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

[Falk Herwig]

Alexey, 
of course it depends on what kind of physics situation you have in mind. But simply loosing a lot of mass through the mass loss procedure is maybe an option. Jean-Claude Passy has used this to explore "The Response of Giant Stars to Dynamical-timescale Mass Loss"  (http://adsabs.harvard.edu/abs/2012ApJ...760…90P) and in this calculations he increased the mass loss up to ~1Msun/yr for a star a red giant of ~0.9Msun. These are dynamic mass loss rates in the sense that they represent the expected mass ejection of a common envelope phase in terms of total mass to be ejected and time scale (see http://adsabs.harvard.edu/abs/2012ApJ...744…52P). 
Falk.

On 2013-04-18, at 7:29 AM, Alexey Zinger wrote:

> 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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--
Falk Herwig
Dept of Physics & Astronomy, U of Victoria
fherwig at uvic.ca, tel: +1 (250) 721-7743



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