[mesa-users] The Cepheid phase

[Hideyuki Saio]

Hi,

I am writing about my calculations of Cepheid loops. 
I computed some models to He-exhaustion with a default setting with X=0.72, Z=0.014; mixing length = 1.7Hp, and no mass loss.

For one set of calculations, no overshootings were included. 

For another set I included tiny overshooting from the He-burning core boundary, by just inserting following two lines in my inlist :
         overshoot_f_above_burn_he_core = 0.001
         overshoot_f0_above_burn_he_core = 0.001

I attach to this mail evolution tracks for the two cases, in which He-burning stages are shown by solid lines, while dotted lines are tracks before He ignition. Numbers are stellar masses. 

The first file show tracks without any overshooting, the second one with tiny over shooting from the He-buring core.  As you see, in the first case small cepheid loops occur in most cases, while  well developed loops were obtained for all masses in the second case.

Judging from the results, I guessed, which might be a wrong guess, that MESA code might be determining  convective-core boundary by using the composition just exterior to the boundary.    If that is the case, I am afraid that it would suppress the growth of convective-core size, which should be induced by the increase of C/O abundance (and hence enhanced opacity) in the core as discussed in ‘70s (e.g., Paczynski 1970, AcA 20, 195). 

Overshooting, even if very tiny one, enhances C/O abundance (and hence opacity) just outside the boundary, which would shift the boundary slightly outward in the next time step, so that the convective core size would increase progressively.  I speculate, that is the reason why well developed loops were obtained if even a tiny overshooting was included.

Although I am not familiar with  MESA code, if the core boundary is determined based on the composition (opacity) just outside of the boundary, I wish the method to be modified.    Thanks.

Best regards, 
Hideyuki Saio


     
  


>> Hi,
>> 
>> This loop is really weird. I’ve studied blue loops pretty extensively and I can’t recall seeing anything like it. You can take a look at my paper (2015, MNRAS 447, 2378). We tried to determine whether pulsating B-type supergiants can be on blue loops or not. The considered masses are higher (M >= 13 Ms) but our loops behave better. I intend to publish a paper solely focused on the blue-loop problem in massive stars but that’s that's not going to happen until the end of the year.
> 
> Thanks Jakub.    Perhaps the message is that mesa/star will do weird things if you give it weird parameters!   The real question is whether can we get reasonable blue loops by some settings -- my plot shows how easy it is to get strange results.   In my book, you're the mesa expert on this, so we'll be interested to see what you have found.   
> 
>> Anyway, the problem is extremely complicated so even the easiest questions might be difficult to answer.
> 
> that sums it up nicely!
> 
> Bill
> 

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