[Mesa-users] Convergence issues solved by changing a_mlt

Thu Nov 7 10:48:17 UTC 2024

Hi Jared and Farag,

thanks for your replies! I will try to answer them all.

I'm using MESA-r22.11.1 where Steindl+ 2022 used v12778. The scheme is the same as in that paper, with an initial stellar seed.

I follow your intuition as well; that more efficient convection should help with the small convection zones.

For the MESA inlist controls in question, I can tell the following:
- 'relax_until_radiative_core' is not in my inlist. I guess it's set to 'true' by default? https://docs.mesastar.org/en/22.11.1/reference/star_job.html#pre-ms-relax-to-start-radiative-core
- 'do_conv_premix' is set to 'True'.

- 'prune_bad_cz*' and 'min_convective_gap' have the following controls:

i) prune_bad_cz_min_Hp_height = 0.1
ii) prune_bad_cz_min_log_eps_nuc = 99
iii) min_convective_gap = 0.1

> If I'm understanding correctly, by decreasing 'alpha_mlt' from 'alpha_mlt = 1.9' to 'alpha_mlt = 1.85', you were able to solver most of your convergence issues?

In general yes, but sometimes I had the case where I also increased 'alpha_mlt' slightly and solved my issues. Overall I have a grid of models from 0.5-1.5 solar masses with 35 different accretion scenarios that gives a total of 735 models. I wanted to keep 'alpha_mlt' fixed at 1.9, but ended up varying it between 1.8-2.0 to get better grid coverage. In the end I ended up with ~85% grid coverage.

> Finally, do the tiny spurious convection zones pop up near the base of the convective envelope, or the surface?

As far as I can tell, the zones show up throughout the entire interior.

> I think two plots could help in diagnosing what's happening.

I have attached the plots, although I have plotted the MESA 'schwarzschild criteria' against logT instead of ∇−∇ad, but this should convey the same information right? (I hope my naming scheme makes sense)

One thing I noticed, when examining the Kippenhahn diagrams in the MESA pngs, is that the model that fails with 'alpha_mlt'=1.9 has a larger region of strong burning in the outer parts  than the model with lower 'alpha_mlt' at the same age. At later stages the the model with lower 'alpha_mlt' will also develop strong off-center burning, yet still converges, so I'm not sure this is the cause of the problem in the model with higher 'alpha_mlt'.

> I would also encourage you to stare at power profile plots.

Yes, thanks for the tip, I will get around to these plots as soon as I can, but I wanted to get the first plots to you both right now.

Final question for now: When examining the MESA solver outputs, I believe one column should display the index of the problematic cell? Is there a way to connect a given cell index with a radius or mass coordinate to illustrate what is going in the problematic cell?

Thanks to both of you for taking the time to help me, I really appreciate it!

Cheers, Johannes

________________________________
Fra: Jared Goldberg <jgoldberg at flatironinstitute.org>
Sendt: 6. november 2024 20:38:52
Til: Farag, Ebraheem
Cc: mesa-users at lists.mesastar.org; Joergensen, Johannes Holm
Emne: Re: [Mesa-users] Convergence issues solved by changing a_mlt

Hi Johannes,

I've been playing around with accretion on the pre-main-sequence in support of a student's project this past summer, and we've been breaking MESA in many interesting ways. The tiny convection zones popping in and out of existence definitely become a problem in this regime. Like Eb speculated, in my experience making convection more efficient (increasing alpha_MLT) tended to behave slightly better near the convective boundary, which I justified intuitively by thinking that it brings convective gradients closer to the adiabatic gradient, and also may be a bit better for composition gradients which may cause the tiny convection zones popping in and out of existence. I have less intuition for why a lower alpha_mlt might help the solver -- perhaps this is a sign that some of the numerics are ill-converged in general, and kicking the solver around (e.g. by changing alpha) could help with convergence.

Convective boundaries are a notorious problem, and have had much work in MESA IV and V devoted to doing better and better, though there's still room for improvement. One work-around here is setting prune_bad_cz_* and min_convective_gap to prune these numerical jitters. Similarly do_conv_premix = .true. seems to help with numerical stability. Playing with overshooting may also help to this end.

A few more questions to be more helpful and gauge what's going on:

> First, what MESA revision are you using? Are you using the old revision from Steindl+2022 (9000s, I think?) or are you using a recent revision?
A number of changes have been made since then which have put resolution away from composition gradients, and similarly there have been changes with convective boundary mixing schemes and the turbulence module in general which may impact the setup. There were also resolution controls near the core boundaries / composition boundaries in that old revision (useful for asteroseismology) which Steindl et al used that don't exist in modern revisions.

> Also, what are you initial conditions? (E.g. We found that things are better-converged if relax_until_reach_radiative_core = .false. on the pre-MS.)
Are you starting from a seed stellar model as in Steindl et al, or from one of MESA's built-in constructors for pre-ms initial conditions?

> Finally, do the tiny spurious convection zones pop up near the base of the convective envelope, or the surface?

I agree with Eb that
1- logT vs logRho profile of the whole model (a pgstar will work well, and it shows where convection operates along the profile)
2- superadiabicity (∇−∇ad) vs logT (so we can match the superadiabatic regions with regions in trho profile)
are good plots to make.

I would also encourage you to stare at power profile plots (are Li and Deuterium burning messing things up?) and composition profile plots (does jagged behavior in the composition profile cause local Ledoux-unstable behavior) as a function of mass coordinate. A kippenhahn diagram would also be helpful to see *when* in the star's evolution these spurious convection zones start to appear.

Cheers,
~Jared

On Wed, Nov 6, 2024 at 10:41 AM Farag, Ebraheem via Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>> wrote:
Hello Johannes,

> For context, I am running some "complicated" accreting pre-MS models with the framework developed by Steindl+ 2022.

Very cool! Thomas did some nice work on developing a framework for pre-MS accretion.

> From examining the output of the solver I see that the issues arise in convection zones. By trial and error I solved a lot of my convergence issues by simply changing the mixing length parameter slightly (e.g. from 1.9 to 1.85). My question is: Is there any way to interpret this? My models show interesting interiors with often many tiny convection cells which I imagine are difficult to handle; could a change in a_mlt help in this regard?

If I'm understanding correctly, by decreasing 'alpha_mlt' from 'alpha_mlt = 1.9' to 'alpha_mlt = 1.85', you were able to solver most of your convergence issues?

if decreasing alpha_mlt is indeed solving your issue, are these problematic convective zones becoming radiative in the improved working model? Naively, I would imagine the problem would become worse with lower alpha_mlt unless the zone becomes entirely radiative.

I think two plots could help in diagnosing what's happening.

1- logT vs logRho profile of the whole model (a pgstar will work well, and it shows where convection operates along the profile)
2- superadiabicity (∇−∇ad) vs logT (so we can match the superadiabatic regions with regions in trho profile)

If you can answer these questions, I can attempt to provide an explanation.

-EbF

________________________________
From: Mesa-users <mesa-users-bounces at lists.mesastar.org<mailto:mesa-users-bounces at lists.mesastar.org>> on behalf of Joergensen, Johannes Holm via Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>>
Sent: Wednesday, November 6, 2024 4:50 AM
To: mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org> <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>>
Subject: [Mesa-users] Convergence issues solved by changing a_mlt

Hi everyone,

I am having convergence issues in MESA. For context, I am running some "complicated" accreting pre-MS models with the framework developed by Steindl+ 2022. From examining the output of the solver I see that the issues arise in convection zones. By trial and error I solved a lot of my convergence issues by simply changing the mixing length parameter slightly (e.g. from 1.9 to 1.85). My question is: Is there any way to interpret this? My models show interesting interiors with often many tiny convection cells which I imagine are difficult to handle; could a change in a_mlt help in this regard? I would gladly provide figures that showcase this, or my inlists. Much gratitude for any help!

Cheers, Johannes

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