The Most Metal-Poor Stars. III. The Metallicity Distribution Function and CEMP Fraction

TL;DR

This study analyzes the metallicity distribution and CEMP star fraction in extremely metal-poor stars, revealing a smooth MDF decline down to [Fe/H] = -4.1 and a CEMP fraction around 23-32%, challenging previous findings.

Contribution

It provides a revised MDF for ultra-metal-poor stars and quantifies the CEMP fraction, using high-resolution spectroscopy to address previous survey limitations.

Findings

MDF decreases smoothly down to [Fe/H] = -4.1

CEMP fraction below [Fe/H] = -3.0 is approximately 23-32%

No sudden drop in MDF at [Fe/H] = -3.6 as previously reported

Abstract

We examine the metallicity distribution function (MDF) and fraction of carbon-enhanced metal-poor (CEMP) stars in a sample that includes 86 stars with [Fe/H] < -3.0, based on high-resolution, high-S/N spectroscopy, of which some 32 objects lie below [Fe/H] = -3.5. After accounting for the completeness function, the "corrected" MDF does not exhibit the sudden drop at [Fe/H] = -3.6 that was found in recent samples of dwarfs and giants from the Hamburg/ESO survey. Rather, the MDF decreases smoothly down to [Fe/H] = -4.1. Similar results are obtained from the "raw" MDF. We find the fraction of CEMP objects below [Fe/H] = -3.0 is 23 +/- 6% and 32 +/- 8% when adopting the Beers et al. and Aoki et al. CEMP definitions, respectively. The former value is in fair agreement with some previous measurements, which adopt the Beers et al. criterion.