Spectral analyses of three carbon-enhanced metal-poor stars

TL;DR

This study conducts high-resolution spectral analysis of three carbon-enhanced metal-poor stars, revealing significant 3D effects on carbon abundance and classifying their neutron capture element overabundances, contributing to understanding of early stellar populations.

Contribution

First detailed 3D hydrodynamical modeling of carbon abundances in CEMP stars, highlighting significant 3D effects and expanding classification of neutron capture element patterns in such stars.

Findings

3D corrections on carbon abundance up to --0.7 dex

Two stars classified as CEMP-s, one as CEMP-no/s

Significant impact of 3D models on abundance determinations

Abstract

We are conducting a high-resolution follow-up of candidate EMP stars extracted from the Sloan Digital Sky Survey (SDSS; York et al. 2000) using UVES at the VLT. Three of the programme stars, SDSS J0912+0216, SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted as CEMP stars since a strong $G$ band was evident from the SDSS spectra and the weakness of the Ca {\sc ii} K line testified their very low metallicity. The UVES high resolution follow-up confirmed the original findings ([Fe/H] $<-2.50$) and allowed a more detailed investigation of their chemical composition. We determined the carbon abundance from molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by classical 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. 3D carbon abundances were determined for all three stars, using CO$^5$BOLD 3D hydrodynamical model atmospheres. 3D effects on the carbon abundance are found to be quite significant for these stars, with 3D corrections of up to --0.7 dex. Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit an overabundance of neutron capture elements which classifies them as CEMP-s. Star SDSS J1036+1212, instead belongs to the elusive class of CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date.