[Mesa-users] Convergence issues solved by changing a_mlt

[Jared Goldberg]

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> 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> 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
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
>
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