Ultra metal-poor stars: improved atmospheric parameters and NLTE abundances of magnesium and calcium

Abstract The most metal-poor stars are the oldest objects, they provide a unique opportunity to study the earliest epoch of the Galaxy formation and individual nucleosynthesis events. These stars should be investigated with a scrupulous care, taking into account all available photometric, spectrosco...

Full description

Saved in:
Bibliographic Details
Published in:Monthly notices of the Royal Astronomical Society 2019-05, Vol.485 (3), p.3527-3543
Main Authors: Sitnova, T M, Mashonkina, L I, Ezzeddine, R, Frebel, A
Format: Article
Language:English
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Abstract The most metal-poor stars are the oldest objects, they provide a unique opportunity to study the earliest epoch of the Galaxy formation and individual nucleosynthesis events. These stars should be investigated with a scrupulous care, taking into account all available photometric, spectroscopic, and astrometric informations. We determined atmospheric parameters for 17 ultra metal-poor (UMP) stars, using an extensive method based on colour-Teff calibrations, isochrones, Gaia DR2 trigonometric parallaxes, and non-local thermodynamic equilibrium (NLTE) analysis of the Ca i/Ca ii ionization equilibrium and the Balmer line wings. We updated the model atom of Ca i- ii by including recent quantum-mechanical rate coefficients for the Ca i + H i and Ca ii + H i inelastic collisions. For any line of Ca i and Ca ii in our sample stars, the changes in collisional data result in a shift of smaller than 0.05 dex in the NLTE abundance. We determined magnesium and calcium NLTE and LTE abundances of our sample stars. For 10 stars, we found close-to-solar [Ca/Mg] NLTE abundance ratios. In the remaining stars, magnesium and calcium abundances do not follow each other, such that [Ca/Mg] varies between −3.15 and + 0.36, suggesting a contribution to stellar Mg and Ca abundances from a small number of supernova explosions with different properties. The obtained atmospheric parameters will be used in the forthcoming paper to determine NLTE abundances of chemical elements observed in spectra of the UMP stars.
ISSN:0035-8711
1365-2966
DOI:10.1093/mnras/stz626