[Mesa-users] Convergence issues solved by changing a_mlt

[Farag, Ebraheem]

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





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From: Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of Joergensen, Johannes Holm via Mesa-users <mesa-users at lists.mesastar.org>
Sent: Wednesday, November 6, 2024 4:50 AM
To: mesa-users at lists.mesastar.org <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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