[Mesa-users] Red Giant Evolution

[Jared Goldberg]

Hi Abha,

Speaking generally, this is not a well-defined problem for the MESA users
list to answer easily, especially since we don't know what result you're
trying to reproduce, nor where you're starting from. From your description,
at best, one can speculate on some of the "obvious" ways that a star's
lifetime and radius might be affected.

It might be the case that you are just making 2 different stars. To make
such a comparison, you need to make sure you're starting from the same
place. Are you starting from the same or similar inlists? Are you using the
same nuclear reaction network? Do you have the same values for mixing
length alpha, winds, and core overshooting? Is everything else the same?

There are many ways to affect the radius of the star, its evolutionary
lifetime, etc, so it depends on how detailed a comparison you are trying to
make and how large a discrepancy you are seeing to try to discern what the
meaningful differences are and how to fix them. For example, the core mass
might be affected by core-boundary mixing/overshooting. In general, with
more overshooting, the larger the core becomes, which leads to higher
luminosity and often shorter evolutionary lifetimes. Likewise, changing the
nuclear reaction network may also significantly impact the onset of nuclear
reactions including He burning. Changing the mixing length alpha will
greatly affect the radius of the star, on the RSG branch typically larger
alpha_MLT = smaller radii / larger effective temperature (more flux carried
by convection = smaller R can give off the same L). Choosing a different
convective mixing scheme (e.g. Henyey vs Cox vs TDC) and a different
convective instability criterion (schwartzschild vs ledoux criterion) will
also affect the radius and other properties. If you are using MLT++
or superadiabatic reduction treatment, note that stronger superadiabatic
reduction/MLT++ also causes hotter stars and smaller radii.

A good reference on how wind prescriptions change the evolution of the star
is Renzo+2017
<https://ui.adsabs.harvard.edu/abs/2017A%26A...603A.118R/abstract>, and a
reference for the impact of many other "stellar engineering" choices
(including overshooting and nuclear reaction networks) on RSG properties is
Farmer+2016 <https://ui.adsabs.harvard.edu/abs/2016ApJS..227...22F/abstract>.
For a bit more hands-on intuition-building exercises, I might highlight
that we covered the impacts of various "stellar engineering" choices
(including superadiabatic reduction, mixing length, and stellar winds) in
this year's MESA (Down Under) School
<https://sites.google.com/view/massive-stars-mesa-down-under/> put together
with Ethan Winch, Annachiarra Picco, and Aldana Grichener. These labs are
not exhaustive and don't focus so much on the onset of He burning or
stellar lifetimes, but I think they do illustrate well the sensitivity of
evolved massive star structure to "engineering" choices.

Cheers,
~Jared

On Tue, Jul 23, 2024 at 3:36 PM Abha Vishwakarma <
abha.vishwakarma at niser.ac.in> wrote:

> Hi Jared,
>
> Thank you for your reply. I am using MESA version 23.05.1. I am trying to
> reproduce a stellar profile from a different work, and upon comparison I
> found that my metal mass fraction near the core is much higher than
> theirs, when I compare the models at the same star_age. The final radius of
> the star is also less than their work. I have been playing around with the
> wind schemes to see how they affect the evolution of the star, but I don't
> fully understand them. It seems as though my star starts to burn Helium
> earlier than their model. Please let me know if I am missing something
> here.
>
> Thank you,
> Abha
>
> On Wed, Jul 24, 2024 at 12:36 AM Jared Goldberg <
> jgoldberg at flatironinstitute.org> wrote:
>
>> Hi Abha,
>>
>> A few questions:
>>
>> 1) What MESA revision are you using?
>> 2) What do you mean by "much sooner"? When/where/under what conditions do
>> you want or expect the star to start core He burning?
>>
>> In general, a 12Msun star will become a Red Supergiant, not a red giant,
>> and it may even start core He burning while it is still crossing the
>> Hertzsprung gap.
>>
>> Best,
>> ~Jared
>>
>> On Tue, Jul 23, 2024 at 3:03 PM Abha Vishwakarma via Mesa-users <
>> mesa-users at lists.mesastar.org> wrote:
>>
>>> Dear MESA Users,
>>>
>>> I am trying to evolve a 12 solar mass star with metallicity 0.01 to the
>>> red giant phase. I am able to evolve it, but it is burning Helium into
>>> metals much sooner than I want it to. I am not sure which controls I should
>>> change to make sure this happens later. Is there any way I can achieve such
>>> a model?
>>>
>>> Best,
>>> Abha Vishwakarma
>>> _______________________________________________
>>> mesa-users at lists.mesastar.org
>>> https://lists.mesastar.org/mailman/listinfo/mesa-users
>>>
>>>
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