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2022-07-10
|
Semimetallic spin-density wave state in iron pnictides
|
We examine the existence of semimetallic spin-density wave states in iron
pnictides. In the experimentally observed metallic spin-density wave state, the
symmetry-protected Dirac cones are located away from the Fermi surface giving
rise to tiny pockets and there are also additional Fermi pockets such as one
around $\Gamma$. We find that the location of a pair of Dirac points with
respect to the Fermi surface exhibits significant sensitivity to the orbital
splitting between the $d_{xz}$ and $d_{yz}$ orbitals. Besides, in the presence
of orbital splitting, the Fermi pockets not associated with the Dirac cones,
can be suppressed so that a semimetallic spin-density wave state can be
realized. We explain these finding in terms of difference in the slopes and
orbital contents of the bands which form the Dirac cone, and obtain the
necessary conditions dependent on these two and other parameters for the
coexisting Dirac semimetallic and spin-density wave states. Additionally, the
topologically protected edge states are studied in the ribbon geometry when the
same are oriented either along $x$ or $y$ axes.
|
2207.04365v1
|
2022-07-22
|
Orbital and magnetic ordering in single-layer FePS3: A DFT+U study
|
Among the numerous 2D system that can be prepared via exfoliation, iron
phosphorus trisulfide (FePS3) attracts a lot of attention recently due to its
broad-range photoresponse, its unusual Ising-type magnetic order and possible
applications in spintronic nano-devices. Despite various experimental and
theoretical-computational reports, there are still uncertainties in identifying
its magnetic ground state. In this paper, we investigate the structural and
magnetic properties of single-layer FePS3 by using Density Functional Theory.
Our findings show that orbital ordering leads to a variation in distance
between pairs of iron atoms by 0.14 Angstrom. These lattice distortions, albeit
small, trigger different (ferromagnetic and antiferromagnetic) exchange
couplings so that the ground state consists of ferromagnetically aligned zigzag
chains along the long Fe-Fe bonds which couple antiferromagnetically along the
shorter Fe-Fe bonds. Within the DFT+U framework, we parameterize a spin
Hamiltonian including Heisenberg, single-ion anisotropy, Dzyaloshinskii-Moriya
and biquadratic interactions. Using U=2.22eV gives a consistent description of
both the electronic band gap and the Neel temperature in 2D FePS3.
|
2207.11283v2
|
2022-07-25
|
Anomalous Shiba states in topological iron-based superconductors
|
We demonstrate the formation of robust zero energy modes close to magnetic
impurities in the iron-based superconductor FeSe$_{1-x}$Te$_x$. We find that
the Zeeman field generated by the impurity favors a spin-triplet inter-orbital
pairing as opposed to the spin-singlet intra-orbital pairing prevalent in the
bulk. The preferred spin-triplet pairing preserves time-reversal symmetry and
is topological, as robust, topologically-protected zero modes emerge at the
boundary between regions with different pairing states. Moreover, the zero
modes form Kramers doublets that are insensitive to the direction of the spin
polarization or to the separation between impurities. We argue that our
theoretical results are consistent with recent experimental measurements on
FeSe$_{1-x}$Te$_x$.
|
2207.12425v2
|
2022-08-16
|
Core-shell enhanced single particle model for lithium iron phosphate batteries: model formulation and analysis of numerical solutions
|
In this paper, a core-shell enhanced single particle model for iron-phosphate
battery cells is formulated, implemented, and verified. Starting from the
description of the positive and negative electrodes charge and mass transport
dynamics, the positive electrode intercalation and deintercalation phenomena
and associated phase transitions are described with the core-shell modeling
paradigm. Assuming two phases are formed in the positive electrode, one rich
and one poor in lithium, a core-shrinking problem is formulated and the phase
transition is modeled through a shell phase that covers the core one. A careful
discretization of the coupled partial differential equations is proposed and
used to convert the model into a system of ordinary differential equations. To
ensure robust and accurate numerical solutions of the governing equations, a
sensitivity analysis of numerical solutions is performed and the best setting,
in terms of solver tolerances, solid phase concentration discretization points,
and input current sampling time, is determined in a newly developed
probabilistic framework. Finally, unknown model parameters are identified at
different C-rate scenarios and the model is verified against experimental data.
|
2208.07485v1
|
2022-08-26
|
Pure nematic state in iron-based superconductor
|
Lattice and electronic states of thin FeSe films on LaAlO$_3$ substrates are
investigated in the vicinity of the nematic phase transition. No evidence of
structural phase transition is found by x-ray diffraction below $T^\ast \sim
90$ K, while results obtained from resistivity measurement and angle-resolved
photoemission spectroscopy clearly show the appearance of a nematic state.
These results indicate formation of a pure nematic state in the iron-based
superconductor and provide conclusive evidence that the nematic state
originates from the electronic degrees of freedom. This pure nematicity in the
thin film implies difference in the electron-lattice interaction from bulk FeSe
crystals. FeSe films provide valuable playgrounds for observing the pure
response of "bare" electron systems free from the electron-lattice interaction,
and should make important contribution to investigate nematicity and its
relationship with superconductivity.
|
2208.12384v4
|
2022-08-27
|
Magnetic bond-order potential for iron-cobalt alloys
|
For large-scale atomistic simulations of magnetic materials, the interplay of
atomic and magnetic degrees of freedom needs to be described with high
computational efficiency. Here we present an analytic bond-order potential
(BOP) for iron-cobalt, an interatomic potential based on a coarse-grained
description of the electronic structure. We fitted BOP parameters to magnetic
and non-magnetic density-functional theory (DFT) calculations of Fe, Co, and
Fe-Co bulk phases. Our BOP captures the electronic structure of magnetic and
non-magnetic Fe-Co phases. It provides accurate predictions of structural
stability, elastic constants, phonons, point and planar defects, and structural
transformations. It also reproduces the DFT-predicted sequence of stable
ordered phases peculiar to Fe-Co and the stabilization of B2 against disordered
phases by magnetism. Our Fe-Co BOP is suitable for atomistic simulations with
thousands and millions of atoms.
|
2208.12973v2
|
2022-08-31
|
The giants that were born swiftly -- Implications of the top-heavy stellar initial mass function on the birth conditions of globular clusters
|
Recent results suggest that the initial mass function (IMF) of globular
clusters (GCs) is metallicity and density dependent. Here it is studied how
this variation affects the initial masses and the numbers of core collapse
supernovae (CCSNe) required to reproduce the observed iron spreads in GCs. The
IMFs of all of the investigated GCs were top-heavy implying larger initial
masses compared to previous results computed assuming an invariant canonical
IMF. This leads to more CCSNe being required to explain the observed iron
abundance spreads. The results imply that the more massive GCs formed at
smaller Galactocentric radii, possibly suggesting in-situ formation of the
population II halo. The time until star formation (SF) ended within a proto-GC
is computed to be 3.5 - 4 Myr, being slightly shorter than the 4 Myr obtained
using the canonical IMF. Therefore, the impact of the IMF on the time for which
SF lasts is small.
|
2209.00045v1
|
2022-09-05
|
Increased muon field at surface and substrate interface of palladium thin films
|
We performed depth-dependent low-energy muon spin spectroscopy ($\mu$SR)
studies on three palladium 100 nm thin films, both undoped and doped with 170
ppm of iron. Muons implanted in the surface and substrate interface region
probe an increased local magnetic field compared to the inner part of the
sample. The field increase extends over a few nanometers, it is
temperature-independent (in the range of 3.7 - 100 K), stronger for the
iron-doped samples and accompanied by an increase in local field inhomogeneity.
We consider various potential origins for this magnetic surface state, such as
adsorbents and supressed d-states. Our conclusion is that orbital moments
induced at the surface / interface by localized spins and charges are the most
likely explanation, potentially accompanied by magnetic moments due to crystal
irregularities.
|
2209.02002v1
|
2022-09-17
|
Onset Temperatures for Superconducting Fluctuations in Te-annealed FeTe$_{1-x}$Se$_x$ Single Crystals: Evidence for the BCS-BEC Crossover
|
Recently, the superconductors' community has witnessed an unsettled debate
regarding whether iron-based superconductors, in particular FeSe and
FeSe$_{1-x}$S$_x$, are in the Bardeen-Cooper-Shrieffer (BCS) - Bose-Einstein
condensation (BEC) crossover regime. Nonetheless, one particular system,
FeTe$_{1-x}$Se$_x$, has been less investigated in this regard owing to the
screening of its intrinsic superconducting properties by the inevitable iron
excess. Herein, the onset temperatures for superconducting fluctuations
($T_{scf}$) are investigated by measuring the magnetoresistance (MR) of
Te-annealed, high-quality FeTe$_{1-x}$Se$_x$ ($x$ = 0.1, 0.2, 0.3, and 0.4)
single crystals. The results reveal very high $T_{scf}$ values for these
crystals. Particularly for $x$ = 0.4, $T_{scf}$ reaches approximately 40 K,
which is 2.7 times larger than $T_c$. This indicates that the superconductivity
of the FeTe$_{1-x}$Se$_x$ system is well within the BCS-BEC crossover regime.
|
2209.08238v1
|
2022-09-19
|
Anisotropic gap structure and sign reversal symmetry in monolayer Fe(Se,Te)
|
The iron-based superconductors are an ideal platform to reveal the enigma of
the unconventional superconductivity and potential topological
superconductivity. Among them, the monolayer Fe(Se,Te)/SrTiO3(001), which is
proposed to be topological nontrivial, shows interface-enhanced
high-temperature superconductivity in the two dimensional limit. However, the
experimental studies on the superconducting pairing mechanism of monolayer
Fe(Se,Te) films are still limited. Here, by measuring quasiparticle
interference in monolayer Fe(Se,Te)/SrTiO3(001), we report the observation of
the anisotropic structure of the large superconducting gap and the sign change
of the superconducting gap on different electron pockets. The results are well
consistent with the 'bonding-antibonding' s+- wave pairing symmetry driven by
spin fluctuations in conjunction with spin-orbit coupling. Our work is of basic
significance not only for a unified superconducting formalism in the iron-based
superconductors, but also for understanding of topological superconductivity in
high-temperature superconductors.
|
2209.08683v2
|
2022-10-03
|
Energetics, electronic states, and magnetism of iron phthalocyanine on pristine and defected graphene layers
|
Transition metal phthalocyanines (TMPc's) are under intense scrutiny in the
field of spintronics, as they may be promising storage devices. The simplicity
and cheapness of such molecules increase their commercial potential. There is
an active study of how the magnetic moment of the metal centre of such
molecules can be changed. Here, we particularly consider the iron
phthalocyanine molecule (FePc) on a graphene layer as a substrate. We study how
graphene defects (the Stone-Wales defect, B-doping, N-doping, S-doping, and
combined B (N, S)-doped Stone-Wales defects) change the FePc electronic
structure. We present ab initio study of the systems, which is done using
several approaches: based on periodic plane wave density functional theory
(DFT), a linear combination of atomic orbitals (LCAO) DFT with a cluster
representation of graphene, and multiconfigurational methods with the pyrene
molecule presented as a miniaturised graphene cluster. The treatment of the
FePc/Graphene hybrid system using multiconfigurational methods was done for the
first time. It was found that the hybrid systems with B- and N- dopings have
quasi-degenerate ground states and it is necessary to go beyond the
approximation of one Slater determinant.
|
2210.01025v1
|
2022-10-19
|
Competing ferromagnetic superconducting states in europium-based iron pnictides
|
In europium-based iron pnictides superconducting Fe-planes can be influenced
by a Zeeman field originated from the neighboring Eu-planes. The field tends to
induce spin-density waves with a ferromagnetic average which coexists with the
superconducting order by forming complementary patterns of the superconducting
and magnetic order parameters in a Fulde-Ferrell-Larkin-Ovchinnikov phase and a
two-dimensional textured-superconducting phase. The hard gap around the Fermi
energy disappears in these fragile inhomogeneous superconducting states, which
features, instead, V-shaped spin-resolved local density of states. The
inhomogeneous states are also competing with either a homogeneous
superconducting or a homogeneous ferromagnetic state, manifesting the
intertwining influences of the magnetic orders in Fe and Eu planes, the
spin-density wave band structure, and the superconducting pairing order.
|
2210.10312v1
|
2022-11-14
|
Rapid Formation of Exoplanetesimals Revealed by White Dwarfs
|
The timing of formation for the first planetesimals determines the mode of
planetary accretion and their geophysical and compositional evolution.
Astronomical observations of circumstellar discs and Solar System geochronology
provide evidence for planetesimal formation during molecular cloud collapse,
much earlier than previously estimated. Here, we present distinct observational
evidence from white dwarf planetary systems for planetesimal formation
occurring during the first few hundred thousand years after cloud collapse in
exoplanetary systems. A significant fraction of white dwarfs have accreted
planetary material rich in iron core or mantle material. In order for the
exo-asteroids accreted by white dwarfs to form iron cores, substantial heating
is required. By simulating planetesimal evolution and collisional evolution we
show that the most likely heat source is short-lived radioactive nuclides such
as Al-2 (half life of approximately 0.7 Myr). Core-rich materials in the
atmospheres of white dwarfs, therefore, provide independent evidence for rapid
planetesimal formation, concurrent with star formation.
|
2211.07244v1
|
2022-12-06
|
Transient gap generation in BaFe$_2$As$_2$ driven by coherent lattice vibrations
|
The electronic structure and the magnetic properties of iron-based
superconductors are highly sensitive to the pnictogen height. Coherent
excitation of the $A_{1g}$ phonon by femtosecond laser directly modulates the
pnictogen height, which has been used to control the physical properties of
iron-based superconductors. Previous studies show that the driven $A_{1g}$
phonon resulted in a transient increase of the pnictogen height in
BaFe$_2$As$_2$, favoring an enhanced Fe magnetic moment. Here, we use
time-resolved broadband terahertz spectroscopy to investigate the dynamics of
BaFe$_2$As$_2$ in the $A_{1g}$ phonon driven state. Below the spin-density wave
(SDW) transition temperature, we observe a transient gap generation at early
time delays. A similar transient feature is observed in the normal state up to
room temperature.
|
2212.03337v1
|
2022-12-28
|
ISAI: Investigating Solar Axion by Iron-57
|
The existence of the axion is a unique solution for the strong CP problem,
and the axion is one of the most promising candidates of the dark matter.
Investigating Solar Axion by Iron-57 (ISAI) is being prepared as a complemented
table-top experiment to confirm the solar axion scenario. Probing an X-ray
emission from the nuclear transitions associated with the axion-nucleon
coupling is a leading approach. ISAI searches for the monochromatic 14.4 keV
X-ray from the first excited state of 57Fe using a state-of-the-art pixelized
silicon detector, dubbed XRPIX, under an extremely low-background environment.
We highlight scientific objectives, experimental design and the latest status
of ISAI.
|
2212.13708v1
|
2023-02-27
|
Predicting elastic and plastic properties of small iron polycrystals by machine learning
|
Deformation of crystalline materials is an interesting example of complex
system behaviour. Small samples typically exhibit a stochastic-like, irregular
response to externally applied stresses, manifested as significant
sample-to-sample variation in their mechanical properties. In this work we
study the predictability of the sample-dependent shear moduli and yield
stresses of a large set of small cube-shaped iron polycrystals generated by
Voronoi tesselation, by combining molecular dynamics simulations and machine
learning. Training a convolutional neural network to infer the mapping between
the initial polycrystalline structure of the samples and features of the
ensuing stress-strain curves reveals that the shear modulus can be predicted
better than the yield stress. We discuss our results in the context of the
sensitivity of the system's response to small perturbations of its initial
state.
|
2302.13745v3
|
2023-04-13
|
Aromatic molecules as sustainable lubricants explored by ab initio simulations
|
In the pursuit of sustainable lubricant materials, the conversion of common
organic molecules into graphitic material has been recently shown to
effectively reduce friction of metallic interfaces. Aromatic molecules are
perfect candidates due to their inertness and possibility to form carbon-based
tribofilms. Among many promising possibilities, we selected a group of common
aromatic compounds and we investigated their capability to reduce the adhesion
of iron interface. Ab initio molecular dynamic simulations of the sliding
interface show that hypericin, a component of St. John's wort, effectively
separates the mating iron surfaces better than graphene. This phenomenon is due
to the size of the molecule, the reactivity of the moieties at its edges and
the possibility to stack several of these structures that can easily slide on
top of each other. The decomposition of the lateral groups of hypericin
observed in the dynamic simulations suggests that the clustering of several
molecules is possible, offering innovative paths to lubricate sliding contacts
with compounds not typically employed in tribology.
|
2304.06501v2
|
2023-04-17
|
Ultrafast Relaxation Dynamics of Spin-Density Wave Order in BaFe$_2$As$_2$ under High Pressures
|
BaFe$_2$As$_2$ is the parent compound for a family of iron-based
high-temperature superconductors as well as a prototypical example of the
spin-density wave (SDW) system. In this study, we perform an optical pump-probe
study of this compound to systematically investigate the SDW order across the
pressure-temperature phase diagram. The suppression of the SDW order by
pressure manifests itself by the increase of relaxation time together with the
decrease of the pump-probe signal and the pump energy necessary for complete
vaporization of the SDW condensate. We have found that the pressure-driven
suppression of the SDW order at low temperature occurs gradually in contrast to
the thermally-induced SDW transition. Our results suggest that the
pressure-driven quantum phase transition in BaFe$_2$As$_2$ (and probably other
iron pnictides) is continuous and it is caused by the gradual worsening of the
Fermi-surface nesting conditions.
|
2304.08080v1
|
2023-04-18
|
Electron correlation effects in paramagnetic cobalt
|
We study the influence of Coulomb correlations on spectral and magnetic
properties of fcc cobalt using a combination of density functional theory and
dynamical mean-field theory. The computed uniform and local magnetic
susceptibilities obey the Curie-Weiss law, which, as we demonstrate, occurs due
to the partial formation of local magnetic moments. We find that the lifetime
of these moments in cobalt is significantly less than in bcc iron, suggesting a
more itinerant magnetism in cobalt. In contrast to the bcc iron, the obtained
electronic self-energies exhibit a quasiparticle shape with the quasiparticle
mass enhancement factor ${m^*/m}\sim$1.8, corresponding to moderately
correlated metal. Finally, our calculations reveal that the static magnetic
susceptibility of cobalt is dominated by ferromagnetic correlations, as
evidenced by its momentum dependence.
|
2304.08980v1
|
2023-04-27
|
Influence of oxygen on electronic correlation and transport in iron in the outer Earth's core
|
Knowing the transport properties of iron under realistic conditions present
in the Earth's core is essential for the geophysical modeling of Earth's
magnetic field generation. Besides by extreme pressures and temperatures,
transport may be influenced importantly also by the presence of light elements.
Using a combination of molecular dynamics, density functional theory, and
dynamical mean-field theory methods we investigate how oxygen impurities
influence the electronic correlations and transport in the liquid outer Earth's
core. We consider a case with an oxygen content of ~10 atomic%, a value that is
believed to be close to the composition of the core. We find that the
electronic correlations are enhanced but their effect on conductivities is
moderate (compared to pure Fe, electrical conductivity drops by 10% and thermal
conductivity by 18%). The effect of electron-electron scattering alone, whereas
not large, is comparable to effects of the compositional disorder. We reveal
the mechanism behind the larger suppression of the thermal conductivity and
associated reduction of the Lorenz ratio and discuss its geophysical
significance.
|
2304.13962v1
|
2023-05-08
|
Uniaxial-Strain Tuning of the Intertwined Orders in BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$
|
An experimental determination of electronic phase diagrams of high-transition
temperature (high-$T_c$) superconductors forms the basis for a microscopic
understanding of unconventional superconductivity. For most high-$T_c$
superconductors, the electronic phase diagrams are established through partial
chemical substitution, which also induces lattice disorder. Here we show that
symmetry-specific uniaxial strain can be used to study electronic phases in
iron-based superconductors, composed of two-dimensional nearly square iron
lattice planed separated by other elements. By applying tunable uniaxial strain
along different high symmetry directions and carrying out transport
measurements, we establish strain-tuning dependent electronic nematicity,
antiferromagnetic (AF) order, and superconductivity of
BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ superconductor. We find that uniaxial strain
along the nearest Fe-Fe direction can dramatically tune the AF order and
superconductivity, producing an electronic phase diagram clearly different from
the chemical substitution-induced one. Our results thus establish strain tuning
as a way to study the intertwined orders in correlated electron materials
without using chemical substitution.
|
2305.04424v1
|
2023-06-19
|
Kinetics of hydrogen and vacancy diffusion in iron: A Kinetic Activation Relaxation technique (k-ART) study
|
We investigate hydrogen (H) and mono and divacancy-hydrogen complexes (VH$_x$
and V$_2$H$_x$) diffusion in body-centered-cubic (BCC) iron using the kinetic
Activation-Relaxation Technique (k-ART), an off-lattice kinetic Monte Carlo
approach with on-the-fly event catalog building, to explore diffusion barriers
and associated mechanisms for these defects. K-ART uncovers complex diffusion
pathways for the bound complexes, with important barrier variations that depend
on the geometrical relations between the position of the inserting Fe atom and
that of the bound H. Since H is small and brings little lattice deformation
around itself, these bound complexes are compact and H is fully unbound at the
second neighbour site already. As more H are added, however, vacancies deform
and affect the lattice over longer distances, contributing to increasing the
VH$_x$ complex diffusion barrier and its impact on its local environment. We
find, moreover, that the importance of this trapping decreases when going from
mono to divacancy complexes, although diffusion barriers for these complexes
increase with the number of trapped H.
|
2306.11176v1
|
2023-06-30
|
Orbital-selective correlations for topology in FeSe$_{x}$Te$_{1-x}$
|
Strong correlations lead to emergent excitations at low energies. When
combined with symmetry constraints, they may produce topological electronic
states near the Fermi energy. Within this general framework, here we address
the topological features in iron-based superconductors. We examine the effects
of orbital-selective correlations on the band inversion in the iron
chalcogenide FeSe$_{x}$Te$_{1-x}$ near its doping of optimal superconductivity,
within a multiorbital model and using a $U(1)$ slave spin theory. The orbital
selectivity of the quasiparticle spectral weight, along with its counterpart of
the energy level renormalization, leads to a band inversion and Dirac node
formation pinned to the immediate vicinity of the Fermi energy. Our work
demonstrates both the naturalness and robustness of the topological properties
in FeSe$_{x}$Te$_{1-x}$, and uncovers a new setting in which strong
correlations and space-group symmetry cooperate in generating strongly
correlated electronic topology.
|
2306.17739v1
|
2023-07-02
|
Thermochemical Stability of Low-Iron, Manganese-Enriched Olivine in Astrophysical Environments
|
Low-iron, manganese-enriched (LIME) olivine grains are found in cometary
samples returned by the Stardust mission to comet 81P/Wild 2. Similar grains
are found in primitive meteoritic clasts and unequilibrated meteorite matrix.
LIME olivine is thermodynamically stable in a vapor of solar composition at
high temperature at total pressures of a millibar to a microbar, but enrichment
of solar composition vapor in a dust of chondritic composition causes the
FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in
primitive materials indicate oxygen fugacities close to that of a very reducing
vapor of solar composition. The compositional zoning of LIME olivines in
amoeboid olivine aggregates is consistent with equilibration with nebular vapor
in the stability field of olivine, without reequilibration at lower
temperatures. A similar history is likely for LIME olivines found in comet
samples and in interplanetary dust particles. LIME olivine is not likely to
persist in nebular conditions in which silicate liquids are stable.
|
2307.00652v1
|
2023-08-10
|
Absence of superconductivity in electron-doped chromium pnictides ThCrAsN$_{1-x}$O$_x$
|
Theoretical studies predicted possible superconductivity in electron-doped
chromium pnictides isostructural to their iron counterparts. Here, we report
the synthesis and characterization of a new ZrCuSiAs-type Cr-based compound
ThCrAsN, as well as its oxygen-doped variants. All samples of
ThCrAsN$_{1-x}$O$_x$ show metallic conduction, but no superconductivity is
observed above 30 mK even though the oxygen substitution reaches 75\%. The
magnetic structure of ThCrAsN is determined to be G-type antiferromagnetic by
magnetization measurements and first-principles calculations jointly. The
calculations also indicate that the in-plane Cr--Cr direct interaction of
ThCrAsN is robust against the heavy electron doping. The calculated density of
states of the orbital occupations of Cr for ThCrAs(N,O) is strongly
spin-polarized. Our results suggest the similarities between chromium pnictides
and iron-based superconductors shouldn't be overestimated.
|
2308.05267v1
|
2023-08-16
|
Phase separation and phase transitions in undoped and Rh3+ doped iron pnictide CaFe2As2: a Raman scattering study
|
Iron-pnictides Ca(Fe1-xRhx)2As2 (x = 0, 0.035, and 0.19) were studied across
the tetragonal-orthorhombic and uncollapsed to collapsed tetragonal phase
transitions using Raman spectroscopy. The effect of the phase separation was
observed in the high-temperature phase for the first time. Two phases with the
low-spin and high-spin states of the Fe2+ ions coexist in the undoped (x = 0)
and doped (x = 0.035) samples at ambient conditions. These two phases are
present on a larger length scale and they are not limited to the local
distortions. Both phases in the undoped sample successfully undergo
tetra-to-ortho phase transitions approximately at the same temperature T0 = 160
K. In doped samples, a cascade of phase transitions is observed at temperatures
T* = 100 K and Tc = 72 K with cooling for the first time. A complex type of the
re-entrant magnetic structure is realized in the x = 0.035 sample at
temperatures below Tc, which is distinct from the usual orthorhombic twofold
one. The overdoped sample (x = 0.19) also shows phase separation at
temperatures 7 - 295 K in Raman spectra similar to these of the remnant phase.
|
2308.08633v1
|
2023-08-23
|
Oxygen-Terminated (1x1) Reconstruction of Reduced Magnetite Fe$_3$O$_4$(111)
|
The (111) facet of magnetite (Fe$_3$O$_4$) has been studied extensively by
experimental and theoretical methods, but controversy remains regarding the
structure of its low-energy surface terminations. Using density functional
theory (DFT) computations, we demonstrate three reconstructions that are more
favorable than the accepted Fe$_{\rm oct2}$ termination in reducing conditions.
All three structures change the coordination of iron in the kagome Fe$_{\rm
oct1}$ layer to tetrahedral. With atomically-resolved microscopy techniques, we
show that the termination that coexists with the Fe$_{\rm tet1}$ termination
consists of tetrahedral iron capped by three-fold coordinated oxygen atoms.
This structure explains the inert nature of the reduced patches.
|
2308.12023v1
|
2023-08-25
|
Causally Sound Priors for Binary Experiments
|
We introduce the BREASE framework for the Bayesian analysis of randomized
controlled trials with a binary treatment and a binary outcome. Approaching the
problem from a causal inference perspective, we propose parameterizing the
likelihood in terms of the baseline risk, efficacy, and adverse side effects of
the treatment, along with a flexible, yet intuitive and tractable jointly
independent beta prior distribution on these parameters, which we show to be a
generalization of the Dirichlet prior for the joint distribution of potential
outcomes. Our approach has a number of desirable characteristics when compared
to current mainstream alternatives: (i) it naturally induces prior dependence
between expected outcomes in the treatment and control groups; (ii) as the
baseline risk, efficacy and risk of adverse side effects are quantities
commonly present in the clinicians' vocabulary, the hyperparameters of the
prior are directly interpretable, thus facilitating the elicitation of prior
knowledge and sensitivity analysis; and (iii) we provide analytical formulae
for the marginal likelihood, Bayes factor, and other posterior quantities, as
well as exact posterior sampling via simulation, in cases where traditional
MCMC fails. Empirical examples demonstrate the utility of our methods for
estimation, hypothesis testing, and sensitivity analysis of treatment effects.
|
2308.13713v2
|
2023-08-30
|
Mechanism of cathodic protection of iron and steel in porous media
|
Cathodic protection (CP) was introduced two centuries ago and since has found
widespread application in protecting structures such as pipelines, offshore
installations, and bridges from corrosion. Despite its extensive use, the
fundamental working mechanism of CP remains debated, particularly for metals in
porous media such as soil. Here, we offer resolution to the long-standing
debate by employing in-situ and ex-situ characterisation techniques coupled
with electrochemical measurements to characterise the spatio-temporal changes
occurring at the steel-electrolyte interface. We show that upon CP, the
interfacial electrolyte undergoes alkalinisation and deoxygenation, and that
depending on polarisation conditions, an iron oxide film can simultaneously
form on the steel surface. We further demonstrate that these changes in
interfacial electrolyte chemistry and steel surface state result in altered
anodic and cathodic reactions and their kinetics. We propose a mechanism of CP
that integrates the long debated theories, based on both concentration and
activation polarisation, complimentarily. Implications of this coherent
scientific understanding for enhancing corrosion protection technologies and
the safe, economic, and environmental-friendly operation of critical
steel-based infrastructures are discussed.
|
2308.15953v1
|
2023-09-08
|
Interband scattering- and nematicity-induced quantum oscillation frequency in FeSe
|
Understanding the nematic phase observed in the iron-chalcogenide materials
is crucial for describing their superconducting pairing. Experiments on
FeSe$_{1-x}$S$_x$ showed that one of the slow Shubnikov--de Haas quantum
oscillation frequencies disappears when tuning the material out of the nematic
phase via chemical substitution or pressure, which has been interpreted as a
Lifshitz transition [Coldea et al., npj Quant Mater 4, 2 (2019), Reiss et al.,
Nat. Phys. 16, 89-94 (2020)]. Here, we present a generic, alternative scenario
for a nematicity-induced sharp quantum oscillation frequency which disappears
in the tetragonal phase and is not connected to an underlying Fermi surface
pocket. We show that different microscopic interband scattering mechanisms -
for example, orbital-selective scattering - in conjunction with nematic order
can give rise to this quantum oscillation frequency beyond the standard Onsager
relation. We discuss implications for iron-chalcogenides and the interpretation
of quantum oscillations in other correlated materials.
|
2309.04237v1
|
2023-09-21
|
Spectral study of neutron star low mass X-ray binary source 1A 1744-361
|
We present X-ray observations of the recent outburst of 2022 from the neutron
star low mass X-ray binary (LMXB) source 1A 1744-361. Spectral properties of
the source have been analyzed using joint NuSTAR and NICER observations. During
our observations, the source happens to be in the banana state (soft state) of
the hardness intensity diagram (HID). In addition to a power-law with a high
energy cutoff, the spectrum is found to exhibit broad iron $K_{\alpha}$
emission along with distinct absorption features. A prominent absorption
feature observed at 6.92 keV may be interpreted as $K_{\alpha}$ absorption line
from hydrogen-like iron. The absorption feature observed at 7.98 keV may be
interpreted as a blend of Fe XXV and Ni XXVII transitions. We have summarized
the evidence of variability of the spectral features observed in the X-ray
continuum by time-resolved spectroscopy.
|
2309.11817v1
|
2023-09-21
|
Interface of Equation-of-State, Atomic Data and Opacities in the Solar Problem
|
Convergence of the Rosseland Mean Opacity (RMO) is investigated with respect
to the equation-of-state (EOS) and the number of atomic levels of iron ions
prevalent at the solar radiative/convection boundary. The "chemical picture"
Mihalas-Hummer-D\"{a}ppen MHD-EOS, and its variant QMHD-EOS, are studied at two
representative temperature-density sets at the base of the convection zone
(BCZ) and the Sandia Z experiment: $(2 \times 10^6K, \ 10^{23}/cc)$ and $(2.11
\times 10^6K, \ 3.16 \times 10^{22}/cc)$, respectively. It is found that
whereas the new atomic datasets from accurate R-matrix calculations for
opacities (RMOP) are vastly overcomplete, involving hundreds to over a thousand
levels of each of the three Fe ions considered -- FeXVII, FeXVIII and FeXIX --
the EOS constrains contributions to RMOs by relatively fewer levels. The RMOP
iron opacity spectrum is quite different from the Opacity Project distorted
wave model and shows considerably more plasma broadening effects. This work
points to possible improvements needed in the EOS for opacities in
high-energy-density (HED) plasma sources.
|
2309.12073v1
|
2023-09-26
|
Polymer amide as a source of the cosmic 6.2 micron emission and absorption
|
Cosmic infrared emission and absorption spectra often carry a well-defined
and invariant 6.2 micron band that has been proposed to emanate from very small
dust grains that may carry polyaromatic hydrocarbons. Hemoglycin, a
well-defined polymer of glycine that also contains iron, has been found in
meteorites of the primordial CV3 class and therefore originated in the solar
protoplanetary disc. In approximate calculations, the principal amide I
infrared absorption band of hemoglycin is at 6.04 microns. Hemoglycin, an
antiparallel beta sheet structure with two 11-mer glycine chains, has an exact
structural analog in antiparallel poly-L-lysine beta sheets which in the
laboratory have an absorption peak at 6.21 microns. This wavelength
coincidence, the demonstrated propensity of hemoglycin 4.9nm rods to form
accreting lattice structures, and its proven existence in the solar
protoplanetary disc strongly suggest that the cosmic 6.2 micron emission and
absorption could be from small grains that are hemoglycin lattices or
shell-like vesicles carrying internal organic molecules of various types.
Calculated hemoglycin ultraviolet absorptions associated with iron in the
molecule match the observed ultraviolet extinction feature at nominal 2175
Angstroms.
|
2309.14914v1
|
2023-10-10
|
Improved iron-tolerance in recycled aluminum alloys via direct strip casting process
|
Recycled aluminum alloys are pivotal for sustainable manufacturing, offering
strength, durability, and environmental advantages. However, the presence of
iron (Fe) impurities poses a major challenge, undermining their properties and
recyclability. Conventional manufacturing processes result in coarse Fe-rich
intermetallic compounds that limit the tolerance of Fe content and negatively
influence performance of advanced aluminum alloys. To address this, rapid
solidification techniques like direct strip casting have been explored. In this
work, a detailed study of the strip cast microstructure was conducted by
scanning electron microscopy, electron backscattered diffraction and atom probe
tomography. Our results reveal that alloys produced by DSC exhibit
significantly refined microstructures and are free from coarse Fe-rich
intermetallics, thereby retaining the majority of Fe in solid solution. These
findings indicate that strip casting significantly enhances Fe-tolerance in
aluminum alloys, making it an attractive process for future aluminum recycling,
with implications for sustainable high-performance applications.
|
2310.06327v1
|
2023-10-16
|
Spectral Properties of GX~339--4 in the Intermediate State Using AstroSat Observation
|
We present the results obtained from the spectral studies of black hole X-ray
binary GX~339--4 using \astrosat~ observations during its 2021 outburst.
\astrosat~ observed the source in the intermediate state for $\sim600$ ks. The
combined spectra of SXT and LAXPC in the $0.7-25$ keV energy range are studied
with phenomenological and physical models. The spectral study reveals a
receding disc and a contracting corona during the observation period. The
outflow rate is found to be increased though the accretion rates did not vary
during the observation period. The X-ray flux decreases as the disc recedes and
the spectrum becomes hard. At the same time, the Comptonized flux decreases
with increasing fraction of thermal emission. This could be plausible that
episodic jet ejection modified the corona and reduced Comptonized flux. An iron
emission line at 6.4 keV is observed in the spectra of all the orbits of
observation. We find that the equivalent width of the iron emission line
correlates with the photon index, indicating a decrease in the reflection
strength as the spectrum becomes hard. We observe that the disc flux does not
follow $F_{\rm DBB}-T^{4}$ relation.
|
2310.10444v1
|
2023-10-16
|
Quantum hardware calculations of the activation and dissociation of nitrogen on iron clusters and surfaces
|
Catalytic processes are vital in the chemical industry, with
nitrogen-to-ammonia conversion being a major industrial process. Designing
catalysts relies on computational chemistry methods like Density Functional
Theory (DFT), which have limitations in accuracy, especially for complex
materials. Quantum computing advancements offer promise for precise ab-initio
methods. We introduce a hybrid quantum-classical workflow to model chemical
reactions on surfaces, demonstrated on nitrogen activation and dissociation on
small Fe clusters and an iron surface. We decoupled key electronic structures
using CASSCF, translated them into qubits, and estimated energies with quantum
and classical simulations, showing potential for quantum computing in catalysis
research as technology scales up.
|
2310.10478v1
|
2023-10-31
|
The Larmor frequency shift of a white matter magnetic microstructure model with multiple sources
|
Magnetic susceptibility imaging may provide valuable information about
chemical composition and microstructural organization of tissue. However, its
estimation from the MRI signal phase is particularly difficult as it is
sensitive to magnetic tissue properties ranging from the molecular to
macroscopic scale. The MRI Larmor frequency shift measured in white matter (WM)
tissue depends on the myelinated axons and other magnetizable sources such as
iron-filled ferritin. We have previously derived the Larmor frequency shift
arising from a dense media of cylinders with scalar susceptibility and
arbitrary orientation dispersion. Here we extend our model to include
microscopic WM susceptibility anisotropy as well as spherical inclusions with
scalar susceptibility to represent subcellular structures, biologically stored
iron etc. We validate our analytical results with computer simulations and
investigate the feasibility of estimating susceptibility using simple iterative
linear least squares without regularization or preconditioning. This is done in
a digital brain phantom synthesized from diffusion MRI (dMRI) measurements of
an ex vivo mouse brain at ultra-high field.
|
2311.00086v3
|
2023-11-19
|
Diverse features of dust particles and their aggregates inferred from experimental nanoparticles
|
Nanometre- to micrometre-sized solid dust particles play a vital role in star
and planet formations. Despite of their importance, however, our understanding
of physical and chemical properties of dust particles is still provisional. We
have conducted a condensation experiment of the vapour generated from a solid
starting material having nearly cosmic proportions in elements. A laser flash
heating and subsequent cooling has produced a diverse type of nanoparticles
simultaneously. Here we introduce four types of nanoparticles as potential dust
particles in space: amorphous silicate nanoparticles (type S); core/mantle
nanoparticles with iron or hydrogenised-iron core and amorphous silicate mantle
(type IS); silicon oxycarbide nanoparticles and hydrogenised silicon oxycarbide
nanoparticles (type SiOC); and carbon nanoparticles (type C), all produced in a
single heating-cooling event. Type IS and SiOC nanoparticles are new for
potential astrophysical dust. The nanoparticles are aggregated to a wide
variety of structures, from compact, fluffy, and networked. A simultaneous
formation of nanoparticles, which are diverse in chemistry, shape, and
structure, prompts a re-evaluation of astrophysical dust particles
|
2311.11213v1
|
2023-12-14
|
Confinement of electron holes via the peroxo group formation in the negative charge-transfer materials on the example of SrFeO3: plane-wave density functional theory predictions
|
The present work puts forward a concept that the thermostable O1s XPS peaks
with energy of about 531 eV in negative charge-transfer SrFeO_{3-\delta}
perovskite are determined by the peroxo-like oxygen species. The peroxo group
forms via coupling two oxygen anions coordinated to iron cations with
d^5\bar-under{L} (\bar-under{L}-oxygen electron hole) configuration. By means
of plane-wave DFT+U approach there have been shown that the peroxo group
represents a metastable state in the absence of oxygen vacancies nearby. The
O-O bonding confines two electron holes freezing the 3+ oxidation state for two
iron cations bridged by peroxide. Increasing the peroxo group numbers makes the
ferrite a semiconductor with charge-transfer gap of about 0.6 eV.
|
2312.08665v1
|
2023-12-18
|
Quantum criticality in quasi-binary compounds of iron-based superconductors
|
In this work, we present the studies of structural phase transitions in
Fe(Se,Te) crystals in the range of about 30% selenium substitution by
tellurium. We found a significant change in the properties of the ordered state
of these compositions compared to the case of pure FeSe. The resistivity at low
temperatures for the studied Fe(Se,Te) is proportional to the square of the
temperature while for pure FeSe below the structural transition it depends
almost linearly on temperature. The NMR data show a noticeable line broadening
below the structural transition and an anomaly in the temperature dependence of
the relaxation rate in the tellurium-substituted compounds, which was not
observed in the pure FeSe. This reveals in quasi-binary compounds of iron-based
superconductors a region of quantum criticality similar to that which exists
when the nematicity of FeSe is suppressed under pressure and which precedes the
emergence of high-temperature superconductivity in FeSe under hydrostatic
pressure.
|
2312.10901v1
|
2024-01-05
|
Comparison of Two Detector Magnetic Systems for the Future Circular Hadron-Hadron Collider
|
The conceptual design study of a Future Circular hadron-hadron Collider
(FCC-hh) to be con-structed at CERN with a center-of-mass energy of the order
of 100 TeV requires superconducting magnetic systems with a central magnetic
flux density of an order of 4 T for the experimental detectors. The developed
concept of the FCC-hh detector involves the use of an iron-free magnetic system
consisting of three superconducting solenoids. A superconducting magnet with a
minimal steel yoke is proposed as an alternative to the baseline iron-free
design. In this study, both magnetic system options for the FCC-hh detector are
modeled with the same electrical parameters using Cobham$'$s program TOSCA. All
the main characteristics of both designs are compared and discussed.
|
2401.02835v1
|
2024-01-08
|
Harvesting nucleating structures in nanoparticle crystallization: The example of gold, silver and iron
|
The thermodynamics and kinetics of crystallization of nanoparticles, as
opposed to bulk phases, may be influenced by surface and size effects. We
investigate the importance of such factors in the crystallization process of
gold, silver, and iron nanodroplets using numerical simulations in the form of
molecular dynamics combined with path sampling. This modeling strategy is
targeted at obtaining representative ensembles of structures located at the
transition state of the crystallization process. A structural analysis of the
transition state ensembles reveals that both the average size and location of
the critical nucleation cluster are influenced by surface and nanoscale size
effets. Furthermore, we also show that transition state structures in smaller
nanodroplets exhibit a more ordered liquid phase, and differentiating between a
well-ordered critical cluster and its surrounding disordered liquid phase
becomes less evident. All in all, these findings demonstrate that
crystallization mechanisms in nanoparticles go beyond the assumptions of
classical nucleation theory.
|
2401.03969v1
|
2024-01-16
|
Influence of temperature, doping, and amorphization on the electronic structure and magnetic damping of iron
|
Hybrid magnonic quantum systems have drawn increased attention in recent
years for coherent quantum information processing, but too large magnetic
damping is a persistent concern when metallic magnets are used. Their intrinsic
damping is largely determined by electron-magnon scattering induced by
spin-orbit interactions. In the low scattering limit, damping is dominated by
intra-band electronic transitions, which has been theoretically shown to be
proportional to the electronic density of states at the Fermi level. In this
work, we focus on body-centered-cubic iron as a paradigmatic ferromagnetic
material. We comprehensively study its electronic structure using
first-principles density functional theory simulations and account for finite
lattice temperature, boron (B) doping, and structure amorphization. Our results
indicate that temperature induced atomic disorder and amorphous atomic
geometries only have a minor influence. Instead, boron doping noticeably
decreases the density of states near the Fermi level with an optimal doping
level of 6.25%. In addition, we show that this reduction varies significantly
for different atomic geometries and report that the highest reduction
correlates with a large magnetization of the material. This may suggest
materials growth under external magnetic fields as a route to explore in
experiment.
|
2401.08076v1
|
2024-01-23
|
Understanding atom probe's analytical performance for iron oxides using correlation histograms and ab initio calculations
|
Field evaporation from ionic or covalently bonded materials often leads to
the emission of molecular ions. The metastability of these molecular ions,
particularly under the influence of the intense electrostatic field (1010
Vm-1), makes them prone to dissociation with or without an exchange of energy
amongst them. These processes can affect the analytical performance of atom
probe tomography (APT). For instance, neutral species formed through
dissociation may not be detected at all or with a time of flight no longer
related to their mass, causing their loss from the analysis. Here, we evaluated
the changes in the measured composition of FeO, Fe2O3 and Fe3O4 across a wide
range of analysis conditions. Possible dissociation reactions are predicted by
density-functional theory (DFT) calculations considering the spin states of the
molecules. The energetically favoured reactions are traced on to the multi-hit
ion correlation histograms, to confirm their existence within experiments,
using an automated Python-based routine. The detected reactions are carefully
analysed to reflect upon the influence of these neutrals from dissociation
reactions on the performance of APT for analysing iron oxides.
|
2401.12784v1
|
2024-01-29
|
HEQuant: Marrying Homomorphic Encryption and Quantization for Communication-Efficient Private Inference
|
Secure two-party computation with homomorphic encryption (HE) protects data
privacy with a formal security guarantee but suffers from high communication
overhead. While previous works, e.g., Cheetah, Iron, etc, have proposed
efficient HE-based protocols for different neural network (NN) operations, they
still assume high precision, e.g., fixed point 37 bit, for the NN operations
and ignore NNs' native robustness against quantization error. In this paper, we
propose HEQuant, which features low-precision-quantization-aware optimization
for the HE-based protocols. We observe the benefit of a naive combination of
quantization and HE quickly saturates as bit precision goes down. Hence, to
further improve communication efficiency, we propose a series of optimizations,
including an intra-coefficient packing algorithm and a quantization-aware
tiling algorithm, to simultaneously reduce the number and precision of the
transferred data. Compared with prior-art HE-based protocols, e.g., CrypTFlow2,
Cheetah, Iron, etc, HEQuant achieves $3.5\sim 23.4\times$ communication
reduction and $3.0\sim 9.3\times$ latency reduction. Meanwhile, when compared
with prior-art network optimization frameworks, e.g., SENet, SNL, etc, HEQuant
also achieves $3.1\sim 3.6\times$ communication reduction.
|
2401.15970v2
|
2024-01-30
|
A New Database of Giant Impacts over a Wide Range of Masses and with Material Strength: A First Analysis of Outcomes
|
In the late stage of terrestrial planet formation, planets are predicted to
undergo pairwise collisions known as giant impacts. Here we present a
high-resolution database of giant impacts for differentiated colliding bodies
of iron-silicate composition, with target masses ranging from 10^-4 M_Earth up
to super-Earths (5 M_Earth). We vary impactor-to-target mass ratio, core-mantle
(iron-silicate) fraction, impact velocity, and impact angle. Strength in the
form of friction is included in all simulations. We find that due to strength,
collisions with bodies smaller than about 2*10^-3 M_Earth can result in
irregular shapes, compound core structures, and captured binaries. We observe
that the characteristic escaping velocity of smaller remnants (debris) is
approximately half of the impact velocity, significantly faster than currently
assumed in N-body simulations of planet formation. Incorporating these results
in N-body planet formation studies would provide more realistic debris-debris
and debris-planet interactions.
|
2401.17356v1
|
2024-02-09
|
Copper phosphate micro-flowers coated with indocyanine green and iron oxide nanoparticles for in vivo localization optoacoustic tomography and magnetic actuation
|
Efficient drug delivery is a major challenge in modern medicine and
pharmaceutical research. Micrometer-scale robots have recently been proposed as
a promising venue to amplify precision of drug administration. Remotely
controlled microrobots sufficiently small to navigate through microvascular
networks can reach any part of the human body, yet real-time tracking is
crucial for providing precise guidance and verifying successful arrival at the
target. In vivo deep-tissue monitoring of individual microrobots is currently
hampered by the lack of sensitivity and/or spatio-temporal resolution of
commonly used clinical imaging modalities. We synthesized biocompatible and
biodegradable copper phosphate micro-flowers loaded with indocyanine green and
iron oxide nanoparticles to enable in vivo individual detection with
localization optoacoustic tomography. We demonstrate magnetic actuation and
optoacoustic tracking of these decorated micro-flowers at a per-particle level.
Functional super-resolution imaging achieved via tracking intravenously
injected particles provides a means of identifying microvascular targets and
quantifying blood flow, while the versatile carrying capacity can be further
exploited for transporting multiple types of drug formulations.
|
2402.06749v1
|
2024-03-24
|
Rigorous ESR spectroscopy of $Fe^{3+}$ impurity ion with oxygen vacancy in ferroelectric $SrTiO_3$ crystal at 20 mK
|
Impurity $Fe^{3+}$ ion electron spin resonance (ESR) spectroscopy using
multiple dielectric modes in a $SrTiO_3$ dielectric resonator has been
performed with a tuneable DC magnetic field of up to $1.6~T$. The
$Ti^{4+}(d^0)$ ion is substituted by $Fe^{3+}$ ion forming $FeO_6$ octahedral
complex with an iron-oxygen-vacancy $(Fe^{3+}-V_O)$. In such a metal-ligand
complex, a giant g-factor of $g_{\scriptscriptstyle\parallel F} = 5.51$ was
observed in the ferroelectric phase at $20~mK$. The the change of $Fe^{3+}$ ion
center-symmetry in the $FeO_6$ complex as a soft-mode characteristics of
ferroelectric phase transition and the influences of iron-oxygen-vacancy
$(Fe^{3+}-V_O)$, interactively sensitive to asymmetry in the octahedral
rotational parameter $\Phi$ in $SrTi0_3$.
|
2403.16299v1
|
2024-03-29
|
Synthesis and Characterization of Superparamagnetic Iron Oxide Nanoparticles: A Series of Laboratory Experiments
|
The following laboratory procedure provides students with a hands-on
experience in nanomaterials chemistry and characterization. This three-day
protocol is easy to follow for undergraduates with basic chemistry or materials
science backgrounds and is suitable for inclusion in upper division courses in
inorganic chemistry or materials science. Students use air-free chemistry
procedures to synthesize and separate iron oxide magnetic nanoparticles and
subsequently modify the nanoparticle surface using a chemical stripping agent.
The morphology and chemical composition of the nanoparticles are characterized
using electron microscopy and dynamic light scattering measurements.
Additionally, magnetic characterization of the particles is performed using an
open-source (3D-printed), inexpensive magnetophotometer. Possible modifications
to the synthesis procedure including the incorporation of dopants to modify the
magnetic response and alternative characterization techniques are discussed.
The three-day synthesis, purification, and characterization laboratory will
prepare students with crucial skills for advanced technology industries such as
semiconductor manufacturing, nanomedicine, and green chemistry.
|
2403.19970v1
|
2024-04-10
|
Microbial iron reduction under oxic conditions: implications for subsurface biogeochemistry
|
Iron (Fe) reduction is one of Earth's most ancient microbial metabolisms, but
after atmosphere-ocean oxygenation, this anaerobic process was relegated to
niche anoxic environments below the water and soil surface. However, new
technologies to monitor redox processes at the microscale relevant to microbial
cells have recently revealed that the oxygen (O2) concentrations controlling
the distribution of aerobic and anaerobic metabolisms are more heterogeneous
than previously believed. To explore how O2 levels regulate microbial Fe
reduction, we cultivated a facultative Fe-reducing bacterium using a
cutting-edge microfluidic reactor integrated with transparent planar O2
sensors. Contrary to expectations, microbial growth induced Fe(III)-oxide
(ferrihydrite) reduction under fully oxygenated conditions without forming
O2-depleted microsites. Batch incubations highlighted the importance of the
process at a larger scale, fundamentally changing our understanding of Fe
cycling from the conceptualization of metal and nutrient mobility in the
subsurface to our interpretation of Fe mineralogy in the rock record.
|
2404.07137v1
|
2021-06-15
|
Measurement of the Iron Spectrum in Cosmic Rays from 10 GeV$/n$ to 2.0 TeV$/n$ with the Calorimetric Electron Telescope on the International Space Station
|
The Calorimetric Electron Telescope (CALET), in operation on the
International Space Station since 2015, collected a large sample of cosmic-ray
iron over a wide energy interval. In this Letter a measurement of the iron
spectrum is presented in the range of kinetic energy per nucleon from 10
GeV$/n$ to 2.0 TeV$/n$ allowing the inclusion of iron in the list of elements
studied with unprecedented precision by space-borne instruments. The
measurement is based on observations carried out from January 2016 to May 2020.
The CALET instrument can identify individual nuclear species via a measurement
of their electric charge with a dynamic range extending far beyond iron (up to
atomic number $Z$ = 40). The energy is measured by a homogeneous calorimeter
with a total equivalent thickness of 1.2 proton interaction lengths preceded by
a thin (3 radiation lengths) imaging section providing tracking and energy
sampling. The analysis of the data and the detailed assessment of systematic
uncertainties are described and results are compared with the findings of
previous experiments. The observed differential spectrum is consistent within
the errors with previous experiments. In the region from 50 GeV$/n$ to 2
TeV$/n$ our present data are compatible with a single power law with spectral
index -2.60 $\pm$ 0.03.
|
2106.08036v1
|
2023-04-13
|
Probing magnetic ordering in air stable iron-rich van der Waals minerals
|
In the rapidly expanding field of two-dimensional materials, magnetic
monolayers show great promise for the future applications in nanoelectronics,
data storage, and sensing. The research in intrinsically magnetic
two-dimensional materials mainly focuses on synthetic iodide and telluride
based compounds, which inherently suffer from the lack of ambient stability. So
far, naturally occurring layered magnetic materials have been vastly
overlooked. These minerals offer a unique opportunity to explore air-stable
complex layered systems with high concentration of local moment bearing ions.
We demonstrate magnetic ordering in iron-rich two-dimensional phyllosilicates,
focusing on mineral species of minnesotaite, annite, and biotite. These are
naturally occurring van der Waals magnetic materials which integrate local
moment baring ions of iron via magnesium/aluminium substitution in their
octahedral sites. Due to self-inherent capping by silicate/aluminate
tetrahedral groups, ultra-thin layers are air-stable. Chemical
characterization, quantitative elemental analysis, and iron oxidation states
were determined via Raman spectroscopy, wavelength disperse X-ray spectroscopy,
X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy.
Superconducting quantum interference device magnetometry measurements were
performed to examine the magnetic ordering. These layered materials exhibit
paramagnetic or superparamagnetic characteristics at room temperature. At low
temperature ferrimagnetic or antiferromagnetic ordering occurs, with the
critical ordering temperature of 38.7 K for minnesotaite, 36.1 K for annite,
and 4.9 K for biotite. In-field magnetic force microscopy on iron bearing
phyllosilicates confirmed the paramagnetic response at room temperature,
present down to monolayers.
|
2304.06533v1
|
2023-11-08
|
A human brain atlas of chi-separation for normative iron and myelin distributions
|
Iron and myelin are primary susceptibility sources in the human brain. These
substances are essential for healthy brain, and their abnormalities are often
related to various neurological disorders. Recently, an advanced susceptibility
mapping technique, which is referred to as chi-separation, has been proposed,
successfully disentangling paramagnetic iron from diamagnetic myelin. This
method opened a potential for generating high resolution iron and myelin maps
in the brain. Utilizing this technique, this study constructs a normative
chi-separation atlas from 106 healthy human brains. The resulting atlas
provides detailed anatomical structures associated with the distributions of
iron and myelin, clearly delineating subcortical nuclei, thalamic nuclei, and
white matter fiber bundles. Additionally, susceptibility values in a number of
regions of interest are reported along with age-dependent changes. This atlas
may have direct applications such as localization of subcortical structures for
deep brain stimulation or high-intensity focused ultrasound and also serve as a
valuable resource for future research.
|
2311.04468v3
|
1997-12-18
|
ASCA detection of the FeK edge in the spectrum of the Seyfert 2 galaxy IRAS 04575-7537: a sign of a complex absorber
|
ASCA X-ray spectral analysis of the Seyfert 2 galaxy IRAS 04575-7537 is
presented. The main result is the presence of a significant iron edge at a rest
energy of 7.13^+0.21_-0.16 keV. The spectrum is flat (Gamma = 1.5) with a
substantial absorption Nh = 10^22 cm^-2 and does not require any reflection
component. There is also evidence of a narrow Fe Kalpha emission line, whose
rest energy (E = 6.35^+0.08_ -0.03 keV) and equivalent width (EW = 130+-50 eV)
suggest that the line originates from thick and cold matter. The intensity of
the line and, in particular, the depth of the iron edge are too strong to be
explained by simple transmission through the measured absorption column
density. This strongly suggests that a model more complex than a single
absorbed power law is needed. We propose an absorption model that we call
"leaky-total absorber" which can explain the spectrum flatness, the iron
emission line and the edge absorption feature and has also the advantage of
having a straightforward physical interpretation in the framework of Unified
models. In this model a thick absorber (Nh = 10^23 cm^-2), possibly associated
with broad line region (BLR) clouds, partially (36 %) covers the source
continuum with Gamma fixed to 1.9 as observed in Sey 1 galaxies. The escaping
radiation is then absorbed by a column density Nh = 10^22 cm^-2, which can be
attributed to the torus, seen through its rim.
|
9712250v1
|
1999-07-13
|
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
|
Chemical analyses of late-type stars are usually carried out following the
classical recipe: LTE line formation and homogeneous, plane-parallel,
flux-constant, and LTE model atmospheres. We review different results in the
literature that have suggested significant inconsistencies in the spectroscopic
analyses, pointing out the difficulties in deriving independent estimates of
the stellar fundamental parameters and hence,detecting systematic errors.
The trigonometric parallaxes measured by the HIPPARCOS mission provide
accurate appraisals of the stellar surface gravity for nearby stars, which are
used here to check the gravities obtained from the photospheric iron ionization
balance. We find an approximate agreement for stars in the metallicity range -1
<= [Fe/H] <= 0, but the comparison shows that the differences between the
spectroscopic and trigonometric gravities decrease towards lower metallicities
for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow
upon the abundance analyses for extreme metal-poor stars that make use of the
ionization equilibrium to constrain the gravity. The comparison with the
strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)
confirms that this method provides systematically larger gravities than the
ionization balance. The strong-line gravities get closer to the physical ones
for the stars analyzed by Fuhrmann, but they are even further away than the
iron ionization gravities for the stars of lower gravities in Edvardsson's
sample. The confrontation of the deviations of the iron ionization gravities in
metal-poor stars reported here with departures from the excitation balance
found in the literature, show that they are likely to be induced by the same
physical mechanism(s).
|
9907155v1
|
2000-01-07
|
The variability of the Seyfert galaxy NGC 2992: the case for a revived AGN
|
We report the transition to an active state of the nucleus in the Seyfert 1.9
galaxy NGC 2992, discovered by means of new hard X-ray data. While the 2-10 keV
flux declined by a factor of ~ 20 from 1978 to 1994, two recent BeppoSAX
observations in 1997 and in 1998 caught the nuclear emission raising back to
the same level of activity observed in 1978.
In both BeppoSAX observations the X-ray spectrum of the source is well
represented by a power law with spectral index Gamma ~ 1.7, absorbed by a
column density of N_H ~ 10^{22} cm^{-2} and characterized by a prominent iron
K\alpha line. While in the second BeppoSAX data set the line properties appear
to be consistent with those expected from accretion disc models, in the first
BeppoSAX data set the iron feature is rather peculiar. The broadening is not
significant and the line energy is E_{K\alpha}=6.62 +/- 0.07 keV, indicating
emission from highly ionized iron. The line has too high equivalent width (~
700 eV) to be produced by a hot scattering medium. By comparing these data with
data previously in the literature, we interpret the spectral and flux changes
in terms of different phases of rebuilding an accretion disc. The timescale for
the disc rebuilding is estimated to range between 1 and 5 years.
The X-ray data are complemented with optical and near-infrared followup
spectra taken 1.5 months after the discovery of the X-ray burst. The spectra
are characterized by prominent broad emission lines. There is also evidence for
hot dust emission in the H and K bands that, however, is probably still in the
process of increasing.
|
0001107v1
|
2000-08-31
|
Variable Iron K-alpha Lines in Seyfert 1 Galaxies
|
We find that variability of the iron K-alpha line is common in Seyfert 1
galaxies. Using data from the ASCA archive for objects that have been observed
more than once during the mission, we study the time-averaged spectra from
individual observations, thereby probing variability on timescales that range
from days to years. Since the statistics of the data do not warrant searches
for line variability in terms of a complex physical model, we use a a simple
Gaussian to model the gross shape of the line, and then use the centroid
energy, intensity and equivalent width as robust indicators of changes in the
line profile. We find that ~70% of Seyfert 1s (ten out of fifteen) show
variability in at least one of these parameters: the centroid energy,
intensity, and equivalent width vary in six, four, and eight sources
respectively. Due to the low S/N, limited sampling and time averaging, we
consider these results to represent lower limits to the rate of incidence of
variability. In most cases changes in the line do not appear to track changes
in the continuum. In particular, we find no evidence for variability of the
line intensity in NGC 4151, suggesting an origin in a region larger than the
putative accretion disk, where most of the iron line has been thought to
originate. Mkn 279 is investigated on short timescales. The time-averaged
effective line energy is 6.5 keV in the galaxy rest frame. As the continuum
flux increases by 20% in a few hours, the Fe K line responds with the effective
line energy increasing by 0.22 keV (~10,500 km s^-1). Problems with the ASCA
and Rosat calibration that affect simultaneous spectral fits are discussed in
an appendix.
|
0008522v1
|
2000-09-13
|
First Stellar Abundances in NGC 6822 from VLT-UVES and Keck-HIRES Spectroscopy
|
We have obtained the first high resolution spectra of individual stars in the
dwarf irregular galaxy, NGC 6822. The spectra of the two A-type supergiants
were obtained at the VLT and Keck Observatories, using UVES and HIRES,
respectively. A detailed model atmospheres analysis has been used to determine
their atmospheric parameters and elemental abundances. The mean iron abundance
from these two stars is $<$[Fe/H]$>=-$0.49 $\pm$0.22 ({\it $\pm$0.21}, with Cr
yielding a similar underabundance, $<$[Cr/H]$>=-$0.50 $\pm$0.20 ({\it
$\pm$0.16}). This confirms that NGC 6822 has a metallicity that is slightly
higher than that of the SMC, and is the first determination of the present-day
iron-group abundances in NGC 6822. The mean stellar oxygen abundance,
12+log(O/H)=8.36 $\pm$0.19 ({\it $\pm$0.21}),is in good agreement with the
nebular oxygen results. Oxygen has the same underabundance as iron,
$<$[O/Fe]$>$=+0.02 $\pm$0.20 ({\it $\pm$0.21}). This O/Fe ratio is very similar
to that seen in the Magellanic Clouds, which supports the picture that chemical
evolution occurs more slowly in these lower mass galaxies, although the O/Fe
ratio is also consistent with that observed in comparatively metal-poor stars
in the Galactic disk. Combining all of the available abundance observations for
NGC 6822 shows that there is no trend in abundance with galactocentric
distance. However, a subset of the highest quality data are consistent with a
radial abundance gradient.
|
0009213v1
|
2000-10-17
|
Estimation of relativistic accretion disk parameters from iron line emission
|
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the emission. Here we present an analysis of the geometrical and
kinematic properties of the disk based on the extreme frequency shifts of a
line profile as determined by measurable flux in both the red and blue wings.
The edges of the line are insensitive to the distribution of the X-ray flux
over the disk, and hence provide a robust alternative to profile fitting of
disk parameters. Our approach yields new, strong bounds on the inclination
angle of the disk and the location of the emitting region. We apply our method
to interpret observational data from MCG-6-30-15 and find that the commonly
assumed inclination 30 deg for the accretion disk in MCG-6-30-15 is
inconsistent with the position of the blue edge of the line at a 3 sigma level.
A thick turbulent disk model or the presence of highly ionized iron may
reconcile the bounds on inclination from the line edges with the full line
profile fits based on simple, geometrically thin disk models. The bounds on the
innermost radius of disk emission indicate that the black hole in MCG-6-30-15
is rotating faster than 30 % of theoretical maximum. When applied to data from
NGC 4151, our method gives bounds on the inclination angle of the X-ray
emitting inner disk of 50\pm 10 deg, consistent with the presence of an
ionization cone grazing the disk as proposed by Pedlar et al. (1993). The
frequency extrema analysis also provides limits to the innermost disk radius in
another Seyfert-1 galaxy, NGC 3516.
|
0010318v1
|
2001-03-07
|
Chandra observtaion of A2256 - a cluster at the early stage of merging
|
We present here \chandra observations of the rich cluster of galaxies A2256.
In addition to the known cool subcluster, a new structure was resolved 2$'$
east of the peak of the main cluster. Its position is roughtly at the center of
a low-brightness radio halo. Spectral analysis shows that the "shoulder" has
high iron abundance ($\sim$ 1). We suggest that this structure is either
another merging component or an internal structure of the main cluster. The
X-ray redshifts of several regions were measured. The results agree with the
optical ones and suggest that the main cluster, the subcluster and the
"shoulder" are physically associated and interacting. The subcluster has low
temperature ($\sim$ 4.5 keV) and high iron abundance ($\sim$ 0.6) in the
central 150 kpc. The \chandra image shows a relatively sharp brightness
gradient at the south of the subcluster peak running south-south-east (SSE). A
temperature jump was found across the edge, with higher temperature ahead of
the edge in the low density region. This phenomenon is qualitatively similar to
the "cold fronts" found in A2142 and A3667. If the "shoulder" is ignored, the
temperature map resembles those simulations at the early stage of merging while
the subcluster approached the main cluster from somewhere west. This fact and
the observed edge, in combination with the clear iron abundance contrast
between the center of the subcluster ($\sim$ 0.6) and the main cluster ($\sim$
0.2), all imply that the ongoing merger is still at the early stage. At least
three member galaxies, including a radio head-tail galaxy, were found to have
corresponding X-ray emission.
|
0103103v2
|
2001-03-15
|
Nucleosynthesis of Zinc and Iron-Peak Elements in Pop III Type II Supernovae : Comparison with abundances of Very Metal-Poor Halo Stars
|
We calculate nucleosynthesis in core-collapse explosions of massive Pop III
stars, and compare the results with abundances of metal-poor halo stars to
constrain the parameters of Pop III supernovae. We focus on iron-peak elements
and, in particular, we try to reproduce the large [Zn/Fe] observed in extremely
metal-poor stars. The interesting trends of the observed ratios [Zn, Co, Mn,
Cr, V/Fe] can be related to the variation of the relative mass of the complete
and incomplete Si-burning regions in supernova ejecta. We find that [Zn/Fe] is
larger for deeper mass-cuts, smaller neutron excess, and larger explosion
energies. The large [Zn/Fe] and [O/Fe] observed in the very metal-poor halo
stars suggest deep mixing of complete Si-burning material and a significant
amount of fall-back in Type II supernovae. Furthermore, large explosion
energies (E_51 >~ 2 for M ~ 13 Msun and E_51 >~ 20 for M >~ 20 Msun) are
required to reproduce [Zn/Fe] ~ 0.5. The observed trends of the abundance
ratios among the iron-peak elements are better explained with this high energy
(``Hypernova'') models rather than the simple ``deep'' mass-cut effect, because
the overabundance of Ni can be avoided in the hypernova models. We also present
the yields of pair-instability supernova explosions of M = 130 - 300 Msun
stars, and discuss that the abundance features of very metal-poor stars cannot
be explained by pair-instability supernovae.
|
0103241v4
|
2001-04-10
|
Partially Absorbed Comptonization Spectrum from the Nearly Edge-on Source X 1822-371
|
We report the results of a spectral analysis over the range 0.1-200 keV
performed on the dipping source X 1822-371 observed by BeppoSAX. We find the
best fit to the continuum using a partially covered Comptonization model, due
to scattering off soft seed photons by electrons at a temperature of ~4.8 keV,
without the presence of any soft blackbody emission. The equivalent hydrogen
column obtained for the absorbed component is ~4.5 10^{22} cm^{-2}, an order of
magnitude larger than the Galactic absorption for this source, and the covering
fraction is ~71%. Because the inclination angle of X 1822-371 to the line of
sight is ~85^\circ, this model gives a reasonable scenario for the source: the
Comptonized spectrum could come from an extended accretion disk corona (ADC),
probably the only region that can be directly observed due to the high
inclination. The excess of matter producing the partial covering could be close
to the equatorial plane of the system, above the outer disk, occulting the
emission from the inner disk and the inner part of the ADC. An iron emission
line is also present at ~6.5 keV with an equivalent width of ~150 eV. We argue
that this strong iron line cannot be explained as reflection of the Comptonized
spectrum by the accretion disk. It is probably produced in the ADC. An emission
line at ~1.9 keV (with an equivalent width of ~54 eV) and an absorption edge at
\~8.7 keV (with an optical depth of ~0.1) are also required to fit this
spectrum. These features are probably produced by highly ionized iron (Fe XXIV)
present in the outer part of the ADC, where the plasma density is
\~10^{11}-10^{12} cm^{-3} and ionized plasma is present.
|
0104171v2
|
2001-09-21
|
A Chandra HETG Observation of the Quasar H 1821+643 and Its Surrounding Cluster
|
We present the high-resolution X-ray spectrum of the low-redshift quasar H
1821+643 and its surrounding hot cluster observed with the Chandra High Energy
Transmission Grating Spectrometer (HETGS). An iron emission line attributed to
the quasar at ~6.43 keV (rest frame) is clearly resolved, with an equivalent
width of ~100 eV. Although we cannot rule out contributions to the line from a
putative torus, the diskline model provides an acceptable fit to this iron
line. We also detect a weak emission feature at ~6.9 keV (rest frame). We
suggest that both lines could originate in an accretion disk comprised of a
highly ionized optically thin atmosphere sitting atop a mostly neutral disk. We
search for absorption features from a warm/hot component of the intergalactic
medium along the ~1.5Gpc/h line of sight to the quasar. No absorption features
are detected at or above the 3 sigma level while a total of six OVI intervening
absorption systems have been detected with HST and FUSE. Based on the lack of
OVII and OVIII absorption lines and by assuming collisionally ionization, we
constrain the gas temperature of a typical OVI absorber to 10^5 < T < 10^6 K,
which is consistent with the results from hydrodynamic simulations of the
intergalactic medium. The zeroth order image reveals the extended emission from
the surrounding cluster. We have been able to separate the moderate CCD X-ray
spectrum of the surrounding cluster from the central quasar and find that this
is a hot cluster with a temperature of ~10 keV and a metal abundance of ~0.3
Zo. We also independently obtain the redshift of the cluster, which is
consistent with the optical results. We estimate that the cluster makes
negligible contributions to the 6.9 keV iron K line flux.
|
0109389v1
|
2003-02-21
|
The iron $K_α$-line diagnostics of a rotational black hole metric
|
The original idea to show the spacetime geometry using few geodesics was
developed by Johnson and Ruffini (1974). We used this idea to interpret the
observational data for rotating BH's. We developed the imitation approach to
simulate a propagation of radiation near BH's. An important problem for this
approach is the diagnostics of a black hole metric using X-ray observational
data of the iron $K_\alpha$-line.
Observations of Seyfert galaxies in X-ray region reveal the broad emissiion
lines in their spectra, which can arise in inner parts of accretion disks,
where the effects of General Relativity (GR) must be counted. A spectrum of a
solitary emission line (the $K_\alpha$-line of iron, for example) of a hot spot
in Kerr accretion disk is simulated, depending on the radial coordinate $r$ and
the angular momentum $a=J/M$ of a black hole, under the assumption of an
equatorial circular motion of a hot spot. Using results of numerical
simulations it is shown that the characteristic two-peak line profile with the
sharp edges arises at a large distance, (about $r \approx (3-10)r_g$). The
inner regions emit the line, which is observed with one maximum and extremely
broad red wing. High accuracy future spectral observations, being carried out,
could detect the angular momentum $a$ of the black hole. We analyzed the
different parameters of problems on the observable shape of this line and
discussed some possible kinds of these shapes. The total number of geodesics is
about $10^9$ (to simulate possible shapes of the $K_\alpha$-line), so the
number is great enough, especially in comparison with few geodesics in the
original paper by Johnson and Ruffini (1974).
|
0302454v1
|
2003-09-22
|
The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks
|
We systematically examine how the presence in a binary affects the final core
structure of a massive star and its consequences for the subsequent supernova
explosion. Interactions with a companion star may change the final rate of
rotation, the size of the helium core, the strength of carbon burning and the
final iron core mass. Stars with initial masses larger than \sim 11\Ms that
experiece core collapse will generally have smaller iron cores at the time of
the explosion if they lost their envelopes due to a previous binary
interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger
helium and metal cores if they have a close companion, since the second
dredge-up phase which reduces the helium core mass dramatically in single stars
does not occur once the hydrogen envelope is lost. We find that the initially
more massive stars in binary systems with masses in the range 8 - 11\Ms are
likely to undergo an electron-capture supernova, while single stars in the same
mass range would end as ONeMg white dwarfs. We suggest that the core collapse
in an electron-capture supernova (and possibly in the case of relatively small
iron cores) leads to a prompt explosion rather than a delayed neutrino-driven
explosion and that this naturally produces neutron stars with low-velocity
kicks. This leads to a dichotomous distribution of neutron star kicks, as
inferred previously, where neutron stars in relatively close binaries attain
low kick velocities. We illustrate the consequences of such a dichotomous kick
scenario using binary population synthesis simulations and discuss its
implications. This scenario has also important consequences for the minimum
initial mass of a massive star that becomes a neutron star. (Abbreviated.)
|
0309588v2
|
2003-09-25
|
Close and distant reprocessing media in Mkn509 studied with BeppoSAX
|
We present the broad band analysis of two BeppoSAX observations of the Sy 1
Mkn509. In 2000 the source was in a typical flux state,
F{2-10keV}=2.7x10^{-11}erg cm^{-2} s^{-1}, while in 1998 it was found in a high
flux state, F{2-10keV}=5.7x10^{-11} erg cm^{-2} s^{-1}. A comparison between
the two states shows a energy-dependent flux variation. A soft excess, a narrow
iron line and a Compton reflection hump above 10 keV, are clearly apparent in
the residuals after fitting the spectra with a simple power law. We tested two
alternative models. In the first the iron line and the high energy bump are
well reproduced by reprocessing in a cold and Compton thick material. The
intensity of the iron line (also consistent with a Chandra measurement) as well
as the normalization of the reflection hump are consistent with a constant in
the two epochs: this, combined with the fact that the line is narrow as
observed by Chandra, suggests a common origin from distant and optically thick
matter. This model further requires a component to model the soft excess: the
empirical choice of two black bodies accounts well for the excess in both
observations, their combined strength was a factor about three higher in the
high than in the low flux state defined above. However the relative
contribution of the soft excess is higher in the low flux state. In the second
model we attempted to reproduce all spectral features, except for the narrow
cold line, with reflection from an ionized disc. This model is successful only
in the high flux state,but it fails in the low flux state, when the soft excess
is only partially accounted for. In either models, the slope of the power law
is larger in the high than in the low flux state, in agreement with a behaviour
known to be shared by several objects of the same type.
|
0309699v1
|
2003-10-20
|
Chandra X-ray Observations of NGC 4258: Iron Absorption Lines from the Nucleus
|
We report sub-arcsecond resolution X-ray imaging spectroscopy of the low
luminosity active galactic nucleus of NGC 4258 and its immediate surroundings
with the Chandra X-ray Observatory. NGC 4258 was observed four times, with the
first two observations separated by one month, followed over a year later by
two consecutive observations. The spectrum of the nucleus is well described by
a heavily absorbed, hard X-ray power law of variable luminosity, plus a
constant, thermal soft X-ray component. We do not detect an iron K alpha
emission line with the upper limit to the equivalent width of a narrow, neutral
iron line ranging between 94 and 887 eV (90% confidence) for the different
observations. During the second observation on 2000-04-17, two narrow
absorption features are seen with >99.5% confidence at ~6.4 keV and ~6.9 keV,
which we identify as resonant absorption lines of Fe XVIII - Fe XIX K alpha and
Fe XXVI K alpha, respectively. In addition, the 6.9 keV absorption line is
probably variable on a timescale of ~6000 sec. The absorption lines are
analyzed through a curve of growth analysis, which allows the relationship
between ionic column and kinematic temperature or velocity dispersion to be
obtained for the observed equivalent widths. We discuss the properties of the
absorbing gas for both photo and collisionally ionized models. Given that the
maser disk is viewed at an inclination i = 82 degrees, the gas responsible for
the 6.9 keV absorption line may be in an inner disk, a disk-wind boundary layer
or be thermal gas entrained at the base of the jet. The gas which gives rise to
the photoelectric absorption may be the same as that which causes the 6.4 keV
Fe K alpha absorption provided that the gas has a bulk velocity dispersion of a
few thousand km/s. (Abstract abridged).
|
0310553v1
|
2004-07-22
|
XMM-Newton observation of the Seyfert 1.8 ESO 113-G010: discovery of a highly redshifted iron line at 5.4 keV
|
We present a spectral analysis of the Seyfert 1.8 ESO 113-G010 observed with
XMM-Newton for 4 ks. The spectrum shows a soft excess below 0.7 keV and more
interestingly a narrow emission Gaussian line at 5.4 keV (in its rest-frame),
most probably originating from a redshifted iron Kalpha line. No significant
line at or above 6.4 keV is found contrary to other objects showing redshifted
lines, ruling out a strong blue-wing to the line profile. The line is detected
at 99% confidence, from performing Monte Carlo simulations which fully account
for the range of energies where a narrow iron line is likely to occur. The
energy of the line could indicate emission from relativistic (0.17 - 0.23 c)
ejected matter moving away from the observer, as proposed for Mrk 766 by Turner
et al. (2004). Alternatively, the emission from a narrow annulus at the surface
of the accretion disk is unlikely due to the very small inclination angle (i.e.
less than 10 deg) required to explain the narrow, redshifted line in this
intermediate Seyfert galaxy. However emission from a small, localized hot-spot
on the disk, occurring within a fraction of a complete disk orbit, could also
explain the redshifted line. This scenario would be directly testable in a
longer observation, as one would see significant variations in the energy and
intensity of the line within an orbital timescale.
|
0407472v2
|
2004-08-12
|
The Li Overabundance of J37: Diffusion or Accretion?
|
In September 2002 the discovery of a super Li-rich F-dwarf (J37) in NGC 6633,
an iron poor analogue of the better studied Hyades and Praecepe open clusters,
was announced. This unique star was thought to be the smoking gun for the
action of diffusion, models of which predict a narrow "Li-peak" at
approximately the correct temperature. However, with more detailed studies into
J37s abundance pattern this star provides firm evidence for the accretion of
planetesimals or other material from the circumstellar environment of new born
stars.
Thanks to the specific predictions made about the behaviour of Be abundances,
(the most striking of which being no Be in super-Li-rich dwarfs subject to
diffusion) the opposing diffusion/accretion predictions can be tested.
Initial modelling of the Be line indicates that J37 is as Be rich as it is Li
rich; log N(Be) = 2.25 +/- 0.25, and so is broadly consistent with an
accretion-fuelled enhancement. However, that both Li and Be are enhanced by
much more than the iron-peak elements (as determined in previous studies)
suggests that diffusion also plays a role in increasing the abundances of Li
and Be specifically.
Furthermore, a new data set from the UVES/UT2 combination has allowed the
elemental abundance of Iron to be measured, and the set of preliminary stellar
parameters determined; Teff ~ 7340 K, log g ~ 4.1, microturbulence ~ 4.3 km/s,
[Fe/H] ~ 0.50. This again provides distinct evidence for the effects of
accretion in J37 and requires a new synthesis of the Be doublet.
|
0408223v1
|
2004-09-29
|
Chemical Abundances for Seven Giant Stars in M68 (NGC 4590) : A Globular Cluster with Abnormal Silicon and Titanium Abundances
|
We present a detailed chemical abundance study of seven giant stars in M68
including six red giants and one post-AGB star. We find significant differences
in the gravities determined using photometry and those obtained from ionization
balance, which suggests that non-LTE effects are important for these
low-gravity, metal-poor stars. We adopt an iron abundance using photometric
gravities and Fe II lines to minimize those effects, finding [Fe/H] = -2.16 +/-
0.02. For element-to-iron ratios,we rely on neutral lines vs. Fe I and ionized
lines vs. FeII (except for [O/Fe]) to also minimize non-LTE effects.
We find variations in the abundances of sodium among the program stars.
However, there is no correlation (or anti-correlation) with the oxygen
abundances. Further, the post-AGB star has a normal (low) abundance of sodium.
Both of these facts add further support to the idea that the variations seen
among some light elements within individual globular clusters arises from
primordial variations, and not from deep mixing.
M68, like M15, shows elevated abundances of silicon compared to other
globular clusters and comparable metallicity field stars. But M68 deviates even
more in showing a relative underabundance of titanium. We speculate that in
M68, titanium is behaving like an iron-peak element rather than its more
commonly observed adherence to enhancements seen in the "alpha" elements such
as magnesium, silicon, and calcium. We interpret this result as implying that
the chemical enrichment seen in M68 may have arisen from contributions from
supernovae with somewhat more massive progenitors than contribute normally to
abundances seen in other globular clusters.
|
0409706v1
|
2005-04-14
|
Three-dimensional modeling of Type Ia supernovae - The power of late time spectra
|
Late time synthetic spectra of Type Ia supernovae, based on three-dimensional
deflagration models, are presented. We mainly focus on one
model,"c3_3d_256_10s", for which the hydrodynamics (Roepke 2005) and
nucleosynthesis (Travaglio et al. 2004) was calculated up to the homologous
phase of the explosion. Other models with different ignition conditions and
different resolution are also briefly discussed. The synthetic spectra are
compared to observed late time spectra. We find that while the model spectra
after 300 to 500 days show a good agreement with the observed Fe II-III
features, they also show too strong O I and C I lines compared to the observed
late time spectra. The oxygen and carbon emission originates from the
low-velocity unburned material in the central regions of these models. To get
agreement between the models and observations we find that only a small mass of
unburned material may be left in the center after the explosion. This may be a
problem for pure deflagration models, although improved initial conditions, as
well as higher resolution decrease the discrepancy. The relative intensity from
the different ionization stages of iron is sensitive to the density of the
emitting iron-rich material. We find that clumping, with the presence of low
density regions, is needed to reproduce the observed iron emission, especially
in the range between 4000 and 6000 AA. Both temperature and ionization depend
sensitively on density, abundances and radioactive content. This work therefore
illustrates the importance of including the inhomogeneous nature of realistic
three-dimensional explosion models. We briefly discuss the implications of the
spectral modeling for the nature of the explosion.
|
0504317v1
|
2006-02-25
|
Probing Multiple Sight Lines through the SN 1006 Remnant by UV Absorption Spectroscopy
|
Absorption-line spectroscopy is an effective probe for cold ejecta within an
SNR, provided that suitable background UV sources can be identified. For the SN
1006 remnant we have identified four such sources, in addition to the
much-studied Schweitzer-Middleditch (SM) star. We have used STIS on HST to
obtain UV spectra of all four sources, to study "core samples" of the SN 1006
interior. The line of sight closest to the center of the SNR shell, passing
only 2.0 arcmin away, is to a V = 19.5 QSO at z = 1.026. Its spectrum shows
broad Fe II absorption lines, asymmetric with red wings broader than blue. The
similarity of these profiles to those seen in the SM star, which is 2.8 arcmin
from the center in the opposite direction, confirms the existence of a bulge on
the far side of SN 1006. The Fe II equivalent widths in the QSO spectrum are ~
50% greater than in the SM star, suggesting that somewhat more iron may be
present within SN 1006 than studies of the SM star alone have indicated, but
this is still far short of what most SNIa models require. The absorption
spectrum against a brighter z = 0.337 QSO seen at 57% of the shell radius shows
broad silicon absorption lines but no iron other than narrow, probably
interstellar lines. The cold iron expanding in this direction must be confined
within v <~ 5200 km/s, also consistent with a high-velocity bulge on the far
side only. The broad silicon lines indicate that the silicon layer has expanded
beyond this point, and that it has probably been heated by a reverse shock.
Finally, the spectra of two ~ A0V stars near the southern shell rim show no
broad or unusually strong absorption lines, suggesting that the low-ionization
ejecta are confined within 83% of the shell radius, at least at the azimuths of
these background sources.
|
0602555v1
|
2006-05-15
|
Accretion processes in AGN: The X-ray View
|
We discuss constraints on the physics of the inner accretion disk, as well as
the properties of the black hole itself, that can be derived by a detailed
examination of the relativistically broadened spectral features (especially the
fluorescent iron line) in the Seyfert galaxy MCG-6-30-15. To begin with, we
show that spectral models which purport to eliminate the broad iron line in
MCG-6-30-15 by invoking a moderately high ionization absorber are ruled out by
recent high-resolution spectra from the Chandra High Energy Transmission
Gratings. We then discuss the comparison of XMM-Newton data with accretion disk
models. The "standard'' black hole disk model of Novikov, Page and Thorne
supplemented by the so-called local corona assumption fails to produce
sufficient broadening; this indicates that the real accretion disk in
MCG-6-30-15 has significantly more centrally concentrated pattern of X-ray
irradiation that predicted by this model. We discuss two possible resolutions.
Firstly, the inner disk may be energized from torques imposed by magnetic
connections between the disk-proper and either the plunging region or the
rotating event horizon itself. Secondly, X-ray emission from a high-latitude
source (such as would be the case of the X-ray source is actually the base of a
jet) would be gravitationally focused onto the central portions of the disk. We
discuss how spectral variability may be used to examine these possibilities and
highlight the still outstanding mystery concerning the anti-correlation between
the iron line equivalent width and relative normalization of the Compton
reflection hump. We end with a few words about the exciting future of these
studies in the future era of Constellation-X and LISA.
|
0605368v1
|
2006-11-09
|
Medium resolution spectroscopy in Omega Centauri: abundances of 400 subgiant and turn-off region
|
Medium resolution spectra of more than 400 subgiant and turn-off region stars
in Omega Centauri were analysed. The observations were performed at the
VLT/Paranal with FORS2/MXU. In order to determine the metallicities of the
sample stars, we defined a set of line indices (mostly iron) adjusted to the
resolution of our spectra. The indices as determined for Omega Centauri were
then compared to line indices from stars in the chemically homogeneous globular
cluster M55, in addition to standard stars and synthetic spectra. The
uncertainties in the derived metallicities are of the order of 0.2 dex. Our
study confirms the large variations in iron abundances found on the giant
branch in earlier studies (-2.2 <[Fe/H]< -0.7 dex). In addition, we studied the
alpha-element and CN/CH abundances. Stars of different metallicity groups not
only show distinct ages, but also different behaviours in their relative
abundances. The alpha abundances increase smoothly with increasing metallicity
resulting in a flat [alpha/Fe] ratio over the whole observed metallicity range.
The combined CN+CH abundance increases smoothly with increasing iron abundance.
The most metal-rich stars are CN-enriched. In a CN vs. CH plot, though, the
individual abundances divide into CN- and CH-rich branches. The large abundance
variations observed in our sample of (unevolved) subgiant branch stars most
probably have their origin in the pre-enriched material rather than in internal
mixing effects. Together with the age spread of the different sub-populations,
our findings favour the formation of Omega Centauri within a more massive
progenitor.
|
0611286v1
|
2006-12-28
|
Probing the Disk-jet Connection of the Radio Galaxy 3C120 Observed with Suzaku
|
Broad line radio galaxies (BLRGs) are a rare type of radio-loud AGN, in which
the broad optical permitted emission lines have been detected in addition to
the extended jet emission. Here we report on deep (40ksec x4) observations of
the bright BLRG 3C~120 using Suzaku. The observations were spaced a week apart,
and sample a range of continuum fluxes. An excellent broadband spectrum was
obtained over two decades of frequency (0.6 to 50 keV) within each 40 ksec
exposure. We clearly resolved the iron K emission line complex, finding that it
consists of a narrow K_a core (sigma ~ 110 eV or an EW of 60 eV), a 6.9 keV
line, and an underlying broad iron line. Our confirmation of the broad line
contrasts with the XMM-Newton observation in 2003, where the broad line was not
required. The most natural interpretation of the broad line is iron K line
emission from a face-on accretion disk which is truncated at ~10 r_g. Above 10
keV, a relatively weak Compton hump was detected (reflection fraction of R ~
0.6), superposed on the primary X-ray continuum of Gamma ~ 1.75. Thanks to the
good photon statistics and low background of the Suzaku data, we clearly
confirm the spectral evolution of 3C120, whereby the variability amplitude
decreases with increasing energy. More strikingly, we discovered that the
variability is caused by a steep power-law component of Gamma ~2.7, possibly
related to the non-thermal jet emission. We discuss our findings in the context
of similarities and differences between radio-loud/quiet objects.
|
0612754v1
|
2007-02-16
|
Accretion of chemically fractionated material on a wide binary with a blue straggler
|
The components of the wide binary HIP64030=HD 113984 show a large (about 0.25
dex) iron content difference (Desidera et al.~2006 A&A 454, 581). The positions
of the components on the color magnitude diagram suggest that the primary is a
blue straggler. We studied the abundance difference of several elements besides
iron, and we searched for stellar and substellar companions around the
components to unveil the origin of the observed iron difference. A line-by-line
differential abundance analysis for several elements was performed, while
suitable spectral synthesis was performed for C, N, and Li. High precision
radial velocities obtained with the iodine cell were combined with available
literature data. The analysis of additional elements shows that the abundance
difference for the elements studied increases with increasing condensation
temperature, suggesting that accretion of chemically fractionated material
might have occurred in the system. Alteration of C and N likely due to CNO
processing is also observed. We also show that the primary is a spectroscopic
binary with a period of 445 days and moderate eccentricity. The minimum mass of
the companion is 0.17 Msun. Two scenarios were explored to explain the observed
abundance pattern. In the first, all abundance anomalies arise on the blue
straggler. If this is the case, the dust-gas separation may have been occurred
in a circumbinary disk around the blue straggler and its expected white dwarf
companion, as observed in several RV Tauri and post AGB binaries. In the second
scenario, accretion of dust-rich material occurred on the secondary. This would
also explain the anomalous carbon isotopic ratio of the secondary. Such a
scenario requires that a substantial amount of mass lost by the central binary
has been accreted by the wide component.
|
0702439v1
|
2007-07-17
|
Wide-band spectroscopy of the Compton thick Seyfert 2 galaxy Mrk 3 with Suzaku
|
We obtained a wide-band spectrum of the Compton-thick Seyfert 2 galaxy Mrk 3
with Suzaku. The observed spectrum was clearly resolved into weak, soft
power-law emission, a heavily absorbed power-law component, cold reflection,
and many emission lines. The heavily absorbed component, absorbed by gas with a
column density of 1.1x10^24 cm^-2, has an intrinsic 2--10 keV luminosity of
~1.6x10^43 erg s^-1, and is considered to be direct emission from the Mrk 3
nucleus. The reflection component was interpreted as reflection of the direct
light off cold, thick material; the reflection fraction $R$ was 1.36+/-0.20.
The cold material is inferred to be located > 1 pc from the central black hole
of Mrk 3 due to the low ionization parameter of iron (xi < 1 erg cm s^-1) and
the narrow iron line width (s < 22 eV). A Compton shoulder to the iron line was
detected, but the intensity of the shoulder component was less than that
expected from spherically distributed Compton-thick material. The weak, soft
power-law emission is considered to be scattered light by ionized gas. The
existence of many highly-ionized lines from O, Ne, Mg, Si, S, and Fe in the
observed spectrum indicates that the ionized gas has a broad ionized structure,
with xi=10--1000. The scattering fraction with respect to the direct light was
estimated to be 0.9+/-0.2%, which indicates that the column density of the
scattering region is about 3.6x10^22 cm^-2. This high-quality spectrum obtained
by Suzaku can be considered a template for studies of Seyfert 2 galaxies.
|
0707.2425v1
|
2007-07-19
|
Relativistic Iron K Emission and absorption in the Seyfert 1.9 galaxy MCG-5-23-16
|
We present the results of the simultaneous deep XMM and Chandra observations
of the bright Seyfert 1.9 galaxy MCG-5-23-16, which is thought to have one of
the best known examples of a relativistically broadened iron K-alpha line. The
time averaged spectral analysis shows that the iron K-shell complex is best
modeled with an unresolved narrow emission component (FWHM < 5000 km/s, EW ~ 60
eV) plus a broad component. This latter component has FWHM ~ 44000 km/s and EW
~ 50 eV. Its profile is well described by an emission line originating from an
accretion disk viewed with an inclination angle ~ 40^\circ and with the
emission arising from within a few tens of gravitational radii of the central
black hole. The time-resolved spectral analysis of the XMM EPIC-pn spectrum
shows that both the narrow and broad components of the Fe K emission line
appear to be constant in time within the errors. We detected a narrow sporadic
absorption line at 7.7 keV which appears to be variable on a time-scale of 20
ksec. If associated with Fe XXVI Ly-alpha this absorption is indicative of a
possibly variable, high ionization, high velocity outflow. The variability of
this absorption feature appears to rule out a local (z=0) origin. The analysis
of the XMM RGS spectrum reveals that the soft X-ray emission of MCG-5-23-16 is
likely dominated by several emission lines superimposed on an unabsorbed
scattered power-law continuum. The lack of strong Fe L shell emission together
with the detection of a strong forbidden line in the O VII triplet is
consistent with a scenario where the soft X-ray emission lines are produced in
a plasma photoionized by the nuclear emission.
|
0707.2950v1
|
2007-07-26
|
Different Methods of Forming Cold Fronts in Non-Merging Clusters
|
Sharp edges in X-ray surface brightness with continuous gas pressure called
cold fronts have been often found in relaxed galaxy clusters such as Abell 496.
Models that explain cold fronts as surviving cores of head-on subcluster
mergers do not work well for these clusters and competing models involving gas
sloshing have been recently proposed. Here, we test some concrete predictions
of these models in a combined analysis of density, temperature, metal
abundances and abundance ratios in a deep Chandra exposure of Abell 496. We
confirm that the chemical discontinuities found in this cluster are not
consistent with a core merger remnant scenario. However, we find chemical
gradients across a spiral "arm" discovered at 73 kpc north of the cluster
center and coincident with the sharp edge of the main cold front in the
cluster. Despite the overall SN Ia iron mass fraction dominance found within
the cooling radius of this cluster, the metal enrichment along the arm,
determined from silicon and iron abundances, is consistent with a lower SN Ia
iron mass fraction (51% +- 14%) than that measured in the surrounding regions
(85% +- 14%). The "arm" is also significantly colder than the surroundings by
0.5-1.6 keV. The arm extends from a boxy colder region surrounding the center
of the cluster, where two other cold fronts are found. This cold arm is a
prediction of current high resolution numerical simulations as a result of an
off-center encounter with a less massive pure dark matter halo and we suggest
that the cold fronts in A496 provide the first clear corroboration of such
model, where the closest encounter happened ~ 0.5 Gyr ago. We also argue for a
possible candidate dark matter halo responsible for the cold fronts in the
outskirts of A496.
|
0707.4001v1
|
2007-08-25
|
Modeling the chemical evolution of Omega Centauri using three-dimensional hydrodynamical simulations
|
We present a hydrodynamical and chemical model for the globular cluster Omega
Cen, under the assumption that it is the remnant of an ancient dwarf spheroidal
galaxy (dSph), the bulk of which was disrupted and accreted by our Galaxy ~10
Gyr ago. We highlight the very different roles played by Type II and Type Ia
supernovae (SNe) in the chemical enrichment of the inner regions of the
putative parent dSph. While the SNe II pollute the interstellar medium rather
uniformly, the SNe Ia ejecta may remain confined inside dense pockets of gas as
long as succesive SNe II explosions spread them out. Stars forming in such
pockets have lower alpha-to-iron ratios than the stars forming elsewhere. Owing
to the inhomogeneous pollution by SNe Ia, the metal distribution of the stars
in the central region differs substantially from that of the main population of
the dwarf galaxy, and resembles that observed in Omega Cen. This inhomogeneous
mixing is also responsible for a radial segregation of iron-rich stars with
depleted [alpha/Fe] ratios, as observed in some dSphs. Assuming a star
formation history of ~1.5 Gyr, our model succeeds in reproducing both the iron
and calcium distributions observed in Omega Cen and the main features observed
in the empirical alpha/Fe versus Fe/H plane. Finally, our model reproduces the
overall spread of the color-magnitude diagram, but fails in reproducing the
morphology of the SGB-a and the double morphology of the main sequence.
However, the inhomogeneous pollution reduces (but does not eliminate) the need
for a significantly enhanced helium abundance to explain the anomalous position
of the blue main sequence. Further models taking into account the dynamical
interaction of the parent dwarf galaxy with the Milky Way and the effect of AGB
pollution will be required.
|
0708.3445v1
|
2007-09-10
|
Iron lines from transient and persisting hot spots on AGN accretion disks
|
[abridged] We model the X-ray reprocessing from a strong co-rotating flare
above an accretion disk in active galactic nuclei. We explore the horizontal
structure and evolution of the underlying hot spot. To obtain the spectral
evolution seen by a distant observer, we apply a general relativity ray-tracing
technique. We concentrate on the energy band around the iron K-line, where the
relativistic effects are most pronounced. Persistent flares lasting for a
significant fraction of the orbital time scale and short, transient flares are
considered. In our time-resolved analysis, the spectra recorded by a distant
observer depend on the position of the flare/spot with respect to the central
black hole. If the flare duration significantly exceeds the light travel time
across the spot, then the spot horizontal stratification is unimportant. On the
other hand, if the flare duration is comparable to the light travel time across
the spot radius, the lightcurves exhibit a typical asymmetry in their time
profiles. The sequence of dynamical spectra proceeds from more strongly to less
strongly ionized re-emission. At all locations within the spot the spectral
intensity increases towards edge-on emission angles, revealing the limb
brightening effect. Future X-ray observatories with significantly larger
effective collecting areas will enable to spectroscopically map out the
azimuthal irradiation structure of the accretion disk and to localize
persistent flares. If the hot spot is not located too close to the marginally
stable orbit of the black hole, it will be possible to probe the reflecting
medium via the sub-structure of the iron K-line. Indications for transient
flares will only be obtained from analyzing the observed lightcurves on the
gravitational time scale of the accreting supermassive black hole.
|
0709.1356v1
|
2007-10-31
|
The Proto-neutron Star Phase of the Collapsar Model and the Route to Long-soft Gamma-ray Bursts and Hypernovae
|
Recent stellar evolutionary calculations of low-metallicity massive
fast-rotating main-sequence stars yield iron cores at collapse endowed with
high angular momentum. It is thought that high angular momentum and black hole
formation are critical ingredients of the collapsar model of long-soft
gamma-ray bursts (GRBs). Here, we present 2D multi-group,
flux-limited-diffusion MHD simulations of the collapse, bounce, and immediate
post-bounce phases of a 35-Msun collapsar-candidate model of Woosley & Heger.
We find that, provided the magneto-rotational instability (MRI) operates in the
differentially-rotating surface layers of the millisecond-period neutron star,
a magnetically-driven explosion ensues during the proto-neutron star phase, in
the form of a baryon-loaded non-relativistic jet, and that a black hole,
central to the collapsar model, does not form. Paradoxically, and although much
uncertainty surrounds stellar mass loss, angular momentum transport, magnetic
fields, and the MRI, current models of chemically homogeneous evolution at low
metallicity yield massive stars with iron cores that may have too much angular
momentum to avoid a magnetically-driven, hypernova-like, explosion in the
immediate post-bounce phase. We surmise that fast rotation in the iron core may
inhibit, rather than enable, collapsar formation, which requires a large
angular momentum not in the core but above it. Variations in the angular
momentum distribution of massive stars at core collapse might explain both the
diversity of Type Ic supernovae/hypernovae and their possible association with
a GRB. A corollary might be that, rather than the progenitor mass, the angular
momentum distribution, through its effect on magnetic field amplification,
distinguishes these outcomes.
|
0710.5789v2
|
2008-02-14
|
Complexity on Small Scales III: Iron and alpha Element Abundances in the Carina Dwarf Spheroidal Galaxy
|
We have obtained high-resolution spectroscopy of ten red giants in the Carina
dwarf spheroidal (dSph) with UVES at the ESO/VLT. Here we present the
abundances of O,Na,Mg,Si,Ca,Ti and Fe. By comparing the iron abundances [Fe/H]
with calcium triplet (CaT) metallicities we show that the empirical CaT
technique yields good agreement with the high-resolution data for [Fe/H]>-2
dex, but tends to deviate at lower metallicities. We identify two metal poor
stars with iron abundances of -2.72 and -2.50 dex. These stars are found to
have enhanced [alpha/Fe] ratios similar to those of stars in the Milky Way
halo. However, the bulk of the Carina red giants are depleted in the [alpha/Fe]
abundance ratios with respect to the Galactic halo at a given metallicity. One
of our targets, with a [Fe/H] of -1.5 dex, is considerably depleted in almost
all of the alpha-elements by ~0.5 dex compared to the solar values. Such a low
[alpha/Fe] can be produced by stochastical fluctuations in terms of an
incomplete mixing of single Type Ia and II SNe events into the ISM. Our derived
element ratios are consistent with the episodic and extended SF in Carina known
from its color-magnitude diagram. We find a considerable star-to-star scatter
in the abundance ratios. This suggests that Carina's SF history varies with
position within the galaxy, with incomplete mixing. Alternatively, the SF rate
is so low that the high-mass stellar IMF is sparsely populated, as
statistically expected in low-mass star clusters, leading to real scatter in
the resultant mass-integrated yields. Both ideas are consistent with slow
stochastic SF in dissolving associations, so that one may not speak of a single
SF history at a detailed level (Abridged).
|
0802.2104v1
|
2008-03-04
|
A large population of recently-quenched red-sequence dwarf galaxies in the outskirts of the Coma Cluster
|
We analyse the stellar populations of 75 red-sequence dwarf galaxies in the
Coma cluster, based on high signal-to-noise spectroscopy from the 6.5m MMT. The
sample covers a luminosity range 3-4 magnitudes below M*, in the cluster core
and in a field centred 1 deg to the south-west. We find a strong dependence of
the absorption line strengths with location in the cluster. Galaxies further
from the cluster centre have stronger Balmer lines than inner-field galaxies of
the same luminosity. The magnesium lines are weaker at large radius, while the
iron lines are not correlated with radius. Converting the line strengths into
estimates of stellar age, metallicity and abundance ratios, we find the
gradients are driven by variations in age (>6 sigma significance) and in the
iron abundance Fe/H (~2.7 sigma significance). The light element (Mg, C, N, Ca)
abundances are almost independent of radius. At radius of 0.4-1.3 degree
(~0.3-1.0x the virial radius), dwarf galaxies have ages ~3.8 Gyr on average,
compared to ~6 Gyr near the cluster centre. The outer dwarfs are also ~50% more
iron-enriched, at given luminosity. Our results confirm earlier indications
that the ages of red-sequence galaxies depend on location within clusters, and
in Coma in particular. The exceptionally strong trends found here suggest that
dwarf galaxies are especially susceptible to environmental "quenching", and/or
that the south-west part of Coma is a particularly clear example of recent
quenching in an infalling subcluster.
|
0803.0327v1
|
2008-09-01
|
Abundance profiles in cooling-core clusters: a fossil record of past AGN-driven convection?
|
Central peaks in the iron abundance of intracluster plasma are a common
feature of cooling-core galaxy clusters. These abundance peaks have a much
broader profile than the stars of the central brightest cluster galaxy (BCG),
which produce the excess iron, indicating that metal-enriched plasma is
transported out of the BCG by some process such as turbulent diffusion. At the
same time, cooling-core clusters are likely heated by central active galactic
nuclei (AGNs) by means of jets, cosmic-ray bubbles, and convection. The recent
AGN-driven convection model of Chandran&Rasera predicts the turbulent velocity
profile in a steady-state cluster in which radiative cooling is balanced by
heating from a combination of AGN-driven convection and thermal conduction. We
use the velocity profiles as input into an advection/diffusion model for the
transport of metals in the intracluster medium, taking the iron to be injected
by the BCG. We compare the results of our model to XMM and Chandra observations
of eight clusters. Assuming a constant turbulence level over a cluster's
lifetime, the turbulent velocities in the model can explain the observed
abundance profiles in only five of the eight clusters. However, we go on to
develop an analytic fit of the turbulent velocity profile as a function of the
AGN power. We then deduce for each cluster the average AGN power (during the
past 10 Gyr) required to match the abundance profiles. The required average
values are between 10^43 and 2.10^44 erg/s, while the present AGN powers span a
much larger range from 6.10^41 (Virgo) to 2.10^44 erg/s (Hydra A). Our results
suggest that AGN-driven convection can account for the observed abundance
profiles if the AGN power varies over a cluster's lifetime between Virgo-like
and Hydra-A-like values, with average values in the above-quoted range.
|
0809.0238v1
|
2008-11-04
|
Influence of the rare-earth element on the effects of the structural and magnetic phase transitions in CeFeAsO, PrFeAsO, and NdFeAsO
|
We present results of transport and magnetic properties and heat capacity
measurements on polycrystalline CeFeAsO, PrFeAsO, and NdFeAsO. These materials
undergo structural phase transitions, spin density wave-like magnetic ordering
of small moments on iron, and antiferromagnetic ordering of rare earth moments.
The temperature dependence of the electrical resistivity, Seebeck coefficient,
thermal conductivity, Hall coefficient, and magnetoresistance are reported. The
magnetic behavior of the materials have been investigated using Mossbauer
spectroscopy and magnetization measurements. Transport and magnetic properties
are affected strongly by the structural and magnetic transitions, suggesting
significant changes in the band structure and/or carrier mobilities occur, and
phonon-phonon scattering is reduced upon transformation to the low temperature
structure. Results are compared to recent reports for LaFeAsO, and systematic
variations in properties as the identity of Ln is changed are observed and
discussed. As Ln progresses across the rare-earth series from La to Nd, an
increase in the hole contributions to Seebeck coefficient, and increases in
magnetoresistance and the Hall coefficient are observed in the low temperature
phase. Analysis of hyperfine fields at the iron nuclei determined from
Mossbauer spectra indicates that the moment on Fe in the orthorhombic phase is
nearly independent of the identity of Ln, in apparent contrast to reports of
powder neutron diffraction refinements.
|
0811.0589v3
|
2009-01-29
|
A Galaxy Merger Scenario for the NGC 1550 Galaxy from Metal Distributions in the X-ray Emitting Plasma
|
The elliptical galaxy NGC 1550 at a redshift of $z=0.01239$, identified with
an extended X-ray source RX J0419+0225, was observed with {\it XMM-Newton} for
31 ks. From the X-ray data and archival near infra-red data of Two Micron All
Sky survay, we derive the profiles of components constituting the NGC 1550
system; the gas mass, total mass, metal mass, and galaxy luminosity. The metals
(oxygen, silicon, and iron) are extended to $\sim 200$ kpc from the center,
wherein $\sim$ 70% of the $K$-band luminosity is carried by NGC 1550 itself. As
first revealed with {\it ASCA}, the data reconfirms the presence of a dark
halo, of which the mass ($1.6 \times 10^{13} M_{\odot}$) is typical of a galaxy
group rather than of a single galaxy. Within 210 kpc, the $K$-band
mass-to-light ratio reaches $75 M_{\odot}/L_{\odot}$, which is comparable to
those of clusters of galaxies. The iron-mass-to-light ratio profile (silicon-
and oxygen mass-to-light ratio profiles as well) exhibits about two orders of
magnitude decrease toward the center. Further studies comparing mass densities
of metals with those of the other cluster components reveal that the iron (as
well as silicon) in the ICM traces very well the total gravitating mass,
whereas the stellar component is significantly more concentrated to within
several tens kpc of the NGC 1550 nucleus. Thus, in the central region, the
amount of metals is significantly depleted for the luminous galaxy light. Among
a few possible explanations of this effect, the most likely scenario is that
galaxies in this system were initially much more extended than today, and
gradually fell to the center and merged into NGC 1550.
|
0901.4626v1
|
2009-02-02
|
Type-Ia Supernova-driven Galactic Bulge Wind
|
Stellar feedback in galactic bulges plays an essential role in shaping the
evolution of galaxies. To quantify this role and facilitate comparisons with
X-ray observations, we conduct 3D hydrodynamical simulations with the adaptive
mesh refinement code, FLASH, to investigate the physical properties of hot gas
inside a galactic bulge, similar to that of our Galaxy or M31. We assume that
the dynamical and thermal properties of the hot gas are dominated by mechanical
energy input from SNe, primarily Type Ia, and mass injection from evolved stars
as well as iron enrichment from SNe. We study the bulge-wide outflow as well as
the SN heating on scales down to ~4 pc. An embedding scheme that is devised to
plant individual SNR seeds, allows to examine, for the first time, the effect
of sporadic SNe on the density, temperature, and iron ejecta distribution of
the hot gas as well as the resultant X-ray morphology and spectrum. We find
that the SNe produce a bulge wind with highly filamentary density structures
and patchy ejecta. Compared with a 1D spherical wind model, the non-uniformity
of simulated gas density, temperature, and metallicity substantially alters the
spectral shape and increases the diffuse X-ray luminosity. The differential
emission measure as a function of temperature of the simulated gas exhibits a
log-normal distribution, with a peak value much lower than that of the
corresponding 1D model. The bulk of the X-ray emission comes from the
relatively low temperature and low abundance gas shells associated with SN
blastwaves. SN ejecta are not well mixed with the ambient medium, at least in
the bulge region. These results, at least partly, account for the apparent lack
of evidence for iron enrichment in the soft X-ray-emitting gas in galactic
bulges and intermediate-mass elliptical galaxies.[...]
|
0902.0386v1
|
2009-02-10
|
A Study of the Accuracy of Mass-Radius Relationships for Silicate-Rich and Ice-Rich Planets up to 100 Earth Masses
|
A mass-radius relationship is proposed for solid planets and solid cores
ranging from 1 to 100 Earth-mass planets. It relies on the assumption that
solid spheres are composed of iron and silicates, around which a variable
amount of water is added. The M-R law has been set up assuming that the
planetary composition is similar to the averaged composition for silicates and
iron obtained from the major elements ratio of 94 stars hosting exoplanets.
Except on Earth for which a tremendous amount of data is available, the
composition of silicate mantles and metallic cores cannot be constrained.
Similarly, thermal profiles are poorly known. In this work, the effect of
compositional parameters and thermal profiles on radii estimates is quantified.
It will be demonstrated that uncertainties related to composition and
temperature are of second order compared to the effect of the water amount. The
Super-Earths family includes four classes of planets: iron-rich, silicate-rich,
water-rich, or with a thick atmosphere. For a given mass, the planetary radius
increases significantly from the ironrich to the atmospheric-rich planet. Even
if some overlaps are likely, M-R measurements could be accurate enough to
ascertain the discovery of an earth-like planet .The present work describes how
the amount of water can be assessed from M-R measurements. Such an estimate
depends on several assumptions including i) the accuracy of the internal
structure model and ii) the accuracy of mass and radius measurements. It is
shown that if the mass and the radius are perfectly known, the standard
deviation on the amount of water is about 4.5 %. This value increases rapidly
with the radius uncertainty but does not strongly depend on the mass
uncertainty.
|
0902.1640v1
|
2009-02-20
|
Frustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and Magnetic Phonons in Iron-Pnictides
|
We present a detailed first principles study of Fe-pnictides with particular
emphasis on competing magnetic interactions, structural phase transition, giant
magneto-elastic coupling and its effect on phonons. The exchange interactions
$J_{i,j}(R)$ are calculated up to $\approx 12 $\AA $. We find that $J_{i,j}(R)$
has an oscillatory character with an envelop decaying as $1/R^3$ along the
stripe-direction while it is very short range along the diagonal direction and
antiferromagnetic. A brief discussion of the neutron scattering determination
of these exchange constants from a single crystal sample with orthorhombic
twinning is given. The lattice parameter dependence of the exchange constants,
$dJ_{i,j}/da$ are calculated for a simple spin-Peierls like model to explain
the fine details of the tetragonal-orthorhombic phase transition. We then
discuss giant magneto-elastic effects in these systems. We show that when the
Fe-spin is turned off the optimized c-values are shorter than experimetnal
values by 1.4 \AA $ $ for CaFe$_2$As$_2$, by 0.4 \AA $ $ for BaFe$_2$As$_2$,
and by 0.13 \AA $ $ for LaOFeAs. Finally, we show that Fe-spin is also required
to obtain the right phonon energies, in particular As c-polarized and Fe-Fe
in-plane modes. Since treating iron as magnetic ion always gives much better
results than non-magnetic ones and since there is no large c-axis reduction
during the normal to superconducting phase transition, the iron magnetic moment
should be present in Fe-pnictides at all times. We discuss the implications of
our results on the mechanism of superconductivity in these fascinating
Fe-pnictide systems.
|
0902.3462v1
|
2009-03-20
|
Spectral Analysis of the Accretion Flow in NGC 1052 with Suzaku
|
We present an analysis of the 101 ks, 2007 Suzaku spectrum of the LINER
galaxy NGC 1052. The 0.5-10 keV continuum is well-modeled by a power-law
modified by Galactic and intrinsic absorption, and it exhibits a soft, thermal
emission component below 1 keV. Both a narrow core and a broader component of
Fe K emission centered at 6.4 keV are robustly detected. While the narrow line
is consistent with an origin in material distant from the black hole, the broad
line is best fit empirically by a model that describes fluorescent emission
from the inner accretion disk around a rapidly rotating black hole. We find no
evidence in this observation for Comptonized reflection of the hard X-ray
source by the disk above 10 keV, however, which casts doubt on the hypothesis
that the broad iron line originates in the inner regions of a standard
accretion disk. We explore other possible scenarios for producing this spectral
feature and conclude that the high equivalent width (EW ~ 185 keV) and
full-width-half-maximum velocity of the broad iron line (v ~ 0.37c) necessitate
an origin within d ~ 8 gravitational radii of the hard X-ray source. Based on
the confirmed presence of a strong radio jet in this galaxy nucleus, the broad
iron line may be produced in dense plasma near the base of the jet, implying
that emission mechanisms in the centralmost portions of active galactic nuclei
are more complex than previously thought.
|
0903.3583v1
|
2009-04-06
|
New insight into the physics of iron pnictides from optical and penetration depth data
|
We report theoretical values for the unscreened plasma frequencies Omega_p of
several Fe pnictides obtained from DFT based calculations within the LDA and
compare them with experimental plasma frequencies obtained from reflectivity
data. The sizable renormalization observed for all considered compounds points
to the presence of many-body effects beyond the LDA. From the large empirical
background dielectric constant of about 12-15, we estimate a large arsenic
polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the
polarizabilities of the remaining ions taken from the literature. This large
polarizability can significantly reduce the value of the Coulomb repulsion U_d
about 4 eV on iron known from iron oxides to a level of 2 eV or below. In
general, this result points to rather strong polaronic effects as suggested by
G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et
al.). Possible consequences for the conditions of a formation of bipolarons are
discussed, too. From the extrapolated muon spin rotation penetration depth data
at T= 0 and the experimental Omega_p we estimate the total coupling constant
lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a
single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong
coupling regime and a quasi-clean limit behaviour are found. For a pronounced
multiband case we obtain a constraint for various intraband coupling constants
which in principle allows for a sizable strong coupling in bands with either
slow electrons or holes.
|
0904.0827v1
|
2009-04-07
|
Search for associations containing young stars (SACY):II. Chemical abundances of stars in 11 young Associations in the Solar neighborhood
|
The recently discovered coeval, moving groups of young stellar objects in the
solar neighborhood represent invaluable laboratories to study recent star
formation and to search for high metallicity stars which can be included in
future exo-planet surveys. In this study we derived through an uniform and
homogeneous method stellar atmospheric parameters and abundances for iron,
silicium and nickel in 63 Post T-Tauri Stars from 11 nearby young associations.
We further compare the results with two different pre-main sequence (PMS) and
main sequence (MS) star populations. The stellar atmospheric parameters and the
abundances presented here were derived using the equivalent width of individual
lines in the stellar spectra through the excitation/ionization equilibrium of
iron. Moreover, we compared the observed Balmer lines with synthetic profiles
calculated for model atmospheres with a different line formation code. We found
that the synthetic profiles agree reasonably well with the observed profiles,
although the Balmer lines of many stars are substantially filled-in, probably
by chromospheric emission. Solar metallicity is found to be a common trend on
all the nearby young associations studied. The low abundance dispersion within
each association strengthens the idea that the origin of these nearby young
associations is related to the nearby Star Forming regions (SFR). Abundances of
elements other than iron are consistent with previous results on Main Sequence
stars in the solar neighborhood. The chemical characterization of the members
of the newly found nearby young associations, performed in this study and
intended to proceed in subsequent works, is essential for understanding and
testing the context of local star formation and the evolutionary history of the
galaxy.
|
0904.1221v1
|
2009-06-17
|
On the metallicity gradient of the Galactic disk
|
Aims: The iron abundance gradient in the Galactic stellar disk provides
fundamental constraints on the chemical evolution of this important Galaxy
component. However the spread around the mean slope is, at fixed Galactocentric
distance, larger than estimated uncertainties. Methods: To provide quantitative
constraints on these trends we adopted iron abundances for 265 classical
Cepheids (more than 50% of the currently known sample) based either on
high-resolution spectra or on photometric metallicity indices. Homogeneous
distances were estimated using near-infrared Period-Luminosity relations. The
sample covers the four disk quadrants and their Galactocentric distances range
from ~5 to ~17 kpc. Results: A linear regression over the entire sample
provides an iron gradient of -0.051+/-0.004 dex/kpc. The above slope agrees
quite well, within the errors, with previous estimates based either on Cepheids
or on open clusters covering similar Galactocentric distances. However, once we
split the sample in inner (Rg < 8 kpc) and outer disk Cepheids we found that
the slope (-0.130+/-0.015 dex/kpc) in the former region is ~3 times steeper
than the slope in the latter one (-0.042+/-0.004 dex/kpc). We found that in the
outer disk the radial distribution of metal-poor (MP, [Fe/H]<-0.02 dex) and
metal-rich (MR) Cepheids across the four disk quadrants does not show a clear
trend when moving from the innermost to the external disk regions. We also
found that the relative fractions of MP and MR Cepheids in the 1st and in the
3rd quadrant differ at 8 sigma (MP) and 15 sigma (MR) level.
|
0906.3140v1
|
2009-10-09
|
Critical change in the Fermi surface of iron arsenic superconductors at the onset of superconductivity
|
The phase diagram of a correlated material is the result of a complex
interplay between several degrees of freedom, providing a map of the material's
behavior. One can understand (and ultimately control) the material's ground
state by associating features and regions of the phase diagram, with specific
physical events or underlying quantum mechanical properties. The phase diagram
of the newly discovered iron arsenic high temperature superconductors is
particularly rich and interesting. In the AE(Fe1-xTx)2As2 class (AE being Ca,
Sr, Ba, T being transition metals), the simultaneous structural/magnetic phase
transition that occurs at elevated temperature in the undoped material, splits
and is suppressed by carrier doping, the suppression being complete around
optimal doping. A dome of superconductivity exists with apparent equal ease in
the orthorhombic / antiferromagnetic (AFM) state as well as in the tetragonal
state with no long range magnetic order. The question then is what determines
the critical doping at which superconductivity emerges, if the AFM order is
fully suppressed only at higher doping values. Here we report evidence from
angle resolved photoemission spectroscopy (ARPES) that critical changes in the
Fermi surface (FS) occur at the doping level that marks the onset of
superconductivity. The presence of the AFM order leads to a reconstruction of
the electronic structure, most significantly the appearance of the small hole
pockets at the Fermi level. These hole pockets vanish, i. e. undergo a Lifshitz
transition, at the onset of superconductivity. Superconductivity and magnetism
are competing states in the iron arsenic superconductors. In the presence of
the hole pockets superconductivity is fully suppressed, while in their absence
the two states can coexist.
|
0910.1799v2
|
2009-11-06
|
Accurate Fundamental Parameters or A, F, and G-type Supergiants in the Solar Neighbourhood
|
The following parameters are determined for 63 Galactic supergiants in the
solar neighbourhood: effective temperature Teff, surface gravity log g, iron
abundance log e(Fe), microturbulent parameter Vt, mass M/Msun, age t and
distance d. A significant improvement in the accuracy of the determination of
log g and, all parameters dependent on it, is obtained through application of
van Leeuwens (2007) re-reduction of the Hipparcos parallaxes. The typical error
in the log g values is now +-0.06 dex for supergiants with distances d < 300 pc
and +-0.12 dex for supergiants with d between 300 and 700 pc; the mean error in
Teff for these stars is +-120 K. For supergiants with d > 700 pc parallaxes are
uncertain or unmeasurable, so typical errors in their log g values are 0.2-0.3
dex.
A new Teff scale for A5-G5 stars of luminosity classes Ib-II is presented.
Spectral subtypes and luminosity classes of several stars are corrected.
Combining the Teff and log g with evolutionary tracks, stellar masses and ages
are determined; a majority of the sample has masses between 4 Msun and 15 Msun
and, hence, their progenitors were early to middle B-type main sequence stars.
Using Fe ii lines, which are insensitive to departures from LTE, the
microturbulent parameter Vt and the iron abundance log e(Fe) are determined
from high-resolution spectra. The parameter Vt is correlated with gravity: Vt
increases with decreasing log g. The mean iron abundance for the 48 supergiants
with distances d < 700 pc is log e(Fe)=7.48+-0.09, a value close to the solar
value of 7.45+-0.05, and thus the local supergiants and the Sun have the same
metallicity.
|
0911.1335v2
|
2009-12-09
|
Empirically Derived Integrated Stellar Yields of Fe-Peak Elements
|
We present here the initial results of a new study of massive star yields of
Fe-peak elements. We have compiled from the literature a database of carefully
determined solar neighborhood stellar abundances of seven iron-peak elements,
Ti, V, Cr, Mn, Fe, Co, and Ni and then plotted [X/Fe] versus [Fe/H] to study
the trends as functions of metallicity. Chemical evolution models were then
employed to force a fit to the observed trends by adjusting the input massive
star metallicity-sensitive yields of Kobayashi et al. Our results suggest that
yields of Ti, V, and Co are generally larger as well as anticorrelated with
metallicity, in contrast to the Kobayashi et al. predictions. We also find the
yields of Cr and Mn to be generally smaller and directly correlated with
metallicity compared to the theoretical results. Our results for Ni are
consistent with theory, although our model suggests that all Ni yields should
be scaled up slightly. The outcome of this exercise is the computation of a set
of integrated yields, i.e., stellar yields weighted by a slightly flattened
time-independent Salpeter initial mass function and integrated over stellar
mass, for each of the above elements at several metallicity points spanned by
the broad range of observations. These results are designed to be used as
empirical constraints on future iron-peak yield predictions by stellar
evolution modelers. Special attention is paid to the interesting behavior of
[Cr/Co] with metallicity -- these two elements have opposite slopes -- as well
as the indirect correlation of [Ti/Fe] with [Fe/H]. These particular trends, as
well as those exhibited by the inferred integrated yields of all iron-peak
elements with metallicity, are discussed in terms of both supernova
nucleosynthesis and atomic physics.
|
0912.1816v1
|
2010-01-03
|
Suzaku Monitoring of the Iron K Emission Line in the Type 1 AGN NGC 5548
|
We present 7 sequential weekly observations of NGC 5548 conducted in 2007
with the \textit{Suzaku} X-ray Imaging Spectrometer (XIS) in the 0.2-12 keV
band and Hard X-ray Detector (HXD) in 10-600 keV band. The iron K$\alpha$ line
is well detected in all seven observations and K$\beta$ line is also detected
in four observations. In this paper, we investigate the origin of the Fe K
lines using both the width of the line and the reverberation mapping method.
With the co-added XIS and HXD spectra, we identify Fe K$\alpha$ and K$\beta$
line at 6.396$_{-0.007}^{+0.009}$ keV and 7.08$_{-0.05}^{+0.05}$ keV,
respectively. The width of line obtained from the co-added spectra is
38$_{-18}^{+16}$ eV ($\textrm{FWHM}=4200_{-2000}^{+1800}$ km/s) which
corresponds to a radius of 20$_{-10}^{+50}$ light days, for the virial
production of $1.220\times10^7$ M$_{\odot}$ in NCG 5548.
To quantitatively investigate the origin of the narrow Fe line by the
reverberation mapping method, we compare the observed light curves of Fe
K$\alpha$ line with the predicted ones, which are obtained by convolving the
continuum light curve with the transfer functions in a thin shell and an
inclined disk. The best-fit result is given by the disk case with $i=30^\circ$
which is better than a fit to a constant flux of the Fe K line at the 92.7%
level (F-test). We find that the emitting radius obtained from the light curve
is 25-37 light days, which is consistent with the radius derived from the Fe K
line width. Combining the results of the line width and variation, the most
likely site for the origin of the narrow iron lines is 20-40 light days away
from the central engine, though other possibilities are not completely ruled
out. (abridged)
|
1001.0356v1
|
2010-01-07
|
Supernova 2007bi as a pair-instability explosion
|
Stars with initial masses 10 M_{solar} < M_{initial} < 100 M_{solar} fuse
progressively heavier elements in their centres, up to inert iron. The core
then gravitationally collapses to a neutron star or a black hole, leading to an
explosion -- an iron-core-collapse supernova (SN). In contrast, extremely
massive stars (M_{initial} > 140 M_{solar}), if such exist, have oxygen cores
which exceed M_{core} = 50 M_{solar}. There, high temperatures are reached at
relatively low densities. Conversion of energetic, pressure-supporting photons
into electron-positron pairs occurs prior to oxygen ignition, and leads to a
violent contraction that triggers a catastrophic nuclear explosion. Tremendous
energies (>~ 10^{52} erg) are released, completely unbinding the star in a
pair-instability SN (PISN), with no compact remnant. Transitional objects with
100 M_{solar} < M_{initial} < 140 M_{solar}, which end up as iron-core-collapse
supernovae following violent mass ejections, perhaps due to short instances of
the pair instability, may have been identified. However, genuine PISNe, perhaps
common in the early Universe, have not been observed to date. Here, we present
our discovery of SN 2007bi, a luminous, slowly evolving supernova located
within a dwarf galaxy (~1% the size of the Milky Way). We measure the exploding
core mass to be likely ~100 M_{solar}, in which case theory unambiguously
predicts a PISN outcome. We show that >3 M_{solar} of radioactive 56Ni were
synthesized, and that our observations are well fit by PISN models. A PISN
explosion in the local Universe indicates that nearby dwarf galaxies probably
host extremely massive stars, above the apparent Galactic limit, perhaps
resulting from star formation processes similar to those that created the first
stars in the Universe.
|
1001.1156v1
|
2010-03-29
|
Ferromagnetism in defect-ridden oxides and related materials
|
The existence of high-temperature ferromagnetism in thin films and
nanoparticles of oxides containing small quantities of magnetic dopants remains
controversial. Some regard these materials as dilute magnetic semiconductors,
while others think they are ferromagnetic only because the magnetic dopants
form secondary ferromagnetic impurity phases such as cobalt metal or magnetite.
There are also reports in d0 systems and other defective oxides that contain no
magnetic ions. Here, we investigate TiO2 (rutile) containing 1 - 5% of iron
cations and find that the room-temperature ferromagnetism of films prepared by
pulsed-laser deposition is not due to magnetic ordering of the iron. The films
are neither dilute magnetic semiconductors nor hosts to an iron-based
ferromagnetic impurity phase. A new model is developed for defect-related
ferromagnetism which involves a spin-split defect band populated by charge
transfer from a proximate charge reservoir in the present case a mixture Fe2+
and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how
inhomogeneous Stoner ferromagnetism depends on the total number of electrons
Ntot, the Stoner exchange integral I and the defect bandwidth W; the band
occupancy is governed by the d-d Coulomb interaction U. There are regions of
ferromagnetic metal, half-metal and insulator as well as nonmagnetic metal and
insulator. A characteristic feature of the high-temperature Stoner magnetism is
an an anhysteretic magnetization curve which is practically temperature
independent below room temperature. This is related to a wandering
ferromagnetic axis which is determined by local dipole fields. The
magnetization is limited by the defect concentration, not by the 3d doping.
Only 1-2 % of the volume of the films is magnetically ordered.
|
1003.5558v1
|
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