[mesa-users] The Cepheid phase

Dennis Stello dennis.stello at sydney.edu.au
Tue Aug 30 05:41:58 EDT 2016


I would agree with Pablo on this one.

Cheers
Dennis/

DENNIS STELLO | Associate Professor | ARC Future Fellow
School of Physics | Faculty of Science

THE UNIVERSITY OF SYDNEY
Rm415, A28 | The University of Sydney | NSW | 2006
T +61 2 9036 5108  | F +61 2 9351 7726
E stello at physics.usyd.edu.au  | W http://www.physics.usyd.edu.au/~stello/

On 30/08/2016 5:59 PM, Pablo Marchant wrote:
> I would strongly oppose adding any kind of overshoot by default. This 
> would likely be a source of significant confusion for many, and could 
> possibly result in many publications inadvertently using overshooting 
> they're unaware of. The amount of overshooting used would also be very 
> arbitrary, and choosing a default would result in people using it just 
> because it is a MESA default.
>
> If you would ask me I would have everyone using the Ledoux criterion, 
> but I think the proper approach is for users to actually understand 
> what they're doing and turn that on themselves.
>
> On Tue, Aug 30, 2016 at 7:18 AM, Ehsan Moravveji 
> <e.moravveji at gmail.com <mailto:e.moravveji at gmail.com>> wrote:
>
>     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
>>     <mailto: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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>
> -- 
> Pablo Marchant Campos
> M.Sc on Astrophysics, Universidad Católica de Chile
> PhD student, Argelander-Institut für Astronomie
>
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