[Mesa-users] Blackbody temperature boundary condition

[Farag, Ebraheem]

Hello Abha,

The comment in the documentation you've identified "use_momentum_outer_BC T_surf is set to Tsurf_factor*T_black_body(L_surf,R_surf)" was incorrect/outdated in r15140 through r23.05.1 and now. I have revised it on github here<https://github.com/MESAHub/mesa/pull/971>. When use_momentum_outer_BC = .true., MESA still uses the Temperature boundary condition provided by the atmosphere unless otherwise specified. The momentum outer BC only affects the way the surface Pressure is calculated in the model.

Use_T_black_body_outer_BC was removed from the codebase shortly after r15140, however if you'd like to reimplement it, you can do so by using the "use_other_surface_PT" hook in your run_stars_extras.f90. In fact the example on how to use automatic differentiation in MESA uses this hook, so it's a good resource for how to do this https://docs.mesastar.org/en/25.12.1/auto_diff/example.html.

-EbF

________________________________
From: Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of Vishwakarma, Abha via Mesa-users <mesa-users at lists.mesastar.org>
Sent: Wednesday, April 15, 2026 12:04 AM
To: mesa-users at lists.mesastar.org <mesa-users at lists.mesastar.org>
Subject: [Mesa-users] Blackbody temperature boundary condition

Dear MESA Users,

I am working with MESA version 23.05.1 , and I am trying to implement a black body surface temperature boundary condition at the outer surface of a 12 solar mass red giant star with hydrodynamics switched on. I found the Tsurf_factor control and the description reads ' used when use_momentum_outer_BC T_surf is set to Tsurf_factor*T_black_body(L_surf,R_surf) '. Does this mean that if I switch on use_momentum_outer_BC the surface temperature gets set to Tsurf_factor*T_black_body ?

I also found that in the r15140 documentation, there's a parameter use_T_black_body_outer_BC which seems to do this more straightforwardly. But I don't find this in the 23.05.1 documentation. So, I would just like to know which controls to use to implement this boundary condition.

I would appreciate any help with this issue.

Sincerely,
Abha Vishwakarma
PhD Student
UIUC Astronomy
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