[mesa-users] crazy C shell burning

[Josh Shiode]

Huh. That is weird. Maybe send your inlists. 

I've had very little trouble running a grid that covers 12 -- 100 Msun, metallicity from 0 -- Solar, rotation from 0 -- 0.8 Critical... 

I'm using Ledoux w/alpha=1.5, alpha_sc = 0.1 (though I've done some runs with 0.01 and 1, as we discussed before)... 

I use mesh_delta_coeff = 0.5 up to the end of C-burning and then mesh down to 1.0 to speed the rest of the evolution (though I'm seeing some bad behavior in some of the lower mass models when I change to lower res, so I've been messing with this lately).

I don't think I've ever seen what you're seeing. Certainly not where it gets into an unending loop that avoids moving on to Ne burning.. Is some fresh fuel being mixed downward that causes the star to relax back to some lower rho_c, T_c state? (I'm not really sure how this would work actually..) What's going on outside the core is the question, I guess? Do you have a Kippenhan diagram or similar?

--Josh



>>> Dear MESA users,
>>> 
>>> many of us are now well aware that due to lack of NSE treatment you're likely to get into trouble during Si burning. But in my 2+ year experience with MESA I have seen many times this weird behavior well before Si ignition, during the C burning. The attached plot shows the central T and rho evolution during C burning for two identical 15Msun models with different alpha_mlt. You see the alpha_mlt=2 case goes smoothly, but the alpha_mlt=1 case is doing some crazy stuff. I could wait for thousands of time steps and it would be just doing its weird loops back and forth, and never manage to ignite Ne.
>>> 
>>> With older MESA versions I noticed sometimes even very slight changes in the composition could result into this problem during C burning. If I remember correctly, during the last summer school Josh Shiode also reported similar results when he played with the time step and zoning (?) parameters.
>>> 
>>> I was wondering if anybody had looked into details of why this happens? how to avoid? previously I used to almost 'randomly' play with certain parameters until I find that sweet spot to make it through C burning. But now I am trying to make a detailed comparison with another code, so i need to be a bit careful.
>>> 
>>> thanks,
>>> Tuguldur
>>> 
>>> ***just to clarify, in this plot the alpha_mlt=1 case only shows data since the ignition of C in the core. While the other case shows data from previous evolution.
>>> 
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