[mesa-users] Core hydrogen mass fraction increases in early evolution of 1.5 Msun star

[Conny Aerts]

Hi James,

asteroseismology of such stars will give you the answers... All Kepler 
data are meanwhile public, and a large number of subgiants have been 
observed in high-cadence so this is a goldmine for what you want to do. 
You will have to dive into the data analysis to get the information out, 
though. Or else contact Bill Chaplin, who is chairing this type of 
Kepler activity (with my compliments), and join that team in a 
collaboration (screening ADS for Chaplin will tell you what type of 
results are already out).

Greetings from snowy Belgium,
Conny


On 01/14/2013 09:56 PM, James Lloyd wrote:
> This is a question that quite possibly has important observational consequences.  There's a large number of subgiant stars with planets that have been identified to be 1.5-1.8 Msun based on comparison with stellar evolution models, I have argued far more than you might expect based on the evolution rates, because as subgiants these stars have thermal pressure supported cores and evolve rapidly (cf. http://adsabs.harvard.edu/abs/2011ApJ...739L..49L ).  If there were about 150-200K offset in the observational and theoretical effective temperatures of these stars, that would account for the discrepancy.
>
> It has been countered that this is an unreasonably large systematic error to attribute to stellar evolution (cf. http://adsabs.harvard.edu/abs/2013ApJ...763...53J ).  This question of how much fresh Hydrogen convective overshoot mixes into convective regions smaller than the convective overshoot mixing length is going to change the core Helium mass and therefore slightly change where they are at the terminal main sequence for ~1.2-1.8 Msun stars, and therefore the track through the subgiant phase.
>
> If anybody knows how to characterize the uncertainty in the position of 1.2-1.8 Msun in the HR diagram roughly at the base of the RGB due to uncertainties in convection/convective overshoot in the stellar evolution calculations, I'm very interested in this question.
>
> Best,
>
> James Lloyd
>
> --
> James Lloyd
> Professor of Astronomy
> Professor of Mechanical and Aerospace Engineering
> Cornell University
> Ithaca, NY 14853
> http://astro.cornell.edu/~jpl
> 607-255-4083 (ph)
>
>
>
> On Jan 14, 2013, at 1:25 PM, David Arnett wrote:
>
>> I agree with Cony and Bill. This often happens in stellar codes when
>> convection begins. Whether it is physical is a different question. It
>> seems to be due to the use of the Schwarzschild criterion for mixing,
>> which ignores composition effects. At the same pressure, H is lighter
>> than He, so buoyancy inhibits mixing H down. Entrainment may still do
>> the trick, but that takes us beyond the present algorithm. We show the
>> composition effects in Viallet et al, 2013 (Jan. astroph).
>>
>> -- 
>> David Arnett
>> Regents Professor
>> Steward Observatory
>> University of Arizona
>>
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