Chromospheric Models and the Oxygen Abundance in Giant Stars

TL;DR

This study demonstrates that including chromospheric effects in stellar models significantly alters the inferred oxygen abundance in giant stars, impacting our understanding of stellar evolution and chemical enrichment.

Contribution

It introduces realistic chromospheric models for metal-poor giant stars and shows their impact on oxygen line formation and abundance estimates.

Findings

Chromospheric models produce stronger oxygen lines than equilibrium models.

Oxygen abundance estimates decrease by a factor of ~3 with chromospheric models.

Implications for stellar evolution and galactic chemical evolution models.

Abstract

Realistic stellar atmospheric models of two typical metal-poor giant stars in Omega Centauri that include a chromosphere influence the formation of optical lines of Oxygen I: the forbidden lines (630nm, 636nm) and the infrared triplet (777.1-777.5 nm). One-dimensional semi-empirical non-LTE models are constructed based on observed Balmer lines. A full non-LTE formulation is applied in evaluating line strengths of O I including photoionization by the Lyman continuum and photoexcitation by Ly-alpha and Ly-beta. Chromospheric models (CHR) yield forbidden oxygen transitions that are stronger than in radiative/convective equilibrium (RCE) models. The triplet oxygen lines from high levels also appear stronger than produced in an RCE model. The inferred oxygen abundance from realistic CHR models for these two stars is decreased by factors ~3 as compared to values derived from RCE models. A lower oxygen abundance suggests that intermediate mass AGB stars contribute to the observed abundance pattern in globular clusters. A change in the oxygen abundance of metal-poor field giants could affect models of deep mixing episodes on the red giant branch. Changes in the oxygen abundance can impact other abundance determinations critical to astrophysics including chemical tagging techniques and galactic chemical evolution.