Re: [mesa-users] very thin shell burning

[Stan W. <wo...@uc...>]

Kent, Falk, et al.,

I am on this list because I am attempting to use Mesa in my
graduate class, but I have worried a lot about these same issues
and am currently writing a paper on Pop III pulsational pair SN in the
80 - 150 Msun range. I use the Kepler code for this.

Falk is right about the dredge up of carbon in these massive low
Z stars. It is almost explosive once the first contact is made and the
radius of the star goes  above 2 x 10**14 cm. It hangs by a thread
and my guess is the envelope gets lost - somehow.  BUT

The carbon is converted to nitrogen in the envelope and C << O
so it is not clear that grains will form AND

If the He core is uncovered one switches to a different sort
of mass loss, which Vink has said (for WR stars) depends on the Fe  
abundance,
not the CO abundance. So it could be small.

Also, in most cases this very violent mixing only occurs when
rotational mixing is included and the extent is very sensitive to
an unknown convective undershoot parameter.

Another issue here is that the usual opacity tables don't work
when you have large CNO mass fractions and no Fe.

Another is that the mass loss propto sqrt(Z) is generally for stars that
have Fe and again, these have none.

Best wishes,

    Stan Woosley



On Mar 30, 2011, at 6:56 AM, Kent Budge wrote:

> This is exactly the kind of thing that has piqued my interest. It  
> looks
> like it might be a fruitful topic for research for me.
>
> On Tue, 2011-03-29 at 23:25 -0700, Falk Herwig wrote:
>> On 8-Mar-11, at 9:55 AM, Kent Budge wrote:
>>
>>> So I'm running some Population III calculations at the lower end of
>>> the
>>> mass range, and the very thin shell burning on the giant branch is
>>> taking *forever*.
>>>
>>> What's the latest thinking about mass loss in Population III? I know
>>> it's thought to be rather low, but just how rather low?
>>
>> I think that really depends on the type of stars. There is common
>> scaling out there in which Mdot ~ sqrt(Z) but that applies only to
>> mass loss processes that are driven by radiation pressure on lines,
>> and if there is no self-enrichment of the surface. For AGB stars both
>> assumptions are wrong, since AGB stars will bring lots of C, O and
>> other metals to the surface. I am not an expert on dust formation,  
>> but
>> I think it is not clear that in particular in the C-rich phase for
>> advance AGB stars, even at Pop III you could not have effective dust
>> formation, which would activate the characteristic dust-driven wind
>> mechanism of AGB stars. Observations seem to indicate at least that  
>> in
>> the MCs Mdot is not smaller than in the Galaxy.
>>
>>>
>>> What is the state of the art in modeling very thin shell burning?
>>> Brute
>>> force seems, well, like brute force. Are there methods out there  
>>> that
>>> treat it as a front propagation problem?
>>>
>>
>> In Pop III AGB stars or even very low metallicity AGb stars you will
>> encounter convective-overshoot induced H-flame propagation into the
>> core when the convective envelope gets in touch with the hot CO core.
>> This is not the regular H-shell in AGB stars, but rather a hot  
>> dregde-
>> up after thermal pulses. That is a situation where a flame model  
>> would
>> be needed, but I believe we have not much right now to base this on.
>> Note that this is a flame across a fuel-mix boundary.
>>
>> BEst, Falk.
>>
>>> Interested in any information anyone has.
>>>
>>> -- Kent G. Budge
>>> CCS-2, LANL
>>>
>>>
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>>
>> --
>> Falk Herwig
>> Dept of Physics & Astronomy, U of Victoria
>> fh...@uv..., tel: +1 (250) 721-7743
>>
>>
>>
> -- 
>
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