Carbon-Enhanced Metal-Poor Star Frequencies in the Galaxy: Corrections for the Effect of Evolutionary Status on Carbon Abundances

TL;DR

This study refines the estimated frequency of Carbon-Enhanced Metal-Poor stars in the Galaxy by correcting for surface carbon depletion due to stellar evolution, revealing higher true frequencies at low metallicities.

Contribution

We developed a correction method based on stellar evolution models to accurately determine initial carbon abundances in CEMP stars, improving frequency estimates.

Findings

CEMP star frequency increases with decreasing metallicity.

Corrected frequencies are higher than previous estimates without corrections.

Developed an online tool for applying carbon abundance corrections.

Abstract

We revisit the observed frequencies of Carbon-Enhanced Metal-Poor (CEMP) stars as a function of the metallicity in the Galaxy, using data from the literature with available high-resolution spectroscopy. Our analysis excludes stars exhibiting clear over-abundances of neutron-capture elements, and takes into account the expected depletion of surface carbon abundance that occurs due to CN processing on the upper red-giant branch. This allows for the recovery of the initial carbon abundance of these stars, and thus for an accurate assessment of the frequencies of carbon-enhanced stars. The correction procedure we develope is based on stellar-evolution models, and depends on the surface gravity, log g, of a given star. Our analysis indicates that, for stars with [Fe/H]<=-2.0, 20% exhibit [C/Fe]>=+0.7. This fraction increases to 43% for [Fe/H]<=-3.0 and 81% for [Fe/H]<=-4.0, which is higher than have been previously inferred without taking the carbon-abundance correction into account. These CEMP-star frequencies provide important inputs for Galactic and stellar chemical-evolution models, as they constrain the evolution of carbon at early times and the possible formation channels for the CEMP-no stars. We also have developed a public online tool with which carbon corrections using our procedure can be easily obtained.