[Mesa-users] Blue loops

[Radek Smolec]

You don't need helium burning to get Cepheid. If the model enters the
instability strip then it can pulsate as Cepheid.
For evolutionary scenario with a loop we have three crossings of the
instability strip. During the first crossing we have a very fast evolution
towards the RGB with no central nuclear energy source (shell hydrogen
burning).
This evolutionary phase is very fast; first crossing is ~2 orders of
magnitude shorter than the second (blueward evolution) and the third (again
redward) crossings that occur during the central helium burning
 and blue loop evolution. So we expect most of the observed Cepheids are
core helium burning stars.

For more massive models, like 13Ms, we may have only one crossing on the
way to RGB and I think central helium burning may already take place - but
again, you have to check with your model (look for central he column in
your history file).
I haven't played with such massive models so I cannot tell more precisely
what is the mass above which central helium ignition takes place already
during the evolution towards RGB and how it depends on other model
parameters.
Maybe other users can share their experience here.
Cheers,
Radek


pt., 5 kwi 2019 o 16:45 Jeremy Sakstein <sakstein at sas.upenn.edu> napisał(a):

> Thanks Radek,
>
> Does that mean that some Cepheids observed (say 13M) are crossing the
> instability strip on their journey to the RGB or do I need to put special
> parameters into my I list to delay helium core burning?
>
> Cheers,
>
> Jeremy
>
> On Thu, Apr 4, 2019 at 2:21 AM Radek Smolec <smolec at camk.edu.pl> wrote:
>
>>
>> One final question: I can’t  seem to get blue loops that cross the
>>> instability strip for say 13M models.
>>>
>> Above some mass loops disappear in most evolutionary calculations I'm
>> aware off. Above 10Ms loops may be difficult to get, independent what
>> parameters you are using.
>> Check when core helium burning starts in your model. For massive models
>> central helium burning starts already on the way to RGB and loops do not
>> develop.
>> Cheers,
>> Radek
>>
>>
>>>
>>> Do I need to change the controls in the inlist due to other physical
>>> processes kicking in?
>>>
>>> Cheers,
>>>
>>> Jeremy
>>>
>>> On Wed, Apr 3, 2019 at 12:54 PM Jeremy Sakstein <sakstein at sas.upenn.edu>
>>> wrote:
>>>
>>>> Thanks Radek,
>>>>
>>>> It looks very similar to the inlist I am using so that is good. The
>>>> only difference is the mixing length scheme. I think for my purposes as
>>>> long as I am consistent this should not matter too much.
>>>>
>>>> Cheers,
>>>>
>>>> Jeremy
>>>>
>>>> On Wed, Apr 3, 2019 at 11:34 AM Radek Smolec <smolec at camk.edu.pl>
>>>> wrote:
>>>>
>>>>> Jeremy,
>>>>> for 5M_cepheid_blue_loop see the recent public release.
>>>>> I attach the relevant inlist for your reference.
>>>>>
>>>>> You may also take a look at Section 2.4.2 of the instrument paper:
>>>>> https://arxiv.org/abs/1903.01426
>>>>> Tracks in Figure 13 were computed using this test_suite example.
>>>>> Cheers,
>>>>> Radek
>>>>>
>>>>>
>>>>> śr., 3 kwi 2019 o 17:25 Jeremy Sakstein <sakstein at sas.upenn.edu>
>>>>> napisał(a):
>>>>>
>>>>>> Thanks Radek,
>>>>>>
>>>>>> Comments below.
>>>>>>
>>>>>> Cheers,
>>>>>>
>>>>>> Jeremy
>>>>>>
>>>>>> On Wed, Apr 3, 2019 at 11:21 AM Radek Smolec <smolec at camk.edu.pl>
>>>>>> wrote:
>>>>>>
>>>>>>> Dear Jeremy,
>>>>>>>
>>>>>>> śr., 3 kwi 2019 o 16:55 Jeremy Sakstein <sakstein at sas.upenn.edu>
>>>>>>> napisał(a):
>>>>>>>
>>>>>>>> Dear Mesa community,
>>>>>>>>
>>>>>>>> I've been using MESA to calculate blue loops for Z=0.0006 Cepheids
>>>>>>>> in the mass range [image: 5-13 M_\odot] and had a couple of
>>>>>>>> questions I was hoping you could answer.
>>>>>>>>
>>>>>>>> Nominally, I am a theorist so my approach so far has been to use
>>>>>>>> the inlist for 7M pre MS to AGB with
>>>>>>>>
>>>>>>> You may also start with 5M_cepheid_blue_loop in the test_suite.
>>>>>>>
>>>>>>
>>>>>> Thanks. I didn't see this in the test suit (I am using 10398*)*
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> mixing_length_alpha = 1.73
>>>>>>>> initial_z = 0.0006
>>>>>>>> alpha_semiconvection = 0.1
>>>>>>>> use_Ledoux_criterion = .true.
>>>>>>>>
>>>>>>>> and cary the initial mass over this range.
>>>>>>>>
>>>>>>>> Initially, I was only using 7-9 [image: M_\odot] but now I need to
>>>>>>>> push to a larger range and am getting funny results e.g. the loops cross.
>>>>>>>>
>>>>>>> What do you mean by "the loops cross"? That single evolutionary
>>>>>>> track passes through the same (Teff, L) point? For larger masses this is
>>>>>>> expected.
>>>>>>>
>>>>>>
>>>>>> Yes, that is what I mean. Good to know this is normal.
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> I am wondering if what I am doing is sensible given that the
>>>>>>>> physics of these objects could be very different depending on mass. In
>>>>>>>> particular, I am not sure what people typically use for e.g. the
>>>>>>>> semi-convection parameter at higher and lower masses.
>>>>>>>>
>>>>>>> I'm not sure what is the range of semi-convection parameter
>>>>>>> considered in the literature. 0.1 seems raesonable.
>>>>>>> Note that the shape and extent of the loops is very sensitive to
>>>>>>> many parameters: chemical composition, overshooting from the convective
>>>>>>> regions, rotation...
>>>>>>> Cheers,
>>>>>>> Radek
>>>>>>>
>>>>>>>
>>>>>>>> I'd be grateful for any help you can give.
>>>>>>>>
>>>>>>>> Cheers,
>>>>>>>>
>>>>>>>> Jeremy
>>>>>>>> _______________________________________________
>>>>>>>> mesa-users at lists.mesastar.org
>>>>>>>> https://lists.mesastar.org/mailman/listinfo/mesa-users
>>>>>>>>
>>>>>>>>
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