[Mesa-users] MLT++ for massive MS stars?

[Francis Timmes]

hi giovanni,


> ... I just want to reach hydrogen depletion for these stars, ...

ok. it would be useful to examine/show a few more diagnostics.
what does the hr diagram look like from the start of the model
to the point where convergence is not reached? what are the density
and temperature profiles, especially in the outer regions?


> I have already written to you a little while ago about this ...

presumably the "Massive stars and timestep management" thread you started on 12nov2020.

my reply in the thread "Timestep and convergence problems for massive stars" 
may (or may not) contain a useful path forward for version 12778.

fxt





> On Nov 25, 2020, at 3:58 AM, GM Mirouh via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> Dear all, 
> 
> I have already written to you a little while ago about this, and I am sorry to insist. 
> I am computing massive stars on the main sequence stopping once in a while to save a model (with mesa 12115). It works nicely for M<320Msun, but I am hitting all kinds of numerical difficulties for anything above that:
> 
> 1) at some point during evolution, the timestep drops to almost nothing, and I think this is due to the luminosity hitting the Eddington limit (at Z=0.02, M=500, center_h1=0.45, my star has Teff=7600, logTcntr = 7.6 and logL=7.2 and evolves with log_dt=-0.3 ). It keeps running for a little while though, albeit very slowly, until...
> 
> 2) my runs end up crashing with errors such as
> 
> " retry  193406
>                      mlt_partials(mlt_cv_var,mlt_gradT)         792                       NaN
>  retry  193406
>                      mlt_partials(mlt_cv_var,mlt_gradT)         792                       NaN
> backup  193406
>                      mlt_partials(mlt_cv_var,mlt_gradT)         774                       NaN
>  retry  193405
>                      mlt_partials(mlt_cv_var,mlt_gradT)         774                       NaN"
> 
> Can anyone enlighten me about what is happening here? I am going out some interpolation table? is there something I can do?
> 
> 3) I have tried playing with some numerical parameters, such as the starting timestep or the varcontrol value, and I have varied mixing parameters such as semiconvection or overshooting efficiencies, to no avail: the code as I have set it up seems to be digging a hole it cannot get out of.
> 
> 4) Some people have mentioned MLT++ as a possible solution, while some others told me to avoid it like the plague. Recent events show people do not really avoid the plague, so I thought I could give MLT++ a try: I have turned "okay_to_reduce_gradT_excess = .true." but it does not seem to change much in terms of convergence (I have the exact same issues, only earlier on the MS). Do you know of a manual or do you have general advice for MLT++? Would it make sense to compute the MS as long as it works, save a model and then turn MLT++ on and proceed? Or maybe MLT++ is not the right approach at all? 
> 
> At this point, I just want to reach hydrogen depletion for these stars, and I am ready to make reasonable concessions about the accuracy of my models as long as I know what is happening in there. 
> I am sorry I don't know what kind of files to attach here, but I can provide anything you think can help you to help me. Thank you for any advice you can give me. 
> 
> Please take care,
> Best.
> -- 
> Giovanni Mirouh
> 
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