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Non-local Thermodynamic Equilibrium Stellar Spectroscopy with 1D and 3D Models. II. Chemical Properties of the Galactic Metal-poor Disk and the Halo

Bergemann, Maria; Collet, Remo; Schönrich, Ralph; Andrae, Rene; Kovalev, Mikhail; Ruchti, Greg LU ; Hansen, Camilla Juul and Magic, Zazralt (2017) In Astrophysical Journal 847(1).
Abstract

From exploratory studies and theoretical expectations it is known that simplifying approximations in spectroscopic analysis (local thermodynamic equilibrium (LTE), 1D) lead to systematic biases of stellar parameters and abundances. These biases depend strongly on surface gravity, temperature and, in particular, for LTE versus non-LTE (NLTE), on metallicity of the stars. Here we analyze the [Mg/Fe] and [Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtained using 1D hydrostatic models and averaged models. We show that compared to the NLTE benchmark, the other three methods display increasing biases toward lower metallicities, resulting in false trends of [Mg/Fe] against [Fe/H], which have profound implications for... (More)

From exploratory studies and theoretical expectations it is known that simplifying approximations in spectroscopic analysis (local thermodynamic equilibrium (LTE), 1D) lead to systematic biases of stellar parameters and abundances. These biases depend strongly on surface gravity, temperature and, in particular, for LTE versus non-LTE (NLTE), on metallicity of the stars. Here we analyze the [Mg/Fe] and [Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtained using 1D hydrostatic models and averaged models. We show that compared to the NLTE benchmark, the other three methods display increasing biases toward lower metallicities, resulting in false trends of [Mg/Fe] against [Fe/H], which have profound implications for interpretations by chemical evolution models. In our best NLTE model, the halo and disk stars show a clearer behavior in the [Mg/Fe]-[Fe/H] plane, from the knee in abundance space down to the lowest metallicities. Our sample has a large fraction of thick disk stars and this population extends down to at least [Fe/H] ∼ -1.6 dex, further than previously proven. The thick disk stars display a constant [Mg/Fe] ≈ 0.3 dex, with a small intrinsic dispersion in [Mg/Fe] that suggests that a fast SN Ia channel is not relevant for the disk formation. The halo stars reach higher [Mg/Fe] ratios and display a net trend of [Mg/Fe] at low metallicities, paired with a large dispersion in [Mg/Fe]. These indicate the diverse origin of halo stars from accreted low-mass systems to stochastic/inhomogeneous chemical evolution in the Galactic halo.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Galaxy: abundances, Galaxy: evolution, Galaxy: kinematics and dynamics, radiative transfer, stars: abundances, stars: late-type
in
Astrophysical Journal
volume
847
issue
1
publisher
University of Chicago Press
external identifiers
  • scopus:85030162451
  • wos:000410740100002
ISSN
0004-637X
DOI
10.3847/1538-4357/aa88b5
language
English
LU publication?
yes
id
49e79909-09a6-43dc-a2ec-18d28eedaba3
date added to LUP
2017-11-28 13:04:40
date last changed
2018-01-16 13:26:45
@article{49e79909-09a6-43dc-a2ec-18d28eedaba3,
  abstract     = {<p>From exploratory studies and theoretical expectations it is known that simplifying approximations in spectroscopic analysis (local thermodynamic equilibrium (LTE), 1D) lead to systematic biases of stellar parameters and abundances. These biases depend strongly on surface gravity, temperature and, in particular, for LTE versus non-LTE (NLTE), on metallicity of the stars. Here we analyze the [Mg/Fe] and [Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtained using 1D hydrostatic models and averaged models. We show that compared to the NLTE benchmark, the other three methods display increasing biases toward lower metallicities, resulting in false trends of [Mg/Fe] against [Fe/H], which have profound implications for interpretations by chemical evolution models. In our best NLTE model, the halo and disk stars show a clearer behavior in the [Mg/Fe]-[Fe/H] plane, from the knee in abundance space down to the lowest metallicities. Our sample has a large fraction of thick disk stars and this population extends down to at least [Fe/H] ∼ -1.6 dex, further than previously proven. The thick disk stars display a constant [Mg/Fe] ≈ 0.3 dex, with a small intrinsic dispersion in [Mg/Fe] that suggests that a fast SN Ia channel is not relevant for the disk formation. The halo stars reach higher [Mg/Fe] ratios and display a net trend of [Mg/Fe] at low metallicities, paired with a large dispersion in [Mg/Fe]. These indicate the diverse origin of halo stars from accreted low-mass systems to stochastic/inhomogeneous chemical evolution in the Galactic halo.</p>},
  articleno    = {16},
  author       = {Bergemann, Maria and Collet, Remo and Schönrich, Ralph and Andrae, Rene and Kovalev, Mikhail and Ruchti, Greg and Hansen, Camilla Juul and Magic, Zazralt},
  issn         = {0004-637X},
  keyword      = {Galaxy: abundances,Galaxy: evolution,Galaxy: kinematics and dynamics,radiative transfer,stars: abundances,stars: late-type},
  language     = {eng},
  month        = {09},
  number       = {1},
  publisher    = {University of Chicago Press},
  series       = {Astrophysical Journal},
  title        = {Non-local Thermodynamic Equilibrium Stellar Spectroscopy with 1D and 3D Models. II. Chemical Properties of the Galactic Metal-poor Disk and the Halo},
  url          = {http://dx.doi.org/10.3847/1538-4357/aa88b5},
  volume       = {847},
  year         = {2017},
}