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

Research output: Contribution to journalArticle

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 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.

Details

Authors
  • Maria Bergemann
  • Remo Collet
  • Ralph Schönrich
  • Rene Andrae
  • Mikhail Kovalev
  • Greg Ruchti
  • Camilla Juul Hansen
  • Zazralt Magic
Organisations
External organisations
  • Max Planck Institute for Astronomy
  • Aarhus University
  • Australian National University
  • University of Oxford
  • Technical University of Darmstadt
  • University of Copenhagen
  • Natural History Museum of Denmark
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Astronomy, Astrophysics and Cosmology

Keywords

  • Galaxy: abundances, Galaxy: evolution, Galaxy: kinematics and dynamics, radiative transfer, stars: abundances, stars: late-type
Original languageEnglish
Article number16
JournalAstrophysical Journal
Volume847
Issue number1
Publication statusPublished - 2017 Sep 20
Publication categoryResearch
Peer-reviewedYes