Population Synthesis of Binary Carbon-enhanced Metal-poor Stars

TL;DR

This study examines the origins of carbon-enhanced metal-poor stars, testing whether low-mass star models can explain their prevalence without altering the initial mass function, and finds partial support for this scenario.

Contribution

It demonstrates that efficient third dredge-up in low-mass, low-metallicity stars can partly explain the high CEMP fraction without increasing NEMP stars, challenging previous models.

Findings

Low-mass, low-metallicity stars can produce CEMP stars through third dredge-up.

Current detailed TPAGB models cannot fully account for the observed CEMP fraction.

The NEMP star fraction remains low, consistent with observations.

Abstract

The carbon-enhanced metal-poor (CEMP) stars constitute approximately one fifth of the metal-poor ([Fe/H] ~< -2) population but their origin is not well understood. The most widely accepted formation scenario, invokes mass-transfer of carbon-rich material from a thermally-pulsing asymptotic giant branch (TPAGB) primary star to a less massive main-sequence companion which is seen today. Recent studies explore the possibility that an initial mass function biased toward intermediate-mass stars is required to reproduce the observed CEMP fraction in stars with metallicity [Fe/H] < -2.5. These models also implicitly predict a large number of nitrogen-enhanced metal-poor (NEMP) stars which is not seen. We investigate whether the observed CEMP and NEMP to extremely metal-poor (EMP) ratios can be explained without invoking a change in the initial mass function. We confirm earlier findings that with current detailed TPAGB models the large observed CEMP fraction cannot be accounted for. We find that efficient third dredge up in low-mass (less than 1.25Msun), low-metallicity stars may offer at least a partial explanation to the large observed CEMP fraction while remaining consistent with the small observed NEMP fraction.