Re: [mesa-users] main sequence problem

[Matteo C. <ca...@ki...>]

Hi Jakub,

> For the last few weeks I've been studying some problems I found and I'd like to start with presenting the most serious one, probably connected to the others.
> 
> There is a serious problem with discontinuities of the main sequence of more massive models (M > 10 M_S), when you use Schwarzschild criterion without overshooting. Take a look at this HR diagram for 20 M_S star (red line).
> <20m-hr-ms.pdf>
> I can find here four discontinuities. In my opinion this is something which can't be present in any evolution code.

I am a bit puzzled by this statement. Why you say that? Just because it doesn't look smooth? 

> I attached an inlist for this case, so you can see which settings I use. I calculated A LOT of models of 15 and 20 M_S with many different parameters to find if something can solve this problem but I got stucked. Nothing helps. So either I don't know which others settings I could use or there is a serious problem deeper in MESA's equations, solver or something like that.

If you want to understand what's going on, I suggest you plot profiles of composition across the convective boundary before and after your little discontinuities.
And you do this for models with different spatial resolution as well. You can do it with bare Schwarzschild and then adding a little overshooting.

> So for me it looks like additional mixing or smoothing the composition which are introduced by overshooting or semiconvection help to mask some significant problem which I don't understand and can't get fixed on my own.
> I think this is very important to fix it because without good main sequence it is completely pointless to study more advanced stages of the evolution when errors propagate extremely fast - you know that the helium burning is seriously affected.

Again, you speak like you know this has to be an error in the code. But at the same time you just said you don't know what's going on. 
I suggest you investigate first the reason for a certain behavior before asking the MESA community for a (n eventual) fix.
Otherwise I find more appropriate to just point out an interesting behavior, admit you do not understand what's going on, and ask if anybody else had a similar experience / has suggestions towards a better understanding.

Your claim that this makes pointless to study late phases of evolution is also puzzling. I guess your point is that those tiny jumps in L and Teff, that you only get when evolving a massive star with bare Schwarzschild, MLT and all the limitations in your 1D prescriptions, are the biggest sources of uncertainties in the study of advances stages of evolution? 
Even if it turns out you are right in saying MESA has a problem in the treatment of convective boundaries in the bare Schwarzschild case, from your plots it seems hard this might impact in a serious way post MS results. But of course I might be wrong here. It's just that your statement 'helium burning is seriously affected' is not backed up by any quantitative plot.
  

> In my opinion at this moment even the old and outdated Warsow - New Jersey code is much better in calculating simple hydrogen burning evolution than 'state of the art' MESA.

It might well be the case!  If so, it would be very nice to understand where the differences in the treatment of the convective boundaries arise between the two codes.
MESA is open source, and so comparisons are possible. 

> 
> There is one more thing that could be related to this problem. The behavior of main sequence improves with overshooting and with lowering the initial mass of model. The other thing that improve with these parameters in the same way is the presence of small loops after TAMS. Take a look at these tracks for 15 M_S (calculated with default zams - so no discontinuities on MS again) for different values of overshooting (step overshooting has been used).
> <15m-hr-ov-step-tams.pdf>
> Track from WNJ is shown for comparison. The presence of this loop is expected

Is it? Do you have observations of stars with a bare Schwarzschild core showing these objects do a loop at the TAMS???

Jakub, I am not trying to say this is not an interesting problem. And I truly appreciate the effort you are putting into sharing your findings with the community.
I just believe you should be a little more cautious with your statements, as sometimes these might misguide people. 
Keep up with the good work, and let us know what you find with the evolution of the composition profiles at core boundary and, eventually, the comparison with the WNJ code.

Matteo


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Dr. Matteo Cantiello ------------------------
Kavli Institute for Theoretical Physics 
Room 1226 Kohn Hall CA 93106-4030
University of California, Santa Barbara
---------------- http://matteocantiello.com/