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

Pablo Marchant pamarca at gmail.com
Tue Dec 15 10:45:20 EST 2015


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
>>>
>>>
>>> ------------------------------------------------------------------------------
>>>
>>> _______________________________________________
>>> mesa-users mailing list
>>> mesa-users at lists.sourceforge.net
>>> https://lists.sourceforge.net/lists/listinfo/mesa-users
>>>
>>>
>>
>
>
> --
> 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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