Envelope overshooting in low-metallicity intermediate- and high-mass stars: a test with the Sagittarius dwarf irregular galaxy

We check the performance of the Padova TRieste Stellar Evolution Code (PARSEC) tracks in reproducing the blue loops of intermediate age and young stellar populations at very low metallicity. We compute new evolutionary PARSEC tracks of intermediate- and high-mass stars from 2 to 350 M_⊙ with enhanced envelope overshooting (EO), EO = 2H_P and 4H_P, for very low metallicity, Z = 0.0005. The input physics, including the mass-loss rate, has been described in PARSEC, version V1.2. By comparing the synthetic colour-magnitude diagrams (CMDs) obtained from the different sets of models with EO = 0.7H_P (the standard PARSEC tracks), 2H_P and 4H_P, with deep observations of the Sagittarius dwarf irregular galaxy (SagDIG), we find that the overshooting scale EO = 2H_P best reproduces the observed loops. This result is consistent with that obtained by Tang et al. for Z in the range 0.001-0.004. We also discuss the dependence of the blue loop extension on the adopted instability criterion. Contrary to what has been stated in the literature, we find that 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. Thus, we confirm that, in the presence of core overshooting during the H-burning phase, a large EO is needed to reproduce the main features of the central He-burning phase of intermediate- and high-mass stars.