Envelope Overshooting in Low Metallicity Intermediate- and High-mass Stars: a test with the Sagittarius Dwarf Irregular Galaxy

TL;DR

This study tests stellar evolution models with enhanced envelope overshooting at very low metallicity by comparing synthetic color-magnitude diagrams to observations of the Sagittarius Dwarf Irregular Galaxy, finding EO=2 ext{ extperiodcentered}HP best reproduces observed blue loops.

Contribution

It introduces new PARSEC stellar evolution tracks with increased envelope overshooting at low metallicity and validates them against observed stellar populations in SagDIG.

Findings

EO=2 ext{ extperiodcentered}HP best reproduces observed blue loops.

Schwarzschild criterion favors blue loop development over Ledoux.

Large envelope overshooting is necessary for accurate modeling of He-burning phases.

Abstract

We check the performance of the {\sl\,PARSEC} tracks in reproducing the blue loops of intermediate age and young stellar populations at very low metallicity. We compute new evolutionary {\sl\,PARSEC} tracks of intermediate- and high-mass stars from 2\Msun to 350\Msun with enhanced envelope overshooting (EO), EO=2\HP and 4\HP, for very low metallicity, Z=0.0005. The input physics, including the mass-loss rate, has been described in {\sl\,PARSEC}~V1.2 version. By comparing the synthetic color-magnitude diagrams (CMDs) obtained from the different sets of models with envelope overshooting EO=0.7\HP (the standard {\sl\,PARSEC} tracks), 2\HP and 4\HP, with deep observations of the Sagittarius dwarf irregular galaxy (SagDIG), we find an overshooting scale EO=2\HP to best reproduce the observed loops. This result is consistent with that obtained by \citet{Tang_etal14} for Z in the range 0.001-0.004. We also discuss the dependence of the blue loop extension on the adopted instability criterion and find that, contrary to what stated in literature, the Schwarzschild criterion, instead of the Ledoux criterion, favours the development of blue loops. Other factors that could affect the CMD comparisons such as differential internal extinction or the presence of binary systems are found to have negligible effects on the results. We thus confirm that, in presence of core overshooting during the H-burning phase, a large envelope overshooting is needed to reproduce the main features of the central He-burning phase of intermediate- and high-mass stars.