New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star forming dwarf galaxies

TL;DR

This paper extends the PARSEC stellar evolution models to massive stars at low metallicity, testing their predictions against observations of dwarf galaxies and exploring the impact of overshooting on stellar evolution features.

Contribution

The study introduces new low-metallicity massive star models with updated physics and examines the effects of overshooting on stellar evolutionary tracks and observed color-magnitude diagrams.

Findings

Models reproduce observed CMDs fairly well.

Blue loop predictions are too cool with canonical overshooting.

Enhanced overshooting improves agreement with observations.

Abstract

We extend the {\sl\,PARSEC} library of stellar evolutionary tracks by computing new models of massive stars, from 14\Msun to 350\Msun. The input physics is the same used in the {\sl\,PARSEC}~V1.1 version, but for the mass-loss rate which is included by considering the most recent updates in literature. We focus on low metallicity, $Z$=0.001 and $Z$=0.004, for which the metal poor dwarf irregular star forming galaxies, Sextans A, WLM and NCG6822, provide simple but powerful workbenches. The models reproduce fairly well the observed CMDs but the stellar colour distributions indicate that the predicted blue loop is not hot enough in models with canonical extent of overshooting. In the framework of a mild extended mixing during central hydrogen burning, the only way to reconcile the discrepancy is to enhance the overshooting at the base of the convective envelope (EO) during the first dredge-UP. The mixing scales required to reproduce the observed loops, EO=2\HP or EO=4\HP, are definitely larger than those derived from, e.g., the observed location of the RGB bump in low mass stars. This effect, if confirmed, would imply a strong dependence of the mixing scale below the formal Schwarzschild border, on the stellar mass or luminosity. Reproducing the features of the observed CMDs with standard values of envelope overshooting would require a metallicity significantly lower than the values measured in these galaxies. Other quantities, such as the star formation rate and the initial mass function, are only slightly sensitive to this effect. Future investigations will consider other metallicities and different mixing schemes.