[Mesa-users] Density inversion in outer envelope of classical nova simulation

[Arman Aryaeipour]

Dear MESA-users,

I'm using MESA revision 21.12.1 to simulate classical nova outbursts. I'm using a 1 solar mass carbon-oxygen white dwarf and accreting solar composition material atop it. Once the WD-envelope interface temperature reaches 10^8K I use the other_mlt_results hook in the run_star_extras routine to artificially dredge-up WD material into the envelope until the envelopes metallicity reaches 20%. Once the envelopes composition is homogenised, I stop using the other_mlt_results hook and go back to using the standard Cox MLT prescription for convection in the star.

The problem I'm experiencing is that once the star starts expanding, a density inversion occurs in the outer envelope which causes the timestep to reduce to ~10^-2 seconds and the timestep never increases again. To clarify, when the envelope is expanding and the density inversion occurs in the outer envelope, I am no longer using the other_mlt_results hook and the MLT prescription being used is the standard Cox prescription.

I was wondering if anyone has suggestions on the best way stop the density inversion occurring or smoothing the density inversion once it does occur.

Would using the Rayleigh-Taylor instability help? The reason I'm not sure using RTI would help is because, if I understand correctly, the RTI routines would calculate a diffusion coefficient that would only mix the stars chemical composition, and the structure and density inversion would remain unchanged. Please correct me If I am wrong about this though!

I would greatly appreciate any advice or suggestions and thank you for taking the time to read this email.

All the best,
Arman
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