[Mesa-users] wd_diffusion doesn't work?

Tue Oct 3 14:21:53 EDT 2017

Hi Evan,

Thanks for your help!  I was trying to analyze diffusion in white dwarves, and didn't realize that wd_diffusion is completely inappropriate for this.

Following the release notes, I made a new inlist that uses OPAL opacities, sets diffusion_use_cgs_solver = .true., and enables radiative acceleration.  I used it on the 0.604_from_2.3_z2m2.mod model that came pre-packaged with MESA, and found that it takes around 200,000 years for a noticeable amount of heavy elements to settle out of the atmosphere.  This is far longer than what I would expect, and far longer than the diffusion timescales I've seen in papers (e.g. https://arxiv.org/pdf/0903.1499.pdf)

I'm not sure if I'm doing something wrong, but the inlist (attached) is very simple.  Any help is appreciated!

Michael

________________________________
From: Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of Evan Bauer <ebauer at physics.ucsb.edu>
Sent: Monday, October 2, 2017 11:59:50 AM
To: Zhang, Zhaoxi M. (Michael)
Cc: mesa-users at lists.mesastar.org
Subject: Re: [Mesa-users] wd_diffusion doesn't work?

Hi Michael,

Short answer: yes, this is expected behavior. You’re right to call attention to this sort of thing, but I wouldn’t worry about this particular test right now.

Long answer: this test case is out of date. It isn’t meant to test diffusion timescales, but conditions for diffusive equilibrium, so the length of timesteps shouldn’t be relevant for the physics being tested. It starts from a model that is already near diffusive equilibrium, and then runs for long enough (a few Gyr) that many diffusion timescales should have occurred in the atmosphere and H/He should have achieved diffusive equilibrium. It then tests against an analytic expression for H/He abundance profiles in diffusive equilibrium. See figure 23 in the second instrument paper for more details.

All of this is out of date because the wd_diffusion routines assume ideal gas for diffusion, as do the analytics that they test against. They agree very nicely, but the ideal gas EOS is not the correct thing to use in a WD, so this test isn’t very useful.

If you want to do diffusion in a WD, I strongly recommend going through the release notes for 9793: https://sourceforge.net/p/mesa/mailman/message/35869775/
mesa / Mailing Lists - SourceForge<https://sourceforge.net/p/mesa/mailman/message/35869775/>
sourceforge.net
Hello MESA Users, We are happy to announce a new MESA public release version: 9793. Release notes below. Cheers, Evan (on behalf of Pablo, Rich, Josiah, Rob, Aaron ...

If you still have convergence issues after that, let me know and I can send more suggestions.

Cheers,
Evan

On Oct 2, 2017, at 11:42 AM, Zhang, Zhaoxi M. (Michael) <mz at astro.caltech.edu<mailto:mz at astro.caltech.edu>> wrote:

Hello everyone,

In MESA 10000, I ran wd_diffusion in the test suites.  It worked, but its timesteps were almost 100 million years, which I thought was far higher than the diffusion timescale.    So I decreased the timesteps to see how the elements diffuse initially:

max_years_for_timestep = 1d3

and the test stopped working.  It struggles to converge, and fails with:

stopping because of problems dt < min_timestep_limit
terminated evolution: cannot find acceptable model

Is it expected behaviour for this test to fail due to smaller timesteps?  Why would having a smaller timestep create convergence problems?

Cheers,

Michael

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