[Mesa-users] Density discontinuity causing timestep problems

[Arman Aryaeipour]

Dear MESA-Users,

I'm having a problem with a density discontinuity that occurs in the region of a opacity bump in my white dwarfs envelope during a classical novae outburst.

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.

After I dredge-up white dwarf material into the envelope, the star begins to expand. When the star reaches ~17 solar radii, a portion of the outer envelope begins to have negative velocity and in-falls on the region of the envelope directly beneath it, which causes a density discontinuity which makes the timestep reduce to hundredths of a second.

I've attached two plots that show this. The first plot is called before_density_discontinuity.pdf and shows a profile of the star before this density discontinuity occurs while the star is at ~15 solar radii. The second is called after_density_discontinuity.pdf which shows the density discontinuity at cell ~410.

It looks like the opacity bump is causing this density discontinuity which makes sense since that region would absorb more of the luminosity from the burning regions at the base of the envelope and expand more rapidly causing a compression on the region above the opacity bump. However, I let the simulation continue for another 500,000 timesteps after this density discontinuity forms but the timestep never increases again and the simulation never goes anywhere.

Does anybody have any ideas on how I can overcome this issue? I have tried increasing the resolution in the region of the discontinuity, but this does not help. If anybody has any suggestions, I would be grateful.

My apologies for such a long email and thank you to anyone who took the time to read it!

Kindest regards,
Arman Aryaeipour
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