[Mesa-users] "Mirror star nuggets" in MESA with modified gravity and heating rate

[Jeremy Sakstein]

Hi Franco,

I would have thought that a combination of other_cgrav and
other_energy_implict would be enough for what you are trying to simulate.

What values of G are you using?

My experience with scaling G by a factor >1.5 is that these types of errors
occur. My solution is to increase it slowly from standard_cgrav to your
required value over some time-scale T where T is chosen to be long enough
to allow convergence but shorter than the timescale of the physics you are
interested in.

An example that I have used in the past is G(t) = G0 (1 + a[1-Exp(-t/T)])
where G0 is standard_cgrav, a is whatever scaling factor you want to apply,
and t is star_age. This ensures that G=G0 at times shorter than T but
G=G0(1+alpha) by the time the star has reached the phase you are interested
in.

Cheers,

Jeremy

On Sun, Jan 5, 2025 at 2:26 PM Franco Cabral via Mesa-users <
mesa-users at lists.mesastar.org> wrote:

> Dear all,
>
> I am reaching out for advice on an issue I am encountering while trying to
> model optically thick "mirror star nuggets" in MESA (version 24.03.1) as
> part of a dark matter study. These nuggets are intended to represent
> compact objects with around the following specifications:
>
>    1. Mass: 10^20 g
>    2. Radius: 10^9 cm
>    3. Heating Rate: 10^10 erg/g/s (implemented via extra_power_source in
>    the inlist), driven by suppressed interactions with dark matter.
>    4. Modified gravity: Gravitational acceleration set by a mirror star
>    core density of around 150 g/cm^3.
>
> I have modified create_initial_model.f90 to change the local gravity
> definition there and its associated differential equations. With these
> changes, MESA is able to converge on an initial model with specified mass
> and radius. However, once this initial model is passed to the main MESA
> solver, it encounters tolerance and convergence errors in the first
> simulation step.
>
> Here are my attempts so far in changing the gravity which have been
> unsuccessful:
>
>    1. Modifying MESA's global m_grav definition in star_utils.f90.
>    2. Using the other_cgrav hook.
>    3. Using the other_momentum_implicit hook.
>
> I suspect that the nugget is evolving too rapidly from its initial model.
> Adjusting certain tolerance switches may help, but I’m unsure which ones
> are most relevant or if the issue lies elsewhere. I would greatly
> appreciate any advice on this matter.
>
> I am attaching a GitHub link with my code changes and a sample run output
> using the other_cgrav hook to show the errors.
>
> https://github.com/MESAHub/mesa/compare/main...fmcabral123:mesa:main
> <https://urldefense.com/v3/__Https://github.com/MESAHub/mesa/compare/main...fmcabral123:mesa:main__;!!PvDODwlR4mBZyAb0!RyMJojKiPqrEenO8JWmYKvVhJQq-AESDNilmRuqMqRfNSGxUEgM_8pLXegWmj8aAQ7oPnxFKY4l1IH2s6EmHgMrcAbY$>
>
> Sincerely,
> Franco Cabral
> _______________________________________________
> mesa-users at lists.mesastar.org
>
> https://urldefense.com/v3/__https://lists.mesastar.org/mailman/listinfo/mesa-users__;!!PvDODwlR4mBZyAb0!RyMJojKiPqrEenO8JWmYKvVhJQq-AESDNilmRuqMqRfNSGxUEgM_8pLXegWmj8aAQ7oPnxFKY4l1IH2s6EmHsGLAdK0$
>
>

-- 
Jeremy Sakstein (He/Him/His)
Assistant Professor of Theoretical Physics
Department of Physics & Astronomy
University of Hawaiʻi at Mānoa
Watanabe Hall
2505 Correa Road
Honolulu, HI, 96822, USA
http://www.jeremysakstein.com
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