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

Jared Brooks brooksjaredc at gmail.com
Tue Dec 15 13:33:40 EST 2015


Hey Pablo,

I don't have any solutions for you, but you asked if anyone had seen a
similar thing. Although I was working with much lower mass helium stars
(<0.5 Msun sdB stars) I found that when they were in the AM CVn phase,
donating mass slowly to a more massive WD companion, when the
once-convective/helium burning core is exposed an C and O appear at the
surface, the model experiences big temperature jumps, but usually would
return to the temperature before the jump, if it survived convergence
issues.

This was in our AM CVn paper, but it was a small point and I'm not sure if
I discussed it with you or not.

Not sure if what I'm saying here is relevant, because the difference in
mass between our models is quite large, but it still seemed familiar enough
to mention it.

-Jared

On Tue, Dec 15, 2015 at 7:45 AM, Pablo Marchant <pamarca at gmail.com> wrote:

> I'm still running into issues with many models, so I wanted to see if
> anyone could have any extra suggestions. Although I can soften the jumps in
> temperature a bit by including extra mixing (like exponential overshooting
> and a minimum D_mix), It is very common that after getting only a slight
> enhancement in CO rich material (but with Z still less than 1d-3) my models
> dive face first into convergence issues. In here I include a very massive
> model (200 msun) but I also get this for models at about 40 msun, which is
> actually the lowest masses I'm working with.
>
> I get the impression that there is a spurious numerical effect here, which
> I can't pinpoint. This is mainly because I can circumvent the problem by
> shedding ~1-2% of the mass using relax_mass, after doing that the star
> evolves without issues starting from a point at a slightly higher
> temperature than before CO rich material reached the surface. Also the
> large changes are produced just by tiny amounts of CO enriched material
> reaching the surface. My main culprits would be sudden changes in the eos
> or the opacity, but have not had much success verifying that. Forcing the
> use of Type2 tables all the time, simply using Type1 tables, or using Type1
> tables with fixed Z and X does not improve things. In terms of the EOS I
> tried calling the eos routines with fixed Z and X or using HELM for the
> whole star, still not helping much.
>
> Anyone else working with Wc stars has run into this?
>
> On Sat, Dec 12, 2015 at 2:59 PM, Pablo Marchant <pamarca at gmail.com> wrote:
>
>> 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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>>>>
>>>>
>>>
>>
>>
>> --
>> Pablo Marchant Campos
>> M.Sc on Astrophysics, Universidad Católica de Chile
>> PhD student, Argelander-Institut für Astronomie
>>
>
>
>
> --
> Pablo Marchant Campos
> M.Sc on Astrophysics, Universidad Católica de Chile
> PhD student, Argelander-Institut für Astronomie
>
>
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