[Mesa-users] Cooling of WD with C/O atmosphere

[Evan Bauer]

Hi Mike,

I think your intuition about interacting with convection is correct. It turns out that diffusion can interact with regions marked as convective even when they aren’t doing any mixing though. Diffusion will try to ignore zones contained in a surface convective region to save resources. That’s part of what sets “nzlo” in the diffusion terminal output. Diffusion won’t operate in zones near the surface with k < nzlo, regardless of the convective diffusion coefficient there.

Instead of setting mixing to zero, I think you’ll have better success if you set this:

	mlt_option = 'none'

That way there will be no convection at all. I think it should be safe to set this at least until the Helium floats upward to form a pure He atmosphere in the outer 1e-12 solar masses or so.

When I ran your model, I also found that the residual winds were interacting with diffusion very near the surface, and things ran more smoothly if I just turned the winds off. They shouldn’t be needed anymore at this stage right?

Lastly, I found it helpful to change the diffusion substepping controls somewhat so that diffusion doesn’t spend too much time forcing retries when it hits a difficult patch:

      diffusion_steps_limit = 20
      diffusion_steps_hard_limit = 50
      diffusion_dt_limit = 1d-99

With those controls, and turning off mlt and winds altogether, I was able to get the atmosphere in your model to stratify fairly quickly.

Cheers,
Evan


> On Oct 25, 2021, at 11:49 AM, Montgomery, Michael H via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> Hi Everyone,
> 
> I gave some undergrads a project to calculate the cooling (and element diffusion) of a white dwarf that has Carbon and Oxygen throughout its interior, all the way to the surface. 
> 
> I mixed in a little Helium so that we could watch it diffuse up to the surface. At least that was what was supposed to happen. 
> 
> Without diffusion turned on this model goes from 150,000 K to 20,000 in about 25 seconds. But when I turn on diffusion, the diffusion of helium upward seems to interact with some narrow carbon convection zones, and this slows things down immensely. 
> 
> I thought that this might be due to the convective mixing interacting with the diffusive mixing, so I set “mix_factor = 0.0” which I think turns off all chemical mixing due to convection. The resulting profiles are smoother, but still quite slow. Evolving down to 38,000 K took about 10 hours, which is 1000 times slower than the non-diffusive situation. 
> 
> So to sum up, I feel I must be doing something wrong but I don’t know what it is.
> 
> I updated to version 15140 of MESA to see if that would help, but the results are the same. 
> 
> I’m attaching my inlist and input model (both for MESA version 15140) in case anyone can spot what I’m doing wrong.
> 
> Thanks!
> 
> Mike
> 
> 
> 
> 
> _____________________________________
> 
> Michael Houston Montgomery
> Research Scientist
> Deputy Director of CAPP
> Research Educator, FRI Program
> Department of Astronomy
> University of Texas
> mikemon at astro.as.utexas.edu
> www.as.utexas.edu/~mikemon/ <http://www.as.utexas.edu/~mikemon/>
> _____________________________________
> 
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