934 research outputs found
The make-up of stars
The chemical composition of stars contain vital clues not only about the stars themselves but also about the conditions prevailing before their births. As such, stellar spectroscopy plays a key role in contemporary astrophysics and cosmology by probing cosmic, galactic, stellar and planetary evolution. In this review I will describe the theoretical foundations of quantitative stellar spectroscopy: stellar atmosphere models and spectral line formation. I will focus mainly on more recent advances in the field, in particular the advent of realistic time-dependent, 3D, (magneto-)hydrodynamical simulations of stellar surface convection and atmospheres and non-LTE radiative transfer relevant for stars like the Sun. I will also discuss some particular applications of this type of modelling which have resulted in some exciting break-throughs in our understanding and with wider implications: the solar chemical composition, the chemical signatures of planet formation imprinted in stellar abundances, the cosmological Li problem(s) and where the first stars may be residing today.financial support from the Organizing Committee of the XVIII School, the Australian
Research Council (e.g. grants FL110100012, DP120100991) and the Australian National University
The stability of late-type stars close to the Eddington limit
Super-Eddington luminosities in hydrostatic model atmospheres manifest
themselves by the presence of gas pressure inversions. Such inversions are not
an artifact of the assumption of hydrostatic equilibrium but can also be
present in hydrodynamical model atmospheres. Only for very large mass loss
rates hardly realized in supergiants will the inversions be removed.
Instabilities may, however, still be present in such inversions, which is
investigated for both H-rich and H-deficient late-type supergiant model
atmospheres. A local, non-adiabatic, linear stability analysis reveals that
sound waves can be amplified due to the strong radiative forces. However,
despite the super-Eddington luminosities, the efficiency of the radiative
instabilities is fairly low compared to for early-type stars with growth rates
of .Comment: 11 pages; accepted for publication in Astronomy & Astrophysic
The light elements in the light of 3D and non-LTE effects
In this review we discuss possible systematic errors inherent in classical 1D
LTE abundance analyses of late-type stars for the light elements (here: H, He,
Li, Be and B). The advent of realistic 3D hydrodynamical model atmospheres and
the availability of non-LTE line formation codes place the stellar analyses on
a much firmer footing and indeed drastically modify the astrophysical
interpretations in many cases, especially at low metallicities. For the
Teff-sensitive hydrogen lines both stellar granulation and non-LTE are likely
important but the combination of the two has not yet been fully explored. A
fortuitous near-cancellation of significant but opposite 3D and non-LTE effects
leaves the derived 7Li abundances largely unaffected but new atomic collisional
data should be taken into account. We also discuss the impact on 3D non-LTE
line formation on the estimated lithium isotopic abundances in halo stars in
light of recent claims that convective line asymmetries can mimic the presence
of 6Li. While Be only have relatively minor non-LTE abundance corrections, B is
sensitive even if the latest calculations imply smaller non-LTE effects than
previously thought.Comment: 10 pages, invited review for IAU Symposium 268 "Light elements in the
Universe", C. Charbonnel, M. Tosi, F. Primas, C. Chiappini (editors
Does SEGUE/SDSS indicate a dual Galactic halo?
We re-examine recent claims of observational evidence for a dual Galactic
halo in SEGUE/SDSS data, and trace them back to improper error treatment and
neglect of selection effects. In particular, the detection of a vertical
abundance gradient in the halo can be explained as a metallicity bias in
distance. A similar bias, and the impact of disk contamination, affect the
sample of blue horizontal branch stars. These examples highlight why non-volume
complete samples require forward-modelling from theoretical models or extensive
bias-corrections. We also show how observational uncertainties produce the
specific non-Gaussianity in the observed azimuthal velocity distribution of
halo stars, which can be erroneously identified as two Gaussian components. A
single kinematic component yields an excellent fit to the observed data, when
we model the measurement process including distance uncertainties. Furthermore,
we show that sample differences in proper motion space are the direct
consequence of kinematic cuts, and are enhanced when distance estimates are
less accurate. Thus, their presence is neither a proof for a separate
population, nor a measure of reliability for the applied distances. We conclude
that currently there is no evidence from SEGUE/SDSS that would favour a dual
Galactic halo over a single halo full of substructure.Comment: 13 pages, 10 figures accepted for publication in Ap
The detection and treatment of distance errors in kinematic analyses of stars
We present a new method for detecting and correcting systematic errors in the
distances to stars when both proper motions and line-of-sight velocities are
available. The method, which is applicable for samples of 200 or more stars
that have a significant extension on the sky, exploits correlations between the
measured U, V and W velocity components that are introduced by distance errors.
We deliver a formalism to describe and interpret the specific imprints of
distance errors including spurious velocity correlations and shifts of mean
motion in a sample. We take into account correlations introduced by measurement
errors, Galactic rotation and changes in the orientation of the velocity
ellipsoid with position in the Galaxy. Tests on pseudodata show that the method
is more robust and sensitive than traditional approaches to this problem. We
investigate approaches to characterising the probability distribution of
distance errors, in addition to the mean distance error, which is the main
theme of the paper. Stars with the most overestimated distances bias our
estimate of the overall distance scale, leading to the corrected distances
being slightly too small. We give a formula that can be used to correct for
this effect. We apply the method to samples of stars from the SEGUE survey,
exploring optimal gravity cuts, sample contamination, and correcting the used
distance relations.Comment: published in MNRAS 14 pages, 8 figures, 2 tables, corrected eq.(35),
minor editin
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