133 research outputs found
Non-LTE radiative transfer in cool stars. Theory and applications to the abundance analysis for 24 chemical elements
The interpretation of observed spectra of stars in terms of fundamental
stellar properties is a key problem in astrophysics. For FGK-type stars, the
radiative transfer models are often computed using the assumption of local
thermodynamic equilibrium (LTE). Its validity is often questionable and needs
to be supported by detailed studies, which build upon the consistent framework
of non-LTE. In this review, we outline the theory of non-LTE. The processes
causing departures from LTE are introduced qualitatively by their physical
interpretation, as well as quantitatively by their impact on the models of
stellar spectra and element abundances. We also compile and analyse the most
recent results from the literature. In particular, we examine the non-LTE
effects for 24 chemical elements for six late-studied FGK-type stars.Comment: 19 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
Fundamental stellar parameters and metallicities from Bayesian spectroscopy. Application to low- and high-resolution spectra
We present a unified framework to derive fundamental stellar parameters by
combining all available observational and theoretical information for a star.
The algorithm relies on the method of Bayesian inference, which for the first
time directly integrates the spectroscopic analysis pipeline based on the
global spectrum synthesis and allows for comprehensive and objective error
calculations given the priors. Arbitrary input datasets can be included into
our analysis and other stellar quantities, in addition to stellar age,
effective temperature, surface gravity, and metallicity, can be computed on
demand. We lay out the mathematical framework of the method and apply it to
several observational datasets, including high- and low-resolution spectra
(UVES, NARVAL, HARPS, SDSS/SEGUE). We find that simpler approximations for the
spectroscopic PDF, which are inherent to past Bayesian approaches, lead to
deviations of several standard deviations and unreliable errors on the same
data. By its flexibility and the simultaneous analysis of multiple independent
measurements for a star, it will be ideal to analyse and cross-calibrate the
large ongoing and forthcoming surveys, like Gaia-ESO, SDSS, Gaia and LSST.Comment: 20 pages, 18 figures, 2 tables, accepted for publication in MNRA
NLTE modelling of integrated light spectra. Abundances of barium, magnesium, and manganese in a metal-poor globular cluster
We study the effects of non-local thermodynamic equilibrium (NLTE) on the
abundance analysis of barium, magnesium, and manganese from integrated light
spectroscopy, as typically applied to the analysis of extra-galactic star
clusters and galaxies. In this paper, our reference object is a synthetic
simple stellar population (SSP) representing a mono-metallic alpha-enhanced
globular cluster with the metallicity [Fe/H]=-2.0 and the age of 11 Gyr. We
used the MULTI2.3 program to compute LTE and NLTE equivalent widths of spectral
lines of Mg I, Mn I, and Ba II ions, which are commonly used in abundance
analyses of extra-galactic stellar populations. We used ATLAS12 model
atmospheres for stellar parameters sampled from a model isochrone to represent
individual stars in the model SSP. The NLTE and LTE equivalent widths
calculated for the individual stars were combined to calculate the SSP NLTE
corrections. We find that the NLTE abundance corrections for the integrated
light spectra of the the metal-poor globular cluster are significant in many
cases, and often exceed 0.1 dex. In particular, LTE abundances of Mn are
consistently under-estimated by 0.3 dex for all optical lines of Mn I studied
in this work. On the other hand, Ba II, and Mg I lines show a strong
differential effect: the NLTE abundance corrections for the individual stars
and integrated light spectra are close to zero for the low-excitation lines,
but they amount to -0.15 dex for the strong high-excitation lines. Our results
emphasise the need to take NLTE effects into account in the analysis of spectra
of individual stars and integrated light spectra of stellar populations.Comment: 15 pages, accepted for publication in A&
Bayesian analysis of ages, masses, and distances to cool stars with non-LTE spectroscopic parameters
For studies of Galactic evolution, the accurate characterization of stars in
terms of their evolutionary stage and population membership is of fundamental
importance. A standard approach relies on extracting this information from
stellar evolution models but requires the effective temperature, surface
gravity, and metallicity of a star obtained by independent means. In previous
work, we determined accurate effective temperatures and non-LTE logg and [Fe/H]
(NLTE-Opt) for a large sample of metal-poor stars, -3<[Fe/H]<-0.5, selected
from the RAVE survey. As a continuation of that work, we derive here their
masses, ages, and distances using a Bayesian scheme and GARSTEC stellar tracks.
For comparison, we also use stellar parameters determined from the widely-used
1D LTE excitation-ionization balance of Fe (LTE-Fe). We find that the latter
leads to systematically underestimated stellar ages, by 10-30%, but
overestimated masses and distances. Metal-poor giants suffer from the largest
fractional distance biases of 70%. Furthermore, we compare our results with
those released by the RAVE collaboration for the stars in common (DR3, Zwitter
et al. 2010, Seibert et al. 2011). This reveals -400 to +400 K offsets in
effective temperature, -0.5 to 1.0 dex offsets in surface gravity, and 10 to
70% in distances. The systematic trends strongly resemble the correlation we
find between the NLTE-Opt and LTE-Fe parameters, indicating that the RAVE DR3
data may be affected by the physical limitations of the 1D LTE synthetic
spectra. Our results bear on any study, where spectrophotometric distances
underlie stellar kinematics. In particular, they shed new light on the debated
controversy about the Galactic halo origin raised by the SDSS/SEGUE
observations.Comment: 13 pages and 15 figures. Accepted for publication in MNRA
Observational constraints on the origin of the elements. VII. NLTE analysis of Y II lines in spectra of cool stars and implications for Y as a Galactic chemical clock
Yttrium (Y), a key s-process element, is commonly used in nucleosynthesis
studies and as a Galactic chemical clock when combined with magnesium (Mg). We
study the applicability of the previously assumed LTE line formation assumption
in Y abundance studies of main-sequence and red giant stars, and probe the
impact of NLTE effects on the [Y/Mg] ratio, a proposed stellar age indicator.
We derive stellar parameters, ages, and NLTE abundances of Fe, Mg, and Y for 48
solar analogue stars from high-resolution spectra acquired within the Gaia-ESO
survey. For Y, we present a new NLTE atomic model. We determine a solar NLTE
abundance of A(Y) dex, dex higher than LTE.
NLTE effects on Y abundance are modest for optical Y II lines, which are
frequently used in Sun-like stars diagnostics. NLTE has a small impact on the
[Y/Mg] ratio in such stars. For metal-poor red giants, NLTE effects on Y II
lines are substantial, potentially exceeding dex. For the
Gaia/4MOST/WEAVE benchmark star, HD 122563, we find the NLTE abundance ratio of
[Y/Fe] dex with consistent abundances obtained from
different Y II lines. NLTE has a differential effect on Y abundance diagnostics
in late-type stars. They notably affect Y II lines in red giants and very
metal-poor stars, which are typical Galactic enrichment tracers of
neutron-capture elements. For main-sequence stars, NLTE effects on optical
diagnostic Y II lines remain minimal across metallicities. This affirms the
[Y/Mg] ratio's reliability as a cosmochronometer for Sun-like stars.Comment: 12 pages, 10 figures, accepted by MNRA
The elemental composition of the Sun II. The iron group elements Sc to Ni
We redetermine the abundances of all iron group nuclei in the Sun, based on
neutral and singly-ionised lines of Sc, Ti, V, Mn, Fe, Co and Ni in the solar
spectrum. We employ a realistic 3D hydrodynamic model solar atmosphere,
corrections for departures from local thermodynamic equilibrium (NLTE),
stringent line selection procedures and high quality observational data. We
have scoured the literature for the best quality oscillator strengths,
hyperfine constants and isotopic separations available for our chosen lines. We
find , , ,
, , ,
and . Our uncertainties factor in both statistical
and systematic errors (the latter estimated for possible errors in the model
atmospheres and NLTE line formation). The new abundances are generally in good
agreement with the CI meteoritic abundances but with some notable exceptions.
This analysis constitutes both a full exposition and a slight update of the
preliminary results we presented in Asplund, Grevesse, Sauval & Scott
(arXiv:0909.0948), including full line lists and details of all input data we
employed.Comment: 10 figures, 24 pages + 10 online-only pages of tables. v2. Matches
version accepted by A&
The elemental composition of the Sun III. The heavy elements Cu to Th
We re-evaluate the abundances of the elements in the Sun from copper ()
to thorium (). Our results are mostly based on neutral and singly-ionised
lines in the solar spectrum. We use the latest 3D hydrodynamic solar model
atmosphere, and in a few cases also correct for departures from local
thermodynamic equilibrium (LTE) using non-LTE (NLTE) calculations performed in
1D. In order to minimise statistical and systematic uncertainties, we make
stringent line selections, employ the highest-quality observational data and
carefully assess oscillator strengths, hyperfine constants and isotopic
separations available in the literature, for every line included in our
analysis. Our results are typically in good agreement with the abundances in
the most pristine meteorites, but there are some interesting exceptions. This
analysis constitutes both a full exposition and a slight update of the relevant
parts of the preliminary results we presented in Asplund, Grevesse, Sauval &
Scott (arXiv:0909.0948), including full line lists and details of all input
data that we have employed.Comment: 5 figures, 18 pages + 6 online-only pages of tables. v2. Matches
version accepted by A&
<3D> NLTE line formation in the atmospheres of red supergiants
Red supergiants with their enormous brightness at J-band are ideal probes of
cosmic chemical composition. It is therefore crucial to have realistic models
of radiative transfer in their atmospheres, which will permit determination of
abundances accurate to 0.15 dex, the precision attainable with future telescope
facilities in galaxies as distant as tens of Mpc. Here, we study the effects of
non-local thermodynamic equilibrium (NLTE) on the formation of iron, titanium,
and silicon lines, which dominate J-band spectra of red supergiants. It is
shown that the NLTE radiative transfer models enable accurate derivation of
metallicity and effective temperature in the J-band. We also discuss
consequences for RSG spectrum synthesis in different spectral windows,
including the heavily TiO-blanketed optical region, and atmospheric structure.
We then touch upon challenges of NLTE integration with new generation of 3D
hydrodynamical RSG models and present the first calculations of NLTE spectra
with the mean 3D model of Betelgeuse.Comment: 7 pages, proceedings of the Betelgeuse Workshop, Paris, 201
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