Metal monoxide abundances as a function of the C/O ratio

TL;DR

This paper presents a model explaining how the relative abundance of certain metal monoxides in stars varies with the carbon-to-oxygen ratio, based on their unique ionization and dissociation properties.

Contribution

The paper introduces a simple model that accounts for the observed abundance patterns of metal monoxides as a function of the C/O ratio in stars.

Findings

Metal monoxides have ionization energies below their dissociation limits.

Cation formation via associative ionization is efficient for these monoxides.

The model successfully explains observed abundance variations.

Abstract

The diatomic metal monoxides whose optical spectra define the classification of stars on the asymptotic giant branch (AGB), that is, TiO, YO, ZrO, and LaO, have the unusual property that their ionization energy is below their dissociation limit. The cations of these metal monoxides can be efficiently produced via associative ionization of their constituent ground state atoms and are long-lived. We present a simple model that can explain the observed relative abundance of these metal oxides as a function of the C/O ratio.