radio quiet QSO
|
We present a 30 ks XMM-Newton observation of the z = 2.35 Type II radio quiet
quasar RX J1343.4+0001. These data provide the first good quality X-ray
spectrum for this object. We measured a continuum slope Gamma = 1.85+/-0.10
with only an upper limit on the column density of the absorbing material of
Nh(z) <~ 10^22 cm^-2 as well as an Fe Kalpha emission line at the 3 sigma
confidence level. We do not find therefore a highly absorbed object nor a truly
flat spectrum as suggested on the basis of previous less sensitive ROSAT and
ASCA measurements. The Nh(z) upper limit is fully consistent with the optical
extinction 3 <A_V <10 inferred from IR observations. The Fe Kalpha line is
consistent with fluorescence from neutral iron and, noteworthy, is one of the
most distant observed so far. The X-ray spectral properties of RX J1343.4+0001
agree well with the steep continuum slope (Gamma ~ 1.9) being independent of
increasing redshift and luminosity as inferred by X-ray studies of large
samples of RQ QSOs.
|
0412360v1
|
2004-12-15
|
Determining the grain composition of the interstellar medium with high resolution X-ray spectroscopy
|
We investigate the ability of high resolution X-ray spectroscopy to directly
probe the grain composition of the interstellar medium. Using iron K-edge
experimental data of likely ISM dust candidates taken at the National
Synchrotron Light Source at Brookhaven National Laboratory and the Advanced
Photon Source at Argonne National Laboratory, we explore the prospects for
determining the chemical composition of astrophysical dust and discuss a
technique for doing so. Focusing on the capabilities of the AstroE2 XRS
micro-calorimeters, we assess the limiting effects of spectral resolution and
noise for detecting significant X-ray absorption fine structure signal in
astrophysical environments containing dust. We find that given sufficient
signal, the resolution of the XRS will allow us to definitively distinguish gas
from dust phase absorption, and certain chemical compositions.
|
0412393v1
|
2004-12-16
|
The Metallicity of Post-T Tauri Stars: A preliminary approach to the understanding of the metal enrichment of stars harboring planets
|
The metallicity of young low mass Post-T Tauri stars in coeval associations
is practically unknown. This work is the beginning of a systematic measurement
of these metallicities based on high resolution spectra of low rotating members
of these associations. Here, we present an application by examining the
behavior of the Iron abundance with stellar mass and temperature of some
members of an association 30 Myr old. This will test the possibility of
explaining the high metallic content of stars with planets by means of
injection of planetesimals during this early stage of evolution.
|
0412398v1
|
2004-12-19
|
Extreme Test of Quantum Theory with Black Holes
|
We propose an extreme test of quantum theory using astrophysical black holes
and entangled photons from atomic cascades. The identification of a cascade
emission close to a black-hole event horizon would allow us to observe photons
entangled with partners that have fallen behind the horizon. The experiment
involves testing the characteristic cos^2(theta) modulation of photon
transmission through a pair of polarisers at relative angle theta (Malus' law).
For single photons, Malus' law is a remarkable feature of quantum theory: it is
equivalent to expectation additivity for incompatible observables, and is
generically violated for hidden-variables theories with nonstandard probability
distributions. An experiment with entangled states straddling an event horizon
is motivated by the Hawking information loss puzzle, as well as on general
grounds. In principle, one could test the currently observed X-ray photons in
iron lines from black-hole accretion discs. However, only a small fraction
(~0.6%) have cascade partners, and current X-ray polarimetry does not permit
successive measurements on a single X-ray photon. A realisable experiment
requires the identification of an appropriate cascade in a more convenient
frequency band.
|
0412503v1
|
2005-01-20
|
Superfluidity in the Solar Interior: Implications for Solar Eruptions and Climate
|
Efforts to understand unusual weather or abrupt changes in climate have been
plagued by deficiencies of the standard solar model (SSM). While it assumes
that our primary source of energy began as a homogeneous ball of hydrogen (H)
with a steady, well-behaved H-fusion reactor at its core, observations instead
reveal a very heterogeneous, dynamic Sun. As examples, the upward acceleration
and departure of H+ ions from the surface of the quiet Sun and abrupt climatic
changes, including geomagnetic reversals and periodic magnetic storms that
eject material from the solar surface are not explained by the SSM. The present
magnetic fields are probably deep-seated remnants of very ancient origin. These
could have been generated from two mechanisms. These are: a) Bose-Einstein
condensation of iron-rich, zero-spin material into a rotating, superfluid,
superconductor surrounding the solar core and/or b) superfluidity and quantized
vortices in nucleon-paired Fermions at the core.
|
0501441v1
|
2005-01-31
|
New Elemental Abundances for V1974 Cygni
|
We present a new analysis of existing optical and ultraviolet spectra of the
ONeMg nova V1974 Cygni 1992. Using these data and the photoionization code
Cloudy, we have determined the physical parameters and elemental abundances for
this nova. Many of the previous studies of this nova have made use of incorrect
analyses and hence a new study was required. Our results show that the ejecta
are enhanced, relative to solar, in helium, nitrogen, oxygen, neon, magnesium
and iron. Carbon was found to be subsolar. We find an ejected mass of ~2x10e-4
solar masses. Our model results fit well with observations taken at IR, radio,
sub-millimeter and X-ray wavelengths.
|
0501648v1
|
2005-02-28
|
Nuclear Astrophysics
|
Nuclear physics has a long and productive history of application to
astrophysics which continues today. Advances in the accuracy and breadth of
astrophysical data and theory drive the need for better experimental and
theoretical understanding of the underlying nuclear physics. This paper will
review some of the scenarios where nuclear physics plays an important role,
including Big Bang Nucleosynthesis, neutrino production by our sun,
nucleosynthesis in novae, the creation of elements heavier than iron, and
neutron stars. Big-bang nucleosynthesis is concerned with the formation of
elements with A <= 7 in the early Universe; the primary nuclear physics inputs
required are few-nucleon reaction cross sections. The nucleosynthesis of
heavier elements involves a variety of proton-, alpha-, neutron-, and
photon-induced reactions, coupled with radioactive decay. The advent of
radioactive ion beam facilities has opened an important new avenue for studying
these processes, as many involve radioactive species. Nuclear physics also
plays an important role in neutron stars: both the nuclear equation of state
and cooling processes involving neutrino emission play a very important role.
Recent developments and also the interplay between nuclear physics and
astrophysics will be highlighted.
|
0502588v1
|
2005-03-08
|
One-armed Spiral Instability in a Low T/|W| Postbounce Supernova Core
|
A three-dimensional, Newtonian hydrodynamic technique is used to follow the
postbounce phase of a stellar core collapse event. For realistic initial data
we have employed post core-bounce snapshots of the iron core of a 20 solar mass
star. The models exhibit strong differential rotation but have centrally
condensed density stratifications. We demonstrate for the first time that such
postbounce cores are subject to a so-called low-T/|W| nonaxisymmetric
instability and, in particular, can become dynamically unstable to an m=1 -
dominated spiral mode at T/|W| ~ 0.08. We calculate the gravitational wave
emission by the instability and find that the emitted waves may be detectable
by current and future GW observatories from anywhere in the Milky Way.
|
0503187v2
|
2005-03-23
|
Abundance trends in kinematical groups of the Milky Way's disk
|
We have compiled a large catalogue of metallicities and abundance ratios from
the literature in order to investigate abundance trends of several alpha and
iron peak elements in the thin disk and the thick disk of the Galaxy. The
catalogue includes 743 stars with abundances of Fe, O, Mg, Ca, Ti, Si, Na, Ni
and Al in the metallicity range -1.30 < [Fe/H] < +0.50. We have checked that
systematic differences between abundances measured in the different studies
were lower than random errors before combining them. Accurate distances and
proper motions from Hipparcos, and radial velocities from several sources have
been retreived for 639 stars and their velocities (U,V,W) and galactic orbits
have been computed. Ages of 322 stars have been estimated with the Bayesian
method of isochrone fitting developped by Pont & Eyer (2004). Two samples
kinematically representative of the thin and thick disks have been selected,
taking into account the Hercules stream which is intermediate in kinematics,
but with a probable dynamical origin. [Abridged...]
|
0503498v1
|
2005-03-30
|
X-ray line tomography of AGN-induced motion in clusters of galaxies
|
The thermal broadening of emission lines of heavy ions is small enough such
that Doppler shifts due to bulk motions may be detected with the next
generation of X-ray observatories. This opens up the possibility to study gas
velocities in the intra-cluster medium. Here we study the effect of bulk
motions induced by a central active galactic nucleus (AGN) on the emission
lines around the FeXXV complex. We have modelled the evolution of AGN-induced
bubbles in a realistic cosmological framework and studied the resulting FeXXV
line profiles. We found that in clusters with AGN feedback, motions induced by
the inflation of bubbles and their buoyant rise lead to distinct features in
the iron emission lines that are detectable with a spectral resolution of about
10 eV. These observations will help to determine the mechanical energy that
resides in the bubbles and thereby the kinetic luminosity of the AGN.
|
0503656v2
|
2005-03-30
|
On the fine structure of the sunspot penumbrae. II. The nature of the Evershed flow
|
We investigate the fine structure of the sunspot penumbra by means of a model
that allows for a flux tube in horizontal pressure balance with the magnetic
background atmosphere in which it is embedded. We apply this model to
spectropolarimetric observations of two neutral iron lines at 1.56 $\mu$m and
invert several radial cuts in the penumbra of the same sunspot at two different
heliocentric angles. In the inner part of the penumbra we find hot flux tubes
that are somewhat inclined to the horizontal. They become gradually more
horizontal and cooler with increasing radial distance. This is accompanied by
an increase in the velocity of the plasma and a decrease of the gas pressure
difference between flux tube and the background component. At large radial
distances the flow speed exceeds the critical speed and evidence is found for
the formation of a shock front. These results are in good agreement with
simulations of the penumbral fine structure and provide strong support for the
siphon flow as the physical mechanism driving the Evershed flow.
|
0503677v1
|
2005-04-12
|
The temperature structure in the core of Sersic 159-03
|
We present results from a new 120 ks XMM-Newton observation of the cluster of
galaxies Sersic 159-03. In this paper we focus on the high-resolution X-ray
spectra obtained with the Reflection Grating Spectrometer (RGS). The spectra
allow us to constrain the temperature structure in the core of the cluster and
determine the emission measure distribution as a function of temperature. We
also fit the line widths of mainly oxygen and iron lines.
|
0504262v1
|
2005-05-06
|
Dust Scattering in Miras R Car and RR Sco resolved by optical interferometric polarimetry
|
We present optical interferometric polarimetry measurements of the Mira-like
variables R Car and RR Sco, using the Sydney University Stellar Interferometer.
By making visibility measurements in two perpendicular polarisations, the
relatively low-surface brightness light scattered by atmospheric dust could be
spatially separated from the bright Mira photospheric flux. This is the first
reported successful use of long-baseline optical interferometric polarimetry.
Observations were able to place constraints on the distribution of
circumstellar material in R Car and RR Sco. The inner radius of dust formation
for both stars was found to be less than 3 stellar radii: much closer than the
expected innermost stable location for commonly-assumed astrophysical ``dirty
silicate'' dust in these systems (silicate dust with a significant iron
content). A model with the dust distributed over a shell which is geometrically
thin compared to the stellar radius was preferred over an outflow. We propose
dust components whose chemistry and opacity properties enable survival at these
extreme inner radii.
|
0505112v1
|
2005-05-14
|
Family ties: abundances in Terzan 7, a Sgr dSph globular cluster
|
We study the chemical composition of 5 giant stars in the globular cluster
Terzan 7 associated with the Sagittarius dwarf spheroidal galaxy (Sgr dSph),
based on high resolution UVES-VLT spectra. We confirm the metallicity found by
previous high resolution investigations: [Fe/H] ~ -0.6. We also show that this
cluster displays the same low alpha-element to iron ratio found in Sgr dSph
field stars of similar metallicity, as well as the same low Ni/Fe ratio. These
chemical signatures are characteristic of the Sgr dSph system, and appear to be
shared both by the globular cluster Pal 12, which was most likely stripped from
Sgr by tidal interaction, and by Pal 5, which may also have belonged in the
past to the Sgr dSph system. Intriguingly even globular cluster Ruprecht 106,
although not associated to Sgr, displays similar characteristics.
|
0505307v1
|
2005-05-19
|
Early star formation in the Galaxy from beryllium and oxygen abundances
|
We investigate the evolution of the star formation rate in the early Galaxy
using beryllium and oxygen abundances in metal poor stars. Specifically, we
show that stars belonging to two previously identified kinematical classes (the
so-called ``accretion'' and ``dissipative'' populations) are neatly separated
in the [O/Fe] vs. Log(Be/H) diagram. The dissipative population follows the
predictions of our model of Galactic evolution for the thick disk component,
suggesting that the formation of this stellar population occurred on a
timescale significantly longer (by a factor ~5-10) than the accretion
component. The latter shows a large scatter in the [O/Fe] vs. Log(Be/H)
diagram, probably resulting from the inhomogeneous enrichment in oxygen and
iron of the protogalactic gas. Despite the limitation of the sample, the data
suggest that the combined use of products of spallation reactions (like
beryllium) and elemental ratios of stellar nucleosynthesis products (like
[O/Fe]) can constrain theoretical models for the formation and early evolution
of our Galaxy.
|
0505396v1
|
2005-06-20
|
Nitrogen-deficient and iron-rich associated absorbers with oversolar metallicities towards the quasar HE0141-3932
|
HE0141-3932 (zem=1.80) is a bright blue radio-quite quasar which reveals an
emission line spectrum with an unusually weak Ly-alpha line. In addition, large
redshift differences (Delta z=0.05) are observed between high ionization and
low ionization emission lines. Absorption systems identified at z=1.78, 1.71,
and 1.68 show mild oversolar metallicities (Z ~= 1-2Zsolar) and can be
attributed to the associated gas clouds ejected from the circumnuclear region.
The joint analysis of the emission and absorption lines leads to the conclusion
that this quasar is seen almost pole-on. Its apparent luminosity may be Doppler
boosted by ~10 times. The absorbing gas shows high abundance of Fe, Mg, and Al
([Fe, Mg, Al/C] ~= 0.15+/-0.10) along with underabundance of N ([N/C]<=-0.5).
This abundance pattern is at variance with current chemical evolution models of
QSOs predicting [N/C]>0 and [Fe/C]<0 at Z ~ Zsolar. Full details of this work
are given in Reimers et al. (2005).
|
0506446v1
|
2005-06-24
|
Kappa-mechanism excitation of retrograde mixed modes in rotating B-type stars
|
I examine the stability of retrograde mixed modes in rotating B-type stars.
These modes can be regarded as a hybrid between the Rossby modes that arise
from conservation of vorticity, and the Poincare modes that are gravity waves
modified by the Coriolis force. Using a non-adiabatic pulsation code based
around the traditional approximation, I find that the modes are unstable in
mid- to late-B type stars, due to the same iron-bump opacity mechanism usually
associated with SPB and $\beta$ Cep stars. At one half of the critical rotation
rate, the instability for $m=1...4$ modes spans the spectral types B4 to A0.
Inertial-frame periods of the unstable modes range from 100 days down to a
fraction of a day, while normalized growth rates can reach in excess of
$10^{-5}$.
I discuss the relevance of these findings to SPB and pulsating Be stars, and
to the putative Maia class of variable star. I also outline some of the
questions raised by this discovery of a wholly-new class of pulsational
instability in early-type stars.
|
0506580v3
|
2005-06-25
|
The Ital-FLAMES survey of the Sagittarius dwarf Spheroidal galaxy. I. Chemical abundances of bright RGB stars
|
We present iron and $\alpha$ element (Mg, Ca, Ti) abundances for a sample of
15 Red Giant Branch stars belonging to the main body of the Sagittarius dwarf
Spheroidal galaxy. Abundances have been obtained from spectra collected using
the high resolution spectrograph FLAMES-UVES mounted at the VLT. Stars of our
sample have a mean metallicity of [Fe/H]=-0.41$\pm$0.20 with a metal poor tail
extending to [Fe/H]=-1.52. The $\alpha$ element abundance ratios are slightly
subsolar for metallicities higher than [Fe/H]\gtsima-1, suggesting a slow star
formation rate. The [$\alpha$/Fe] of stars having [Fe/H]$<$-1 are compatible to
what observed in Milky Way stars of comparable metallicity.
|
0506622v2
|
2005-07-08
|
The relativistic shift of narrow spectral features from black-hole accretion discs
|
Transient spectral features have been discovered in the X-ray spectra of
Active Galactic Nuclei, mostly in the 5--7 keV energy range. Several
interpretations were proposed for the origin of these features. We examined a
model of Doppler boosted blue horns of the iron line originating from a spot in
a black hole accretion disc, taking into account different approximations of
general relativistic light rays and the resulting shift of energy of photons.
We provide a practical formula for the blue horn energy of an intrinsically
narrow line and assess its accuracy by comparing the approximation against an
exact value, predicted under the assumption of a planar accretion disc. The
most accurate approximation provides excellent agreement with the spot orbital
radius down to the marginally stable orbit of a non-rotating black hole.
|
0507196v1
|
2005-07-08
|
Probing the outer edge of an accretion disk: A Her X-1 turn-on observed with RXTE
|
We present the analysis of Rossi X-ray Timing Explorer (RXTE) observations of
the turn-on phase of a 35 day cycle of the X-ray binary Her X-1. During the
early phases of the turn-on, the energy spectrum is composed of X-rays
scattered into the line of sight plus heavily absorbed X-rays. The energy
spectra in the 3-17 keV range can be described by a partial covering model,
where one of the components is influenced by photoelectric absorption and
Thomson scattering in cold material plus an iron emission line at 6.5 keV. In
this paper we show the evolution of spectral parameters as well as the
evolution of the pulse profile during the turn-on. We describe this evolution
using Monte Carlo simulations which self-consistently describe the evolution of
the X-ray pulse profile and of the energy spectrum.
|
0507198v1
|
2005-07-14
|
Heavy element abundances in DAO white dwarfs measured from FUSE data
|
We present heavy element abundance measurements for 16 DAO white dwarfs,
determined from Far-Ultraviolet Spectroscopic Explorer {FUSE} spectra. Evidence
of absorption by heavy elements was found in the spectra of all the objects.
Measurements were made using models that adopted the temperatures, gravities
and helium abundances determined from both optical and FUSE data by Good et al.
(2004). Despite the different evolutionary paths that the types of DAO white
dwarfs are thought to evolve through, their abundances were not found to vary
significantly, apart from for the silicon abundances. Abundances measured when
the FUSE derived values of temperature, gravity and helium abundance were
adopted were, in general, a factor 1-10 higher than those determined when the
optical measure of those parameters was used. Satisfactory fits to the
absorption lines were achieved in approximately equal number. The models that
used the FUSE determined parameters seemed better at reproducing the strength
of the nitrogen and iron lines, while for oxygen, the optical parameters were
better.
|
0507341v1
|
2005-07-19
|
The Type Ia Supernova Rate
|
We explore the idea that the Type Ia supernovae (SNe Ia) rate consists of two
components: a prompt piece that is proportional to the star formation rate
(SFR) and an extended piece that is proportional to the total stellar mass. We
fit the parameters of this model to the local observations of Mannucci and
collaborators and then study its impact on three important problems. On cosmic
scales, the model reproduces the observed SNe Ia rate density below z=1, and
predicts that it will track the measured SFR density at higher redshift,
reaching a value of 1-3.5 X 10^-4 per yr per Mpc^3 at z=2. In galaxy clusters,
a large prompt contribution helps explain the iron content of the intracluster
medium. Within the Galaxy, the model reproduces the observed stellar [O/Fe]
abundance ratios if we allow a short (approximately 0.7 Gyr) delay in the
prompt component. Ongoing medium-redshift SN surveys will yield more accurate
parameters for our model
|
0507456v2
|
2005-07-20
|
UHECR Composition Measurements Using the HiRes-II Detector
|
While stereo measurements of extensive air showers allow a more precise
determination of the depth of shower maximum and hence the composition of
UHECR's, monocular measurements allow one to go much lower in energy. Since the
composition of UHECR seems to constant throughout the HiRes stereo energy range
but changing just below it, this is not a trivial lowering of the energy
threshold. We fit the observed Xmax distribution to a combination of expected
proton and iron Xmax distributions, using two different interaction models, to
determine the relative fraction of light and heavy components throughout the
HiRes monocular energy range. Using a two component fit allows both the mean
Xmax and the width of the Xmax distribution to contribute composition
measurement and allows us to deal with the Xmax acceptance bias caused by
limited elevation coverage. An updated analysis from a larger data set will be
presented in Pune.
|
0507483v1
|
2005-07-27
|
Gamma-Rays from Single Lobe Supernova Explosions
|
Multi-dimensional simulations of the neutrino-driven mechanism behind
core-collapse supernovae have long shown that the explosions from this
mechanism would be asymmetric. Recently, detailed core-collapse simulations
have shown that the explosion may be strongest in a single direction. We
present a suite of simulations modeling these ``single-lobe'' supernova
explosions of a 15 solar mass red supergiant star, focusing on the effect these
asymmetries have on the gamma-ray emission and the mixing in the explosion. We
discuss how asymmetries in the explosion mechanism might explain many of the
observed ``asymmetries'' of supernovae, focusing on features of both supernova
1987A and the Cas A supernova remnant. In particular, we show that single-lobe
explosions provide a promising solution to the redshifted iron lines of
supernova 1987A. We also show that the extent of mixing for explosive burning
products depends sensitively on the angular profile of the velocity asymmetry
and can be much more extensive than previously assumed.
|
0507651v1
|
2005-08-10
|
Relativistic outflow in CXO CDFS J033260.0-274748
|
In this letter we report the detection of a strong and extremely blueshifted
X-ray absorption feature in the 1 Ms Chandra spectrum of CXO CDFS
J033260.0-274748, a quasar at z = 2.579 with L_2-10keV ~ 4x10^44 ergs/s. The
broad absorption feature at ~ 6.3 keV in the observed frame can be fitted
either as an absorption edge at 20.9 keV or as a broad absorption line at 22.2
keV rest frame. The absorber has to be extremely ionized with an ionization
parameter \xi ~ 10^4, and a high column density N_H >5x10^23 cm^-2. We reject
the possibility of a statistical or instrumental artifact. The most likely
interpretation is an extremely blueshifted broad absorption line or absorption
edge, due to H or He--like iron in a relativistic jet-like outflow with bulk
velocity of ~ 0.7-0.8 c. Similar relativistic outflows have been reported in
the X-ray spectra of several other AGNs in the past few years.
|
0508225v1
|
2005-08-19
|
The magnetic structure of neutron stars and their surface-to-core temperature relation
|
We study the relation between the mean effective surface temperature T_s and
the internal temperature T_b for magnetic neutron stars, assuming that the
magnetic field near the surface has a presumably small-scale structure. The
heavy-element (iron) and light-element (accreted) heat-blanketing envelopes are
considered, and the results are compared with the case of a dipole magnetic
field. We argue that the difference in the T_b(T_s)-relation for different
magnetic configurations is always much smaller than a possible difference
caused by variations of the chemical composition in the envelope.
|
0508415v2
|
2005-08-29
|
Highly ionized iron absorption lines from outflowing gas in the X-ray spectrum of NGC 1365
|
We present the discovery of four absorption lines in the X-ray spectrum of
the Seyfert Galaxy NGC 1365, at energies between 6.7 and 8.3 keV. The lines are
detected with high statistical confidence (from >20sigma for the strongest to
\~4sigma for the weakest) in two XMM-Newton observations 60 ksec long. We also
detect the same lines, with lower signal-to-noise (but still >2sigma for each
line) in two previous shorter (~10 ksec) XMM observations. The spectral
analysis identifies these features as FeXXV and FeXXVI Kalpha and Kbeta lines,
outflowing with velocities varying between ~1000 to ~5000 km/s among the
observations. These are the highest quality detections of such lines so far.
The high equivalent widths (EW(Kalpha)~100 eV) and the Kalpha/Kbeta ratios
imply that the lines are due to absorption of the AGN continuum by a highly
ionized gas with column density N_H~5x10^23 cm^{-2}
|
0508608v1
|
2005-09-07
|
An Observationally Motivated Framework for AGN Heating of Cluster Cores
|
The cooling-flow problem is a long-standing puzzle that has received
considerable recent attention, in part because the mechanism that quenches
cooling flows in galaxy clusters is likely to be the same mechanism that
sharply truncates the high end of the galaxy luminosity function. Most of the
recent models for halting cooling in clusters have focused on AGN heating, but
the actual heating mechanism has remained mysterious. Here we present a
framework for AGN heating derived from a Chandra survey of gas entropy profiles
within cluster cores. This set of observations strongly suggests that the inner
parts of cluster cores are shock-heated every ~10^8 years by intermittent AGN
outbursts, driven by a kinetic power output of ~ 10^45 erg/sec and lasting at
least 10^7 years. Beyond ~30 kpc these shocks decay to sound waves, releasing
buoyant bubbles that heat the core's outer parts. Between heating episodes,
cooling causes the core to relax toward an asymptotic pure-cooling profile. The
density distribution in this asymptotic profile is sufficiently peaked that the
AGN shock does not cause a core entropy inversion, allowing the cluster core to
retain a strong iron abundance gradient, as observed.
|
0509176v1
|
2005-09-09
|
A Comparison of the Chemical Evolutionary Histories of the Galactic Thin Disk and Thick Disk Stellar Populations
|
We have studied 23 long-lived G dwarfs that belong to the thin disk and thick
disk stellar populations. Abundances have been derived for 24 elements: O, Na,
Mg, Al, Si, Ca, Ti, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce,
Nd, and Eu. We find that the behavior of [alpha/Fe] and [Eu/Fe] vs. [Fe/H] are
quite different for the two populations. As has long been known, the thin disk
O, Mg, Si, Ca, and Ti ratios are enhanced relative to iron at the lowest
metallicities, and decline toward solar values as [Fe/H] rises above -1.0. For
the thick disk, the decline in [alpha/Fe] and [Eu/Fe] does not begin at [Fe/H]
= -1.0, but at -0.4. Other elements share this behavior, including Sc, Co, and
Zn, suggesting that at least in the chemical enrichment history of the thick
disk, these elements were manufactured in similar-mass stars. Combining our
results for the oldest and longest-lived stars with prior work, we find clear
signs for an independent origin for the Galactic thick disk. (Abridged)
|
0509267v1
|
2005-09-13
|
Formation of the First Stars
|
How and when did the first generation of stars form at the end of the cosmic
dark ages? Quite generically, within variants of the cold dark matter model of
cosmological structure formation, the first sources of light are expected to
form in ~ 10^{6} M_sun dark matter potential wells at redshifts z > 20. I
discuss the physical processes that govern the formation of the first stars.
These so-called Population~III stars are predicted to be predominantly very
massive, and to have contributed significantly to the early reionization of the
intergalactic medium. Such an early reionization epoch is inferred from the
recent measurement of the Thomson optical depth by the {\it WMAP} satellite. I
address the importance of heavy elements in bringing about the transition from
an early star formation mode dominated by massive stars, to the familiar mode
dominated by low mass stars, at later times, and present possible observational
probes. This transition could have been gradual, giving rise to an
intermediate-mass population of still virtually metal-free stars (``Population
II.5''). These stars could have given rise to the peculiar class of black-hole
forming supernovae inferred from the abundance pattern of extremely iron-poor
stars.
|
0509354v1
|
2005-09-15
|
Evolution of Galaxies and the Tully-Fisher Relation
|
We study the evolution of the [O/Fe]-[Fe/H] relation and the dependence of
the iron abundance on distance from the galactic plane z in a one-zone model
for a disk galaxy, starting from the beginning of star formation (Wiebe98). We
obtain good agreement with the observational data, including, for the first
time, agreement for the [Fe/H]-z relation out to heights of 16 kpc. We also
study the influence of the presence of dark matter in the galaxies on the
star-formation rate. Comparison of the observed luminosity of the Galaxy with
the model prediction places constraints on the fractional mass of dark matter,
which cannot be much larger than the fractional mass of visible matter, at
least within the assumed radius of the Galaxy, 20 kpc. We studied the evolution
of disk galaxies with various masses, which should obey the Tully-Fisher
relation, M \propto R^2. The Tully-Fisher relation can be explained as a
combination of a selection effect related to the observed surface brightnesses
of galaxies with large radii and the conditions for the formation for
elliptical galaxies.
|
0509423v2
|
2005-09-16
|
Analysis of extensive air showers with the hybrid code SENECA
|
The ultrahigh energy tail of the cosmic ray spectrum has been explored with
unprecedented detail. For this reason, new experiments are exerting a severe
pressure on extensive air shower modeling. Detailed fast codes are in need in
order to extract and understand the richness of information now available. In
this sense we explore the potential of SENECA, an efficient hybrid
tridimensional simulation code, as a valid practical alternative to full Monte
Carlo simulations of extensive air showers generated by ultrahigh energy cosmic
rays. We discuss the influence of this approach on the main longitudinal
characteristics of proton, iron nucleus and gamma induced air showers for
different hadronic interaction models. We also show the comparisons of our
predictions with those of CORSIKA code.
|
0509503v1
|
2005-09-20
|
Acceptance of fluorescence detectors and its implication in energy spectrum inference at the highest energies
|
Along the years HiRes and AGASA experiments have explored the fluorescence
and the ground array experimental techniques to measure extensive air showers,
being both essential to investigate the ultra-high energy cosmic rays. However,
such Collaborations have published contradictory energy spectra for energies
above the GZK cut-off. In this article, we investigate the acceptance of
fluorescence telescopes to different primary particles at the highest energies.
Using CORSIKA and CONEX shower simulations without and with the new
pre-showering scheme, which allows photons to interact in the Earth magnetic
field, we estimate the aperture of the HiRes-I telescope for gammas, iron
nuclei and protons primaries as a function of the number of simulated events
and primary energy. We also investigate the possibility that systematic
differences in shower development for hadrons and gammas could mask or distort
vital features of the cosmic ray energy spectrum at energies above the
photo-pion production threshold. The impact of these effects on the true
acceptance of a fluorescence detector is analyzed in the context of top-down
production models.
|
0509597v1
|
2005-09-28
|
An abundance analysis of the symbiotic star CH Cyg
|
The photospheric abundances for the cool component of the symbiotic star CH
Cyg were calculated for the first time using high-resolution near-infrared
spectra and the method of of standard LTE analysis and atmospheric models. The
iron abundance for CH Cyg was found to be solar, [Fe/H] = 0.0+/-0.19. The
atmospheric parameters and metallicity for CH Cyg are found to be approximately
equal to those for nearby field M7 giants. The calculated [C/H] = -0.15, [N/H]
= +0.16, [O/H] = -0.07, and the isotopic ratios of 12C/13C and 16O/17O are
close to the mean values for single M giants that have experienced the first
dredge-up. A reasonable explanation for the absence of barium star-like
chemical peculiarities seems to be the high metallicity of CH Cyg. The emission
line technique was explored for estimating CNO ratios in the wind of the giant.
|
0509852v1
|
2005-10-07
|
Dust obscuration of DLA galaxies
|
We show that the extinction of quasar absorbers increases exponentially with
the logarithmic column density of any volatile metal (e.g. zinc), with a
charateristic turning point above which the quasars are severely dimmed. We
derive the relation between extinction, HI column density, metallicity and
fraction of iron in dust in Damped Lyman alpha (DLA) systems. We use this
relation to estimate the effect of dust obscuration on the statistical
distributions of N(HI) and metallicity measured in magnitude-limited surveys of
DLAs. In the redshift range where the measurements of zinc column densities
have sufficient statistics (1.8 </= z </= 3) we find that the obscuration bias
affects the shapes of the observed distributions. The metallicity distribution
is particularly affected by the bias, which hampers the detection of DLA
galaxies with near solar metallicity. Our results perfectly reproduce, without
tuning the dust parameters, the DLA observational threshold log N(ZnII) </~
13.15 found by Boisse' and collaborators in 1998, in terms of a rapid rise of
the obscuration. Our predictions of the effects of the bias on the statistics
of DLAs are consistent with observational results obtained from unbiased
surveys of radio-selected quasars.
|
0510216v1
|
2005-10-14
|
Thermonuclear supernova simulations with stochastic ignition
|
We apply an ad hoc model for dynamical ignition in three-dimensional
numerical simulations of thermonuclear supernovae assuming pure deflagrations.
The model makes use of the statistical description of temperature fluctuations
in the pre-supernova core proposed by Wunsch & Woosley (2004). Randomness in
time is implemented by means of a Poisson process. We are able to vary the
explosion energy and nucleosynthesis depending on the free parameter of the
model which controls the rapidity of the ignition process. However, beyond a
certain threshold, the strength of the explosion saturates and the outcome
appears to be robust with respect to number of ignitions. In the most energetic
explosions, we find about 0.75 solar masses of iron group elements. Other than
in simulations with simultaneous multi-spot ignition, the amount of unburned
carbon and oxygen at radial velocities of a few 1000 km/s tends to be reduced
for an ever increasing number of ignition events and, accordingly, more
pronounced layering results.
|
0510427v1
|
2005-10-18
|
The Chemical Composition of Cepheids in the Milky Way and the Magellanic Clouds
|
We have measured the elemental abundances of 68 Galactic and Magellanic
Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra
in order to establish the influence of the chemical composition on the
properties of these stars (see Romaniello et al. 2005). Here we describe the
robust analytical procedure we have developed to accurately determine them. The
resulting iron abundances span a range between ~ -0.80 dex for stars in the
Small Magellanic Cloud and ~ +0.20 dex for the most metal-rich ones in the
Galaxy. While the average values for each galaxy are in good agreement with
non-pulsating stars of similar age, Cepheids display a significant spread. Thus
it is fundamental to measure the metallicity of individual stars.
|
0510514v1
|
2005-10-19
|
On the fine structure of the sunspot penumbrae. III The vertical extension of penumbral filaments
|
In this paper we study the fine structure of the penumbra as inferred from
the uncombed model (flux tube embedded in a magnetic surrounding) when applied
to penumbral spectropolarimetric data from the neutral iron lines at 6300 \AA.
The inversion infers very similar radial dependences in the physical quantities
(LOS velocity, magnetic field strength etc) as those obtained from the
inversion of the Fe I 1.56 $\mu$m lines. In addition, the large Stokes $V$ area
asymmetry exhibited by the visible lines helps to constrain the size of the
penumbral flux tubes. As we demonstrate here, the uncombed model is able to
reproduce the area asymmetry with striking accuracy, returning flux tubes as
thick as 100-300 kilometers in the vertical direction, in good agreement with
previous investigations.
|
0510586v1
|
2005-10-27
|
Phase-resolved spectroscopy of the helium dwarf nova 'SN 2003aw' in quiescence
|
High time resolution spectroscopic observations of the ultra-compact helium
dwarf nova 'SN 2003aw' in its quiescent state at V=20.5 reveal its orbital
period at 2027.8 +/- 0.5 seconds or 33.80 minutes. Together with the
photometric 'superhump' period of 2041.5 +/- 0.5 seconds, this implies a mass
ratio q of 0.036. We compare both the average and time-resolved spectra of 'SN
2003aw' and SDSS J124058.03-015919.2. Both show a DB white dwarf spectrum plus
an optically thin, helium-dominated accretion disc. 'SN 2003aw' distinguishes
itself from the SDSS source by its strong calcium H & K emission lines,
suggesting higher abundances of heavy metals than the SDSS source. The silicon
and iron emission lines observed in the SDSS source are about twice as strong
in 'SN 2003aw'. The peculiar 'double bright spot' accretion disc feature seen
in the SDSS source is also present in time-resolved spectra of 'SN 2003aw',
albeit much weaker.
|
0510767v1
|
2005-11-17
|
X-ray reflections on AGN
|
X-ray reflection generates much of the spectral complexity in the X-ray
spectra of AGN. It is argued that strong relativistic blurring of the
reflection spectrum should commonly be expected from objects accreting at a
high Eddington rate. The good agreement found between the local density in
massive black holes and the energy density in quasar and AGN light requires
that the accretion which built massive black holes was radiatively efficient,
involving thin discs extending within 6 gravitational radii. The soft excess
found in the spectra of many AGN can be explained by X-ray reflection when such
blurring is included in the spectral analysis. Some of the continuum
variability and in particular the puzzling variability of the broad iron line
can be explained by the strong light bending expected in the region immediately
around a black hole. Progress in understanding this behaviour in the brightest
sources can be made now with long observations using instruments on XMM-Newton
and Suzaku. Future missions like Xeus and Con-X, with large collecting areas,
are required to expand the range of accessible objects and to make
reverberation studies possible.
|
0511537v1
|
2005-12-15
|
The mass of cosmic rays above 1017 eV
|
Interpretation of the energy spectrum and arrival distribution of cosmic rays
is complicated by lack of knowledge of the nature of the primaries. We review
claims for the mass composition above 1017 eV where it can be determined only
indirectly from air-shower observables. Difficulties in comparisons between
data arise because, inevitably, a set of measurements is interpreted using the
best model of hadronic interactions available at the time of analysis. We
discuss the situation and conclude that the evidence for a proton-dominated
mass composition, even at the highest energies, is unconvincing. However, it
may be that there are consistent differences between mass measurements from
optical techniques and those based upon other shower observables. We also find
that iron nuclei of ultra high energy can probably escape from the galaxies
that host GRBs, possible cosmic ray accelerators. The accelerators must lie
nearby.
|
0512408v1
|
2005-12-16
|
Lateral distribution of Cherenkov light in extensive air showers at high mountain altitude produced by different primary particles in wide energy range
|
The general aim of this work is to obtain the lateral distribution of
atmospheric Cherenkov light in extensive air showers produced by different
primary particles precisely by. protons, Helium, Iron, Oxygen, Carbon,
Nitrogen, Calcium, Silicon and gamma quanta in wide energy range at high
mountain observation level of Chacaltaya cosmic ray station. The simulations
are divided generally in two energy ranges 10GeV-10 TeV and 10 TeV-10 PeV. One
large detector has been used for simulations, the aim being to reduce the
statistical fluctuations of the obtained characteristics. The shape of the
obtained lateral distributions of Cherenkov light in extensive air showers is
discussed and the scientific potential for solution of different problems as
well.
|
0512432v1
|
2005-12-16
|
INTEGRAL and RXTE Observations of Centaurus A
|
INTEGRAL and RXTE performed three simultaneous observations of the nearby
radio galaxy Cen A in 2003 March, 2004 January, and 2004 February with the
goals of investigating the geometry and emission processes via the
spectral/temporal variability of the X-ray/low energy gamma ray flux, and
intercalibration of the INTEGRAL instruments with respect to those on RXTE.
When combined with earlier archival RXTE results, we find the power law
continuum flux and the line-of-sight column depth varied independently, and the
iron line flux was essentially unchanging. Taking X-ray spectral measurements
from satellite missions since 1970 into account, we discover a variability in
the column depth between 1.0x10^23 cm^-2 and 1.5x10^23 cm^-2, and suggest that
variations in the edge of a warped accretion disk viewed nearly edge-on might
be the cause.
|
0512451v1
|
2005-12-20
|
An Overview of Extremely Large Telescopes Projects
|
IAU Symposium 232 allows a snapshot of ELTs at a stage when design work in
several critical mass projects has been seriously underway for two to three
years. The status and som eof the main initial design choices are reviewed for
the North American Giant Magellan Telescope (GMT) and the Thirty Meter
Telescope (TMT) projects and the European Euro50 and the Overwhelmingly Large
(OWL) projects. All the projects are drawing from the same "basket" of science
requirements, although each project has somewhat different ambitions. The role
of the project offices in creating the balance between project scope, timeline
and cost, the "iron triangle" of project management, is emphasized with the OWL
project providing a striking demonstration at this meeting. There is a
reasonable case that the very broad range of science would be most effectively
undertaken on several complementary telescopes.
|
0512499v1
|
2005-12-21
|
The Oxygen Abundance of HE 1327-2326
|
From a newly obtained VLT/UVES spectrum we have determined the oxygen
abundance of HE 1327-2326, the most iron-poor star known to date. UV-OH lines
yield a 1D LTE abundance of [O/Fe]_OH = 3.7 (subgiant case) and [O/Fe]_OH = 3.4
(dwarf case). Using a correction of -1.0 dex to account for 3D effects on OH
line formation, the abundances are lowered to [O/Fe] = 2.8 and [O/Fe] = 2.5,
respectively, which we adopt. Without 3D corrections, the UV-OH based abundance
would be in disagreement with the upper limits derived from the OI triplet
lines: [O/Fe]_trip < 2.8 (subgiant) and [O/Fe]_trip < 3.0 (dwarf). We also
correct the previously determined carbon and nitrogen abundances for 3D
effects. Knowledge of the O abundance of HE 1327-2326 has implications for the
interpretation of its abundance pattern. A large O abundance is in accordance
with HE 1327-2326 being an early Population II star which formed from material
chemically enriched by a first generation supernova. Our derived abundances,
however, do not exclude other possibilities such as a Population III scenario.
|
0512543v1
|
2006-01-08
|
Excitation of g modes in Wolf-Rayet stars by a deep opacity bump
|
We examine the stability of l=1 and l=2 g modes in a pair of nitrogen-rich
Wolf-Rayet stellar models characterized by differing hydrogen abundances. We
find that modes with intermediate radial orders are destabilized by a kappa
mechanism operating on an opacity bump at an envelope temperature log T ~ 6.25.
This `deep opacity bump' is due primarily to L-shell bound-free transitions of
iron. Periods of the unstable modes span ~ 11-21 hr in the model containing
some hydrogen, and ~ 3-12 hr in the hydrogen-depleted model. Based on the
latter finding, we suggest that self-excited g modes may be the source of the
9.8 hr-periodic variation of WR 123 recently reported by Lefevre et al. (2005).
|
0601154v2
|
2006-01-18
|
Observable Effects of Dust Formation in Dynamic Atmospheres of M-type Mira Variables
|
The formation of dust with temperature-dependent non-grey opacity is
considered in a series of self-consistent model atmospheres at different phases
of an O-rich Mira variable of mass 1.2 $M_\odot$. Photometric and
interferometric properties of these models are predicted under different
physical assumptions regarding the dust formation. The iron content of the
initial silicate that forms and the availability of grain nuclei are found to
be critical parameters that affect the observable properties. In particular,
parameters were found where dust would form at 2-3 times the average continuum
photospheric radius. This work provides a consistent physical explanation for
the larger apparent size of Mira variables at wavelengths shorter than 1 $\mu$m
than that predicted by dust free fundamental-mode pulsation models.
|
0601383v1
|
2006-02-21
|
Tracing gas motions in the Centaurus Cluster
|
We apply the stochastic model of iron transport developed by Rebusco et. al.
(2005) to the Centaurus cluster. Using this model, we find that an effective
diffusion coefficient D in the range 2x10^28 - 4x10^28 cm^2 s^-1 can
approximately reproduce the observed abundance distribution. Reproducing the
flat central profile and sharp drop around 30-70 kpc, however, requires a
diffusion coefficient that drops rapidly with radius so that D > 4x10^28 cm^2
s^-1 only inside about 25 kpc. Assuming that all transport is due to
fully-developed turbulence, which is also responsible for offsetting cooling in
the cluster core, we calculate the length and velocity scales of energy
injection. These length scales are found to be up to a factor of ~ 10 larger
than expected if the turbulence is due to the inflation and rising of a bubble.
We also calculate the turbulent thermal conductivity and find it is unlikely to
be significant in preventing cooling.
|
0602466v1
|
2006-02-25
|
Luminosity dependent study of the High Mass X-ray Binary Pulsar 4U~0114+65 with ASCA
|
Here we report the spectral characteristics of the high and low states of the
pulsar 4U~0114+65 and examine the change in the parameters of the spectral
model. A power law and a photoelectric absorption by material along the line of
sight together with a high energy cut-off suffice to describe the continuum
spectrum in both the states. A fluorescence iron line at $\sim$6.4 keV is
present in the high as well as in the low state, though it is less intense in
the latter. The photon index, cut-off energy and e-folding energy values hardly
show any discernible change over the states. We compare these spectral
characteristics as observed with ASCA to that with other satellites. We also
compare the spectral characteristics of 4U~0114+650 with other X-ray sources
which show intensity variation at different time scales.
|
0602558v1
|
2006-03-03
|
Metallicity structure in X-ray bright galaxy groups
|
Using Chandra X-ray data of a sample of 15 X-ray bright galaxy groups, we
present preliminary results of a coherent study of the radial distribution of
metal abundances in the hot gas in groups. The iron content in group outskirts
is found to be lower than in clusters by a factor of ~2, despite showing mean
levels in the central regions comparable to those of clusters. The abundance
profiles are used to constrain the contribution from supernovae type Ia and II
to the chemical enrichment and thermal energy of the intragroup medium at
different group radii. The results suggest a scenario in which a substantial
fraction of the chemical enrichment of groups took place in filaments prior to
group collapse.
|
0603085v1
|
2006-03-16
|
Yields of Population III Supernovae and the Abundance Patterns of Extremely Metal-Poor Stars
|
The abundance patterns of extremely metal-poor (EMP) stars provide us with
important information on nucleosynthesis in supernovae (SNe) formed in a Pop
III or EMP environment, and thus on the nature of the first stars in the
Universe. We review nucleosynthesis yields of various types of those SNe,
focusing on core-collapse (black-hole-forming) SNe with various progenitor
masses, explosion energies (including Hypernovae), and asphericity. We discuss
the implications of the observed trends in the abundance ratios among iron-peak
elements, and the large C/Fe ratio observed in certain EMP stars with
particular attention to recently discovered hyper metal-poor (HMP) stars. We
show that the abundance pattern of the HMP stars with [Fe/H] < -5 and other EMP
stars are in good accord with those of black-hole-forming supernovae, but not
pair-instability supernovae. This suggests that black-hole-forming supernovae
made important contributions to the early Galactic (and cosmic) chemical
evolution. Finally we discuss the nature of First (Pop III) Stars.
|
0603433v1
|
2006-04-01
|
Chemical abundances in LMC stellar populations I. The Inner disk sample Based on observations collected at the VLT UT2 telescope
|
The advent of the new class 8 meters telescopes allows for the first time the
detailed study of numerous stars of galaxies other than our own. The Large
Magellanic Cloud is one of the most interesting targets due to its proximity
and its relatively face-on disk, permitting to separate populations pertaining
to the different structures of the galaxy. We have used FLAMES (the Fibre Large
Array Multi Element Spectrograph) at the VLT-UT2 telescope to obtain spectra of
a large sample of stars from the Inner Disk of the LMC, $\sim$2 kpc from the
center of the galaxy. We investigate the chemical abundances of iron-peak
elements, heavy and light $s$-process elements, Cu, Na, Sc and
$\alpha$-elements for a sample of red giant stars. Metallicities for the sample
stars range from [Fe/H] = $-$1.76 to $-$0.02. LMC inner disk stars show a
definitely different chemical pattern when compared to galactic stars of the
same metallicity.
|
0604009v2
|
2006-04-03
|
Abundance analysis of cool extreme helium star: LSS 3378
|
Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is
presented. The abundance analysis is done using LTE line formation and LTE
model atmospheres constructed for EHe stars.
The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and
s-process nucleosynthesis. The derived abundances of iron-peak and
alpha-elements indicate absence of selective fractionation or any other
processes that can distort chemical composition of these elements. Hence, the
Fe abundance (log epsilon(Fe) = 6.1) is adopted as an initial metallicity
indicator. The measured abundances of LSS 3378 are compared with those of R
Coronae Borealis (RCB) stars and with rest of the EHe stars as a group.
|
0604023v1
|
2006-04-03
|
High resolution infrared spectroscopy of the old open cluster NGC6791
|
We report abundance analysis for 6 M giant members of the old open cluster
NGC6791, based on infrared spectroscopy (1.5-1.8 micron) at R=25,000, using the
NIRSPEC spectrograph at the Keck II telescope. We find the iron abundance
<[Fe/H]>= +0.35 +/- 0.02$, confirming the super solar metallicity of this
cluster derived from optical medium-high resolution spectroscopy. We also
measure C, O and other alpha element abundances, finding roughly solar
[alpha/Fe] and <[C/Fe]> = -0.35. Our approach constrains [O/Fe] especially
well, based on the measurement of a number of OH lines near 1.6micron; we find
[O/Fe]=-0.07 +/-0.03. The Solar alpha enhancement is in contrast to the
composition of similar stars in the Galactic bulge. We also find low
12C/13C~10, confirming the presence of extra-mixing processes during the red
giant phase of evolution, up to super solar metallicities.
|
0604030v1
|
2006-04-13
|
The flow field in the sunspot canopy
|
We investigate the flow field in the sunspot canopy using simultaneous Stokes
vector spectropolarimetry of three sunspots ($\theta$ = 27, 50, 75 deg) and
their surroundings in visible (630.15 and 630.25 nm) and near infrared (1564.8
and 1565.2 nm) neutral iron lines.} {To calibrate the Doppler shifts, we
compare an absolute velocity calibration using the telluric $O_2$-line at
630.20 nm and a relative velocity calibration using the Doppler shift of Stokes
V profiles in the umbra under the assumption that the umbra is at rest. Both
methods yield the same result within the calibration uncertainties (~150 m/s).
We study the radial dependence of Stokes V profiles in the directions of disk
center and limb side. Maps of Stokes V profile shifts, polarity, amplitude
asymmetry, field strength and magnetic field azimuth provide strong evidence
for the presence of a magnetic canopy and for the existence of a radial outflow
in the canopy. Our findings indicate that the Evershed flow does not cease
abruptly at the white-light spot boundary, but that at least a part of the
penumbral Evershed flow continues into the magnetic canopy.
|
0604301v1
|
2006-04-27
|
Resonance scattering, absorption and off-centre abundance peaks in clusters of galaxies
|
A possible explanation for the central abundance dips found from
spatially-resolved X-ray spectroscopy of several groups and clusters of
galaxies is resonance scattering. A number of the prominent iron emission lines
are resonance lines. We construct a unique spectral model which takes account
of resonance scattering for several thousand resonance lines, projection
effects, photoelectric absorption, and allows direct spectral fitting. We apply
our model to Chandra observations of two clusters with pronounced central
abundance dips, Centaurus and Abell 2199. The results show that the effect of
resonance scattering on emission from the centre of the cluster can be as much
as 30 per cent for the Fe-K resonance lines, and 10 per cent for several Fe-L
lines, if turbulence is low. The change to the metallicities obtained by
fitting low resolution CCD spectra is at most 10 per cent. Accounting for
resonance scattering does not remove the central dip. Allowing for internal
absorption within the Centaurus significantly improves the quality of the fits,
indicating the presence of absorbing material within the inner 40 kpc of NGC
4696.
|
0604575v1
|
2006-05-04
|
Variable iron-line emission near the black hole of Markarian 766
|
We investigate the link between ionised Fe X-ray line emission and continuum
emission in the bright nearby AGN, Mrk 766. A new long (433 ks) XMM-Newton
observation is analysed, together with archival data from 2000 and 2001. The
contribution from ionised line emission is measured and its time variations on
short (5-20 ks) timescales are correlated with the continuum emission. The
ionised line flux is found to be highly variable and to be strongly correlated
with the continuum flux, demonstrating an origin for the ionised line emission
that is co-located with the continuum emission. Most likely the emission is
ionised reflection from the accretion disc within a few A.U. of the central
black hole, and its detection marks the first time that such an origin has been
identified other than by fitting to spectral line profiles. Future observations
may be able to measure a time lag and hence achieve reverberation mapping of
AGN at X-ray energies.
|
0605130v1
|
2006-06-13
|
Unveiling the nature of the highly absorbed X-ray source SAXJ1748.2-2808 with XMM-Newton
|
We report on the results of an EPIC XMM-Newton observation of the faint
source SAXJ1748.2-2808 and the surrounding field. This source was discovered
during the BeppoSAX Galactic center survey performed in 1997-1998. A spatial
analysis resulted in the detection of 31 sources within the EPIC field of view.
SAXJ1748.2-2808 is clearly resolved into 2 sources in EPIC images with the
brighter contributing almost 80% of the 2-10keV flux. Spectral fits to this
main source are consistent with an absorbed power-law with a photon index of
1.4+/-0.5 and absorption equivalent to 14E22cm-2 together with an iron line at
6.6+/-0.2 keV with an equivalent width of ~780eV. The significantly better
statistics of the XMM-Newton observation, compared with BeppoSAX,allows to
exclude a thermal nature for the X-ray emission. A comparison with other
observations of SAXJ1748.2-2808 does not reveal any evidence for spectral or
intensity long-term variability. Based on these properties we propose that the
source is a low-luminosity high-mass X-ray binary located in the Galactic
center region.
|
0606307v1
|
2006-06-27
|
Outgassing of Ordinary Chondritic Material and Some of its Implications for the Chemistry of Asteroids, Planets, and Satellites
|
We used chemical equilibrium calculations to model thermal outgassing of
ordinary chondritic material as a function of temperature, pressure, and bulk
compositions and use our results to discuss outgassing on asteroids and the
early Earth. The calculations include ~1,000 solids and gases of the elements
Al, C, Ca, Cl, Co, Cr, F, Fe, H, K, Mg, Mn, N, Na, Ni, O, P, S, Si, and Ti. The
major outgassed volatiles from ordinary chondritic material are CH4, H2, H2O,
N2, and NH3(the latter at conditions where hydrous minerals form). Contrary to
widely held assumptions, CO is never the major C-bearing gas during ordinary
chondrite metamorphism. The calculated oxygen fugacity (partial pressure) of
ordinary chondritic material is close to that of the quartz-fayalite-iron (QFI)
buffer. Our results are insensitive to variable total pressure, variable
volatile element abundances, and kinetic inhibition of C and N dissolution in
Fe metal. Our results predict that Earth's early atmosphere contained CH4, H2,
H2O, N2, and NH3; similar to that used in Miller-Urey synthesis of organic
compounds.
|
0606671v1
|
2006-06-27
|
They Came From the Deep in the Supernova: The Origin of TiC and Metal Subgrains in Presolar Graphite Grains
|
A new formation scenario for TiC and Fe,Ni-metal inclusions in presolar
graphite grains of supernova origin is described. The mineralogy and chemistry
require condensation of (Fe,Ni)-titanides from Fe, Ni, and Ti-rich gaseous
ejecta, subsequent carburization to make TiC and metal, and encapsulation into
graphite. Titanides only condense if Si is depleted relative to heavier mass
elements, which requires alpha-rich freeze-out and a deep mass-cut for the
supernova ejecta. This Si-poor core material must remain unmixed with other
supernova zones until the titanides condensed. This can be accomplished by
transport of core ejecta in bipolar jets through the major expanding supernova
zone ejecta. If the jets stall in regions dominated by C-rich ejecta such as
C-He zone where graphite condenses, thermochemically favored in situ
carburization of the titanides - either before or during encapsulation into
condensing graphite - leads to a TiC and metal composite. This scenario agrees
with theoretical models and observations of asymmetric core collapse in
supernovae that are associated with bipolar jets loaded with iron-peak
elements.
|
0606673v1
|
2006-07-03
|
Sulphur abundances in disk stars as determined from the forbidden 10821 A [SI] line
|
Aims: In this paper we aim to study the chemical evolution of sulphur in the
galactic disk, using a new optimal abundance indicator: the [SI] line at 10821
A. Similar to the optimal oxygen indicators, the [OI] lines, the [SI] line has
the virtues of being less sensitive to the assumed temperatures of the stars
investigated and of likely being less prone to non-LTE effects than other
tracers. Methods: High-resolution, near-infrared spectra of the [SI] line are
recorded using the Phoenix spectrometer on the Gemini South telescope. The
analysis is based on 1D, LTE model atmospheres using a homogeneous set of
stellar parameters. Results: The 10821 A [SI] line is suitable for an abundance
analysis of disk stars, and the sulphur abundances derived from it are
consistent with abundances derived from other tracers. We corroborate that, for
disk stars, the trend of sulphur-to-iron ratios with metallicity is similar to
that found for other alpha elements, supporting the idea of a common
nucleosynthetic origin.
|
0607017v1
|
2006-07-06
|
Enrichment in the Centaurus cluster of galaxies
|
We perform a detailed spatially-resolved, spectroscopic, analysis of the core
of the Centaurus cluster of galaxies using a deep Chandra X-ray observation and
XMM-Newton data. The Centaurus cluster core has particularly high metallicity,
upto twice Solar values, and we measure the abundances of Fe, O, Ne, Mg, Si, S,
Ar, Ca and Ni. We map the distribution of these elements in many spatial
regions,and create radial profiles to the east and west of the centre. The
ratios of the most robustly determined elements to iron are consistent with
Solar ratios, indicating that there has been enrichment by both Type Ia and
Type II supernovae. For a normal initial stellar mass function it represents
the products of about 4x10^10 solar masses of star formation. This star
formation can have occured either continuously at a rate of 5 solar masses per
year for the past 8 Gyr or more,or was part of the formation of the central
galaxy at earlier times. Either conclusion requires that the inner core of the
Centaurus cluster has not suffered a major disruption within the past 8 Gyr, or
even longer.
|
0607113v1
|
2006-07-11
|
Atomic and Molecular Opacities for Brown Dwarf and Giant Planet Atmospheres
|
We present a comprehensive description of the theory and practice of opacity
calculations from the infrared to the ultraviolet needed to generate models of
the atmospheres of brown dwarfs and extrasolar giant planets. Methods for using
existing line lists and spectroscopic databases in disparate formats are
presented and plots of the resulting absorptive opacities versus wavelength for
the most important molecules and atoms at representative temperature/pressure
points are provided. Electronic, ro-vibrational, bound-free, bound-bound,
free-free, and collision-induced transitions and monochromatic opacities are
derived, discussed, and analyzed. The species addressed include the alkali
metals, iron, heavy metal oxides, metal hydrides, $H_2$, $H_2O$, $CH_4$, $CO$,
$NH_3$, $H_2S$, $PH_3$, and representative grains. [Abridged]
|
0607211v4
|
2006-07-13
|
The puzzling abundance pattern of HD134439 and HD134440
|
Abundances of 18 elements are determined for the common proper-motion pair,
HD134439 and HD134440, which shows high [Mn/Fe] and low [\alpha/Fe] ratios as
compared to normal halo stars. Moreover, puzzling abundances are indicated from
elements whose origins are normally considered to be from the same
nucleosynthesis history. Particularly, we have found that [Mg/Fe] and [Si/Fe]
are lower than [Ca/Fe] and [Ti/Fe] by 0.1-0.3 dex. When elemental abundances
are interpreted in term of their condensation temperatures (Tc), obvious trends
of [X/Fe] vs. Tc for alpha elements and probably iron-peak elements as well are
shown. The hypothesis that these stars have formed from a dusty environment in
dSph galaxy provides a solution to the puzzling abundance pattern.
|
0607287v1
|
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