[Mesa-users] Post-AGB evolution

Tue Apr 27 15:44:24 UTC 2021

Hi Natalie,

Ah right, I presume Rob Izzard will have pointed you to that literature already :-)

If I am not mistaken, the test-suite inlists that evolve stars to the WD stage use the MLT++ options (i.e. okay_to_reduce_gradT_excess = .true.). This can reduce or remove the convergence errors if you play around with the parameters, at the cost of changing the stellar structure. Increasing the mixing length alpha will probably have the same effect. I think you will have to compromise between computational time and modifying the MLT in the code.

Good luck!
Glenn

From: Natalie Rees<mailto:n.rees at surrey.ac.uk>
Sent: Tuesday, 27 April 2021 11:54
To: Evan Bauer<mailto:evan.bauer.astro at gmail.com>; Glenn-Michael Oomen<mailto:glennmichael.oomen at hotmail.com>; Yuan Qi Ni<mailto:chris.ni at mail.utoronto.ca>
Cc: mesa-users<mailto:mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] Post-AGB evolution

Hi Chris, Glenn, and Evan,

Thank you very much for your responses, they were very helpful indeed!

Chris, in response to your question my AGB models use inlists that I have adapted from the 1M_pre_ms_to_wd and c13_pocket test suite cases. For post-AGB evolution I have written a pretty simple inlist, similar to the AGB but with a few changes (e.g. turning off overshooting). I have attached them in case that is helpful. Perhaps I should have a look at the make_co_wd test_suite to see if there's anything in there I can implement.

Glenn, thank you for the detailed explanation! Indeed, I have read Lau et al (2012) and your PhD thesis (which I very much enjoyed). In fact, I have implemented a routine for the end of the TP-AGB that increases the mixing length parameter in a restricted region at the base of the convective envelope. This is able to successfully push the model through the last couple of thermal pulses. I wasn't sure if it was this same issue plaguing the post-AGB evolution, however having read section 2.2.3 of your thesis again, and Evan's explanation of what it means when "max increase" is limiting the timestep, it is clear that it is. I shall try adapting my alpha mlt routine to see if I can get the timestep down for post-AGB evolution.

Evan, thank you for that information! That does indeed make sense with how the timestep is behaving so I will focus on solving the convergence issues.

Thank you all again,
Natalie

From: Evan Bauer <evan.bauer.astro at gmail.com>
Sent: 26 April 2021 19:13
To: Rees, Natalie R (PG/R - Physics) <n.rees at surrey.ac.uk>
Cc: mesa-users <mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] Post-AGB evolution

Hi Natalie,

"max increase" is governed by this control:
https://docs.mesastar.org/en/latest/reference/controls.html#max-timestep-factor<https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.mesastar.org%2Fen%2Flatest%2Freference%2Fcontrols.html%23max-timestep-factor&data=04%7C01%7Cn.rees%40surrey.ac.uk%7C4c3c50bb42e9491066e808d908def3f1%7C6b902693107440aa9e21d89446a2ebb5%7C0%7C0%7C637550575932556350%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=R0tnDc734JXnLkW2km9ni%2FKp3yl%2FSQCWXJ%2FBZQTNEQM%3D&reserved=0>

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<mailto: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
_______________________________________________
mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>
https://lists.mesastar.org/mailman/listinfo/mesa-users<https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.mesastar.org%2Fmailman%2Flistinfo%2Fmesa-users&data=04%7C01%7Cn.rees%40surrey.ac.uk%7C4c3c50bb42e9491066e808d908def3f1%7C6b902693107440aa9e21d89446a2ebb5%7C0%7C0%7C637550575932566307%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=Uv0TOG4L34QqBsEOtSfRtwFcQaygzk6w%2FJtfeL8TUAo%3D&reserved=0>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20210427/d3e17066/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: A017F212A7A14C669947633BB8937D06.png
Type: image/png
Size: 135 bytes
Desc: A017F212A7A14C669947633BB8937D06.png
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20210427/d3e17066/attachment.png>