[Mesa-users] Ni56 getting converted to Fe isotopes during SN shock breakout

[Mandal, Soham (uqa7ur)]

Thanks a lot for that clarification, Jared! I remember this from our previous discussion. I'm not sure though if I understand why the value of log X_Ni (logarithm of the Ni fraction) goes from about -2 to about -10 in the entire ejecta profile, between shock_part4 and shock_part5.

I expected the nickel to be removed as a result of excising the proto-neutron star, which then wouldn't affect the outer layers.  Should I expect the nickel to convert to Fe52 and Fe54 at this stage, and add as much nickel as desired by x_ctrl(12)? If yes, could you please help me understand why?

Thanks,
Soham
________________________________
From: Jared Goldberg (Guest Researcher) <jgoldberg at flatironinstitute.org>
Sent: Wednesday, April 8, 2026 12:10 PM
To: Mandal, Soham (uqa7ur) <soham at virginia.edu>
Cc: mesa-users at lists.mesastar.org <mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] Ni56 getting converted to Fe isotopes during SN shock breakout

The Ni56 is reset in inlist_shock_part5 to a user-specified value via a set of x_ctrls which are called during the set_nico_mass routine in run_star_extras.f90.

Specifically the lines:
  x_ctrl(12) = 1d-15 ! amount of ni56 to add << This is in Msun
  x_ctrl(35) = -1d+99 ! put ni56 this + m_center
  x_ctrl(36) = -1d+99 ! end ni56 at this + he_core_mass

In short, you're telling it to remove all the nickel. Set x_ctrl(12) to -1 to ignore the switch, or set it to your value of choice (I think ~ 0.03 - 0.07 are typical for partially-stripped IIP's).

In general, overwriting abundances is sometimes but not always the issue in STELLA.

Best,
~Jared

On Wed, Apr 8, 2026 at 11:49 AM Mandal, Soham (uqa7ur) via Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>> wrote:
Dear MESA users,

I have been trying to model core collapse and eventual shock breakout in a massive star, using the 'ccsn_IIp' problem in star/test_suite as a template. The pre core-collapse star is a partially stripped massive star of solar metallicity, with M_ZAMS = 16Msun, and M_env (at core collapse) = 1.27Msun. The eventual goal is to handoff these models to STELLA and produce lightcurves.

STELLA crashes early and produces erroneous lightcurves. Upon investigating the shock breakout phase and looking for sharp abundance changes in the profiles, I find that Ni56 abundances in the model profiles suddenly drop dramatically between inlist_shock_part4 and inlist_shock_part5. This is accompanied by sudden increase in Fe52 and Fe54 abundances. The temperature profile remains around log T[K] ~ 5.5, which rules out nuclear burning. I do not understand (i) why all the Ni56 is getting converted into Fe isotopes, and (ii) if this is related to numerical issues encountered by STELLA.

I'd greatly appreciate if someone is aware of/encountered similar issues with shock breakout in MESA and could chime in! The inlists and a snapshot of the profile abundances for shock_part4 and shock_part5 are attached to the email. Thanks for your attention! Please let me know if more information is required.

Best,
Dr. Soham Mandal
VITA postdoctoral research fellow
Department of Astronomy
University of Virginia
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