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