[Mesa-users] Post-AGB evolution

[Evan Bauer]

Hi Natalie,

"max increase" is governed by this control:
https://docs.mesastar.org/en/latest/reference/controls.html#max-timestep-factor <https://docs.mesastar.org/en/latest/reference/controls.html#max-timestep-factor>

In my experience, when I see many timesteps reporting "max increase" as the reason for the timestep limit, it means that MESA is going through a cycle of solver struggles. It will try to increase the timestep as much as it can, but eventually run into a step where the Newton solver can't converge. This forces several retries that cause the timestep to plummet before the solver recovers, followed by several steps where the timestep slowly grows according to "max increase" until another convergence issue, and the cycle repeats itself.

That cycle is almost always indicative of some more fundamental issue that causes the Newton solver to struggle, so there probably isn't a magic combination of timestep controls alone that will make evolution go more smoothly. Hopefully the advice of others on this thread about TP-AGB evolution can help you find settings that aren't as challenging for the solver.

Cheers,
Evan


> On Apr 26, 2021, at 9:06 AM, Natalie Rees via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> Dear MESA Users,
> 
> I have been evolving AGB stars to the end of the TP-AGB stage and am now trying to evolve them off the AGB and to the white dwarf cooling track. Unfortunately, I am finding the post-AGB evolution to be very slow (it is taking about 140,000 models to go from logTeff = 3.56 to 3.63). So, I am trying to figure out what is limiting the timestep as there is very little change in the structure variables. The terminal output for dt_limit often says "max increase", but this is not listed in the table for dt_limit in controls.defaults. Does anyone know what this means and/or have any tips for faster post-AGB evolution? 
> 
> Thanks,
> Natalie Rees
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