[mesa-users] The Cepheid phase

[Ehsan Moravveji]

Dear Hideyuki,

Regarding the choice of opacity tables, and in the light of CO sensitivity of the core, when growing, you need CO-enhanced opacities. For that purpose, you can, e.g. set the following in your star_jobs
      kappa_CO_prefix = ‘a09_co’
in addition to the following in the controls section:
      use_Type2_opacities = .true.
      Zbase = 0.014

Please read the documentation around the latter section to set other relevant options accordingly.

I hope this helps your cores consistently grow.

Best regards,
Ehsan.


> On 30 Aug 2016, at 02:37, Hideyuki Saio <saio at astr.tohoku.ac.jp> wrote:
> 
> 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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>> 
> 

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