[mesa-users] Extended helium envelopes

[David Arnett]

Hi all,

We are trying to make ad hoc "Wolf-Rayet" models with MESA by stripping
off the hydrogen envelope by hand from massive star models near the
end of helium core burning. We are finding that the resulting "stripped
envelope" models evolve normally for a while, but then are prone to
having the remaining helium envelope expand to red-giant size,
100 R_sun or so. There is no question this can happen in some
circumstances, for instance stars with a substantial helium abundance
and a helium-burning shell atop a degenerate carbon/oxygen core, the
supposed the structure of R Cor Bor stars. I have, however, no
recollection of this expanding helium envelope phenomenon occurring
in more massive, non-degenerate cores. The issue seems more acute
for lower mass cores, a few to 10 solar masses, but may be related to
opacity bumps, Eddington luminosity and radiation pressure. It also
seems to be exacerbated by rotation.

Yes, there is a pronounced tendency to do this.
Lower mass core are most likely to do so. This may have been obscured by
a lack of care in zoning, etc., imho. Rotation gives an outward acceleration
and thus enhances the effect. We need to carefully sort this out, I think.

Any perspective on this issue would be appreciated.

Thanks,

Craig


-- 
David Arnett
Regents Professor
Steward Observatory
University of Arizona

Facts are stubborn, but statistics are more pliable.  Mark Twain
Facts do not cease to exist because they are ignored.  Aldous Huxley
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