[mesa-users] Convergence issues when simulating massive eruptions by modifying .mod file

[Alexey Zinger]

Hi MESA community,

I am simulating rapid massive eruptions (>1 M_sun) from massive stars (~100
M_sun). I do this by artificially modifying the saved model file and
restarting the evolution with loading that file.

The easiest (crudest) way to do this is to simply truncate the model by
removing a specified number of the outer zones. Of course, this new model
is very unnatural and does not converge when plugged into MESA, unless the
amount of mass removed is minuscule. A slightly better approximation is to
create a smooth gradient of parameters (temperature, etc) in the outer
layers, rather than a sharp drop. This works a little better, but still
does not allow a substantial amount of mass to be removed. Finally, the
approach I use now is to put the outer layers in hydrostatic equilibrium
(to facilitate convergence) with the constant specific entropy in that
region (to simulate the star adjustment to a sharp temperature gradient).
This allows for instantaneous removal of over a half-solar mass, but there
are still convergence problems when trying to go to higher masses.

The non-convergence message MESA throws are different depending on the mass
removed - I think I've seen almost all of them (varcontrol,
neg_mass_fraction, etc). I've tried increasing the number of zones in the
.mod file by changing mesh_delta_coeff, decreasing the initial time steps,
and modifying various additional settings to make MESA less finicky about
its convergence requirements, but with little success. I have also tried to
trace the problem in the source code, but have quickly bogged down in it
(the fact that the non-convergence reason is different every time does not
help at all).

Astrophysicists that I've talked to say that this new model is not
unphysical, so, in principle, it should converge.
So my question is: is there a relatively painless way to figure out the
reason for non-convergence (without going through all the source code)? If
I know a physical reason for this behavior, I can adjust my model
accordingly to make it play nice with MESA.


Inlists plus some graphs are in attach. I'm using version 4298 of MESA.


Alexey
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