[mesa-users] Enabling diffusion in Mesa-r8118

[Evan Bauer]

Hi Émile,

Jared’s answer seems to cover the likely culprit, but I took a look at the inlists you attached, and I have a few more suggestions. Unfortunately, diffusion in MESA sometimes requires more commands than just “do_element_diffusion = .true.” if you want reasonable results. It may be worth reading through the diffusion section of controls.defaults (http://mesa.sourceforge.net/controls_defaults.html#element_diffusion <http://mesa.sourceforge.net/controls_defaults.html#element_diffusion>), though there are certainly more controls there than you need to deal with. To get an idea of the other types of commands you might need to play with, I’d recommend looking at two test suite cases:

star/test_suite/1.5M_with_diffusion
star/test_suite/wd_diffusion

I wouldn’t worry too much about the solver controls you see there (unless you have convergence issues), but the “diffusion_class…” controls may be something you care about. 

In addition to the commands found there, since you’re dealing with a neutron star, you may want to consider using the option
diffusion_use_cgs_solver = .true.
With this option turned on, MESA’s diffusion routine will do a better job accounting for the effects of electron degeneracy. 

By the way, your question deserves some sort of award for well-statedness. Including a github repository like you did makes it fantastically easy to diagnose issues and answer questions. (Or pile on and offer unsolicited advice in my case.)

Cheers,
Evan


> On May 20, 2016, at 12:06 PM, Jared Brooks <jcbrooks at physics.ucsb.edu <mailto:jcbrooks at physics.ucsb.edu>> wrote:
> 
> Hi Émile,
> 
> All your timesteps are less than a year, and if you look in controls.defaults, right under do_element_diffusion is a control named diffusion_dt_limit, which is default set to 3.15d7 (1 year in seconds). Try lowering this and see if diffusion starts working.
> 
> -Jared
> 
> On Fri, May 20, 2016 at 10:50 AM, Émile Jetzer <emile.jetzer at mail.mcgill.ca <mailto:emile.jetzer at mail.mcgill.ca>> wrote:
> I have been trying to do simulation of a neutron star based on `ns_h` from the standard test suite, with diffusion. However, I've found that setting `do_element_diffusion` to `.true.` in `controls` did not change the result I got. To make sure, I ran tests with no accretion, & the pgstar output was the same in the case with & the case without diffusion. Is there something I am missing? The full code is available at http://github.com/ejetzer/neutronstars2 <http://github.com/ejetzer/neutronstars2> .
> 
> I have done something similar successfully in a previous version of Mesa, but cannot catch what I might be missing this time. The old code is at http://github.com/ejetzer/neutronstars <http://github.com/ejetzer/neutronstars> .
> 
> Thanks,
> Émile Jetzer.
> 
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