[mesa-users] Jumps in temperature during evolution of helium stars

[Pablo Marchant]

Thanks Aaron, Tuguldur. I know some of these models show a bifurcation of
the convective core, but when it happens it does at a different point in
time than this temperature jump. I also checked the EOS and my profiles are
far from the OPAL/SCVH blending region.

By checking the profiles though I noticed something I missed before. The
jump happens when mass loss digs deep enough to reach regions where helium
has been burnt. It's not a jump in composition but rather a jump in the mu
gradient at the surface, and this seems to cause the big change.

So there is some physical aspect to it in the end. I guess I can soften the
change by using exponential overshooting or extra mixing so that the
mu-gradient does not suddenly change. Guess I'll try that and see how it
works.

On Fri, Dec 11, 2015 at 11:53 PM, Aaron Dotter <aaron.dotter at gmail.com>
wrote:

> Hi Pablo,
>
> I am suspicious of the EOS, particularly the blend between OPAL and SCVH.
> Can you make some T-rho profile plots and compare with the ever-popular
> figure 1 from MESA paper I?  Another way to check this would be to switch
> to only HELM, which can be done in controls.defaults.
>
> Aaron
>
> On Sat, Dec 12, 2015 at 4:48 AM, Pablo Marchant <pamarca at gmail.com> wrote:
>
>> Hello mesa-users!
>>
>> For the last weeks I've been working on models of helium stars formed by
>> chemically homogeneous evolution product of efficient rotational mixing.
>> I've consistently run into an issue though, midway through helium burning
>> my models experience sudden jumps in temperature. Here I attach an example
>> of a 67 msun star with a metallicity of Z=0.00034 settling into the helium
>> ZAMS, for simplicity I removed rotation and any associated mixing effects.
>> When core helium drops to about 0.60, the outer layers experience a sudden
>> change in a few timesteps which I can't seem to resolve (see figure
>> prof.pdf, note that the saved model starts at model 1500). There is not
>> really to my knowledge any fundamental change that could cause a sudden
>> jump in this stage of evolution.
>>
>> I know this star evolves towards the Eddington limit, so I also tried
>> applying MLT++, but it does not make much of a difference, as depicted in
>> the attached HR diagram.
>>
>> Anyone has ran into similar issues while modeling helium stars? Any clue
>> what might be amiss here?
>>
>> FYI, i'm using r7624
>>
>> Cheers!
>>
>> --
>> Pablo Marchant Campos
>> M.Sc on Astrophysics, Universidad Católica de Chile
>> PhD student, Argelander-Institut für Astronomie
>>
>>
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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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