[Mesa-users] Issue with rotation in MESA r22.11.1

[Matteo Cantiello]

>
>
> This is true. But I also wonder why the PARSEC models for a similar mass
> and initial rotation *do not* reach critical rotation at the blue hook,
> whereas MESA does. I've been assuming that PARSEC manages to get rid of
> angular momentum more efficiently at this mass and initial w/w_crit,
> somehow. Still, I agree that having the right observations would be very
> useful.
>
>
Ok, that's a well defined question and worth looking into it. I am not a
PARSEC expert, but I have used other codes than MESA that include the
physics of rotation. I'll assume you have double checked that the models
you are comparing start rigidly and with the same rotation rate, as this
can depend on how rotation is initialized. With that out of the way, one
issue that matters a lot when it comes to reaching critical rotation is how
angular momentum is transported in the model. It is easy to understand why:
stars that have efficient angular momentum transport stay ~rigidly rotating
during the main sequence.  While models with weaker transport develop some
degree of differential rotation. Now this is important because it may
change the ability of stars to reach breakup: if more angular momentum is
stored in the core, the envelope can rotate more slowly.

The most efficient sources of coupling during the MS in MESA are magnetic
torques from Tayler-Spruit dynamo fields, and meridional circulation
(Eddington Sweet). I just looked at one PARSEC paper from 2022 and their
rotating models do not have magnetic torques.  While this is disfavored by
asteroseismic observations that point to efficient angular momentum
transfer in stars, in order to compare to the PARSEC models you correctly
seem to have turned ST off in your inlist. However, if your goal is not a
direct comparison I would argue you should have magnetic torques on.
PARSEC then includes Eddington Sweet and Shear, while MESA has those plus a
few more (all described in the controls and the second instrument paper).
The formulation of the shear diffusivity in PARSEC is different than in
MESA (Talon & Zahn 1997 <-> Heger et al. 2000). But I doubt this should
make much difference. I would then for sure test turning off SH and GSF as
well. All in all I would test with:

    D_ST_factor = 0.0
    D_SH_factor =  0.0
    D_GSF_factor = 0.0
    D_ES_factor =  1.0
    D_SSI_factor = 1.0
    D_DSI_factor = 1.0

In order to see if you get results more similar to PARSEC. And I would run
with

    D_ST_factor = 1.0
    D_SH_factor =  1.0
    D_GSF_factor = 1.0
    D_ES_factor =  1.0
    D_SSI_factor = 1.0
    D_DSI_factor = 1.0

for your science runs.
Cheers,

Matteo


>
>
>
>> On Thu, Jan 23, 2025 at 2:53 PM Torres, Guillermo via Mesa-users <
>> mesa-users at lists.mesastar.org> wrote:
>>
>>> All,
>>>
>>> I am using MESA r22.11.1 to model the evolution of a rapidly rotating
>>> star of 5.04 solar masses (initial w/w_crit = 0.55), and I'm finding an odd
>>> shape for the evolutionary track in the blue hook region, as the star is
>>> contracting. During this phase, I see the model reaching very high values
>>> of w/w_crit up to the limit of 0.99, and as it approaches that value, the
>>> morphology goes haywire, at least compared to a similar track from the
>>> PARSEC v2.0 series (see figure attached, with the odd part highlighted in
>>> red). I'm wondering if this really is an issue with the physics in MESA, or
>>> just that I don't have the right controls in my inlist. It appears the star
>>> would need to get rid of more of its angular momentum to avoid this, but
>>> evidently it is not doing that with the controls I am using. I attach my
>>> inlist for reference. The pre-main-sequence part is computed separately. I
>>> would be thankful for any suggestions.
>>>
>>> Willie
>>> _______________________________________________
>>> mesa-users at lists.mesastar.org
>>> https://lists.mesastar.org/mailman/listinfo/mesa-users
>>>
>>>
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