[mesa-users] The Cepheid phase

Hideyuki Saio saio at astr.tohoku.ac.jp
Mon Aug 29 20:37:42 EDT 2016


Dear Ehsan,

> In the instrument paper II, Fig. 15, the evolution of non-degenerate convective He burning cores using Ledoux and Sch criteria are shown. Indeed, the behaviour of the core depends on two factors: the (extra) mixing at the boundary, and the CO-enhanced opacities.
> Fig. 15 shows that Sch with tiny little overshoot shall give you a growing core; so, the machinery is already there.
Thanks for indicating this. I didn’t know it.

> Two questions: 
> - For a fixed mass, how does you Mcc vs. t_He behave? Can you supply a figure comparing models with and without overshoot?
Sorry, I cannot produce it immediately, because I am not sufficiently familiar with MESA.

>   Do you use Ledoux or Schwarzschild criteria?
I didn’t specify it. So, probably Schwarzschild criterion.
> - Are you using CO-enhanced type II opacity tables?
About opacity, I included only the line:
      kappa_file_prefix = 'a09'

Thanks.
Hideyuki



> 
> 
> 
>> On 29 Aug 2016, at 11:50, Hideyuki Saio <saio at astr.tohoku.ac.jp <mailto:saio at astr.tohoku.ac.jp>> wrote:
>> 
>> 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
>> 
>> 
>> <hrd_ceploop_noheos.pdf>
>> <hrd_ceploop_heos001.pdf>
>>      
>>   
>> 
>> 
>>>> 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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