[Mesa-users] PPISN evolution with large nuclear network: timestep shrinks in late burning stages

[Farag, Ebraheem]

Hello,

Perhaps see section 10.2 in MESA VI, Jermyn et al. 2023<https://ui.adsabs.harvard.edu/abs/2023ApJS..265...15J/abstract>. If the only difference is the nuclear network, and no other issue in the model is being exhibited, the culprit is likely stiff nuclear burning from adopting a 280 isotope nuclear network. Perhaps try using op_split_burn. Controls in the latest version are found here<https://docs.mesastar.org/en/24.08.1/reference/controls.html#split-burn>, but refer to the defaults in r11701 for specifics. I'm not sure how will operator splitting will play out with the extreme hydrodynamics of a ppisn, but it will likely make the burning more manageable. I see you have gold_tolerances = .false. As well, so turning that back on and potentially playing with these might help:

! From your inlist
  relax_use_gold_tolerances = .false.
   use_gold_tolerances = .false.
   maxT_for_gold_tolerances = 5d9

-EbF

________________________________
From: Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of 翟朔 <zhaishuo at bao.ac.cn>
Sent: Monday, June 23, 2025 3:57 AM
To: mesa-users at lists.mesastar.org <mesa-users at lists.mesastar.org>
Subject: [Mesa-users] PPISN evolution with large nuclear network: timestep shrinks in late burning stages

Dear all,


I am currently simulating the evolution of pulsational pair-instability supernova (PPISN) progenitors using MESA version r11701.

My setup is based on the same inlists and run_star_extras.f90 modifications as used in Farmer et al. (2020).

I employ a large nuclear reaction network (280 isotopes) to resolve detailed nucleosynthesis during advanced burning stages.

However, I’ve encountered a critical issue: during late evolution (log Tc > 9.4, log ρc ~ 8), the timestep drops sharply (e.g., log dt ≈ -21),

causing the model to either freeze or progress extremely slowly.

This does not occur when using a smaller reaction network under otherwise identical conditions.

In addition to standard timestep and mesh controls, I have tried tuning the following parameters:
- Pextra_factor
- opacity_max
- mesh_delta_coeff
- max_dq
- min_D_mix
Despite these adjustments, the timestep collapse persists when using the large reaction network.

To facilitate feedback, I’ve uploaded a compressed test folder to Google Drive. It includes:
- the custom nuclear reaction network,
- initial metallicity settings,
- relevant inlists,

- history.data and log.txt from the problematic run.

You can download the test setup here:

https://drive.google.com/file/d/15fuWWYKaEFllDBMEoFTPU1WhEM3IsNg2/view?usp=sharing

If further output (e.g., selected profiles) would be helpful, I’m happy to share.
Any suggestions on how to stabilize timestep behavior in this context would be greatly appreciated.


Thanks and regards,

Shuo Zhai,

The National Astronomical Observatories of the Chinese Academy of Sciences
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