M dwarfs and the fraction of high carbon-to-oxygen stars in the solar neighbourhood

TL;DR

This study assesses the frequency of high carbon-to-oxygen ratio M dwarf stars in the solar neighborhood, finding such stars are very rare, comprising less than one percent of nearby stars, based on spectral analysis.

Contribution

It introduces a method to identify high C/O ratio M dwarfs via spectral features and provides the first observational constraint on their frequency in the solar neighborhood.

Findings

Less than 1% of nearby stars have high C/O ratios.

High C/O M dwarfs exhibit weaker TiO bands in spectra.

Stars with C/O=0.9 and certain metallicities mimic solar metallicity spectra.

Abstract

We investigate the frequency of high carbon-to-oxygen (C/O $= 0.9$) M dwarf stars in the solar neighbourhood. Using synthetic spectra, we find that such M dwarfs would have weaker TiO bands relative to hydride features. Similar weakening has already been detected in M-subdwarf (sdM) stars. By comparing to existing spectroscopic surveys of nearby stars, we show that less than one percent of nearby stars have high carbon-to-oxygen ratios. This limit does not include stars with C/O$=0.9$, [m/H]$>0.3$, and [C/Fe]$>0.1$, which we predict to have low-resolution optical spectra similar to solar metallicity M dwarfs.