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

Abstract

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