[Mesa-users] A question about delta_HR_limit

[Matteo Cantiello]

Hi Levi,

Your rapidly rotating model is likely at the cusp between normal and quasi
chemically homogeneous evolution. See e.g. Maeder 1987 (
https://ui.adsabs.harvard.edu/abs/1987A%26A...173..247M/abstract),  Yoon
&  Langer  2005  (
https://ui.adsabs.harvard.edu/abs/2005A%26A...443..643Y/abstract),  Paxton
et  al.  2013  (
https://ui.adsabs.harvard.edu/abs/2013ApJS..208....4P/abstract). This is a
threshold process,  with  the  amount  of  rotational  mixing  leading to
very different solutions for the stellar structure.  When models are close
to the transition rotational rate between normal and quasi chemically
homogeneous,  results  are  sensitive to resolution and choices  of the
internal physics. The  transition  line between  normal  and  quasi
chemically  homogeneous  evolution shown in e.g. Yoon & Langer 2006  for
models  with  different  metallicity  and  mass  (
https://ui.adsabs.harvard.edu/abs/2006A%26A...460..199Y/abstract,  their
Fig. 3).
Quasi chemically homogeneous evolution also leads to larger cores, again
because of internal mixing. Which explains your findings.

With that said, I don't know the details of how delta_HR_limit plays a role
here, something you maybe can investigate a bit further (e.g. you can see
that both models evolve quasi-chemically homogeneously until ~halfway on
the main sequence). Maybe you can look at plots of rotational diffusivity
and compositional gradients across that transition for the two models, and
determine what is being affected differently and why.

Hope this helps,

Matteo

--
Matteo Cantiello - https://www.stellarphysics.org/
Research Scientist,  CCA, Flatiron Institute
Visiting Associate Researcher, Princeton University



On Tue, Mar 29, 2022 at 1:04 AM, Levi <mesa-users at lists.mesastar.org> wrote:

> Dear Mesa users,
>
> I had a very confusing experience when using MESA version r10398.
> I was recently working on a set of models that went from zero age main
> sequence to core collapse, and in the process of this, I suddenly found
> that the delta_HR_limit option would greatly change the nature of the
> model. At the beginning I set a=b, the delta_HR_limit = 0.005d0 then the
> evolution trajectory moves towards the RSG of the 23Msun in the figure
> below, then I comment out it in the inlist, and the evolution trajectory
> moves towards the BSG such as 20Msun test. In addition, the helium core
> mass of the two models at the time of pre-collapse are 13.7 and 20.2 Msun,
> respectively.
>
> I considered the initial surface speed of 600 km/s and the magnetic
> braking in the model but I don't know why the delta_HR_limit option would
> make such a big difference.
> I was very grateful for any information.
>
> Thanks,
> Levi
>
>
>
>
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
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