[Mesa-users] A very short timestep of the evolution of a He star appears during it towards an AGB star

[Josiah Schwab]

Hi Yaotain,

I'm re-pasting some of the info from another active thread because I think
it applies here too.  I've worked a decent amount on related models (e.g.,
https://ui.adsabs.harvard.edu/abs/2019ApJ...885...27S/abstract) and to the
best of my understanding there aren't any timestep or solver controls that
are going to let you easily go through this phase.

This model has a near- (or even maybe even super-) Eddington luminosity
(b/c of the He shell burning on a massive degenerate core).
Correspondingly, it has developed an inflated envelope with outer layers
that are radiation pressure dominated and have a density inversion.  The
steep entropy gradient at the base of this (inefficient) convection zone is
usually what is responsible for driving the MESA timestep down.

This is similar to the behavior in massive stars that the MLT++ "hack"
described in Section 7 of Paxton et al. (2013) is designed to numerically
circumvent (but not physically solve).  You could consider using
okay_to_reduce_gradT_excess and related controls, depending on what your
goals for the model are.

Josiah
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