[Mesa-users] The evolution of pre-ccsn

[Ebraheem Farag]

Hello Wenyu,

Your Ledoux model has a larger Helium core mass than your schwarzschild
model, therefore it is unsurprising that their nuclear evolution is
different.

In either case, you're entering the regime of propagating flames in a
degenerate O-Ne-Mg, a numerically difficult area. In your ledoux case, you
have encountered a conductively burning neon flame slowly propagating
inward, and in your schwarzschild model, it simply crashes for whatever
reason likely due to numerics (1000 timesteps to get from zams to
Ne-depletion is quite coarse timestep resolution).

Getting through ne-core melting in your schwarzschild model can perhaps be
done by restarting with significantly smaller timesteps. In the case of
your ledoux model, propagating the flame in a MESA evolution calculation is
a numerically challenging practice which has only ever been done by a few
groups of people and for maybe a few models.

For the flame, it can be done, but with hundreds of thousands if not
millions of careful timesteps, with controls designed to prevent the flame
from stalling. Actual flame widths are of the order of < 10^-2 cm,
significantly smaller than the actual width of a zone in a normal MESA run.
Without a sub-grid flame model to know the real flame speed, thermal
diffusion in MESA essentially heats one zone at a time until it is hot
enough to burn. To make matters worse, neutrino emission cools the
degenerate core ahead of the flame, making it even more challenging. The
white dwarf basically acts like an ice cream popsicle, and the solver can
only take small licks without getting a brain freeze :)

See Sam Jones et al. 2013
<https://iopscience.iop.org/article/10.1088/0004-637X/772/2/150>, or more
recently Marco Limongi et al. 2023/2024 <https://arxiv.org/abs/2312.00107> (not
in MESA) for more information on what it takes to get one of the models to
evolve through this difficult phase.

An example of resolving the flame width and speed in a 10g ball can be
found in the conductive flame test suite
<https://docs.mesastar.org/en/release-r24.03.1/test_suite/conductive_flame.html>.
You can also check out https://cococubed.com/research_pages/flames.shtml,
for more information on flame physics.

If you want to avoid propagating flames entirely, I suggest you increase
your initial mass or modify your overshooting scheme to generate a large
He-core. Otherwise, your ledoux model might be able to hit CC, but you'll
need to really hammer the timesteps a bit more.

This 9.5Msun movie
<https://drive.google.com/file/d/14sja1hHXrAXsxk8Op3m3nUIjvEkMzK0I/view?usp=sharing>
was produced by slightly modifying the  12Msun massive star test_suite
<https://docs.mesastar.org/en/release-r24.03.1/test_suite/12M_pre_ms_to_core_collapse.html#m-pre-ms-to-core-collapse>
in
the latest MESA release-r24.03.1. (with approx21 only, not ideal for
degenerate cores dominated by electron captures.)
This is an example of a 9.5Msun model taken to cc through a phase of flames
(in the current release version), however a variety of tweaks had to be
made for this to work, including suppressing overshooting substantially
(like in Jones et al. 2013), and even then the he core mass in this 9.5Msun
model is larger than your 11 Msun model, which only makes your model more
difficult to evolve. The lower the core mass, and the more degenerate and
cold your core gets, the harder this phase becomes to resolve. In fact you
really need a large nuclear network to accurately evolve through this phase
as electron captures among other weak interactions modify the electron
fraction of the core during this phase, which can fundamentally alter the
rho-T of the core and are needed to capture the the urca neutrino cooling
process and generate electron-capture sn progenitors.

Hope this helps!

-EbF

On Tue, Mar 19, 2024 at 2:33 AM 辛文宇 <xinwenyu16 at mails.ucas.ac.cn> wrote:

> Hi everyone,
>
>
> I am evolve a pre-ccsn model using mesa-12778 with a initial mass of 11
> Msun.
>
> But the calculation stop near the Ne burning because the timesteps are too
> small.
>
> The output information in the terminal is attached.
>
> I tried with both Schwartshild and Ledoux criterion and compared them in
>
> the attached figures.
>
>
> Is there anyone can help me to solve my problem?
>
>
> Thanks very much!
>
> Wenyu
> _______________________________________________
> mesa-users at lists.mesastar.org
>
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