[mesa-users] Interesting behavior at the lower edge of the main sequence

[Jon Brase]

On Mon, 24 Oct 2016 09:13:53 -0500, Aaron Dotter <aaron.dotter at gmail.com>  
wrote:

> Hi Jon,
>
> mesa-users accepts attachments, so please send your inlist.

Attached

>
> 2. Any time you're seeing an interplay between rapid rise (or fall) in  
> burning rates coupled with transition >between energy transport  
> dominated by radiation and convection, you have to take with a grain of  
> salt.  We know >from simulations that things don't evolve in 3D like  
> they do in 1D.  Though, again, I'm not aware of any >simulations that  
> focus exactly on the situation you describe.

I assume you're referring to the He3 flash? The He3 flash happens in the  
same region of the star as the convective to radiative transition began,  
and the pre main sequence and early main sequence life of the star is  
characterized by that transition working its way inward through the core,  
but the He3 flash occurs much later in the lifetime of the star, by which  
time the region in which the flash occurs is solidly radiative (until the  
flash creates a temperature gradient that touches off convection, as in a  
traditional He4 flash in a solar mass star).

That's what makes the flash so weird, really. It happens after tens of  
trillions of years of stable burning for no obvious reason. If there were  
some obvious structural transition in the star it would make sense, even  
if the fact that the transition triggered the flash were just a  
simulational artifact.

Just to guess at why both the radiative -> convective transition and the  
He3 flash begin at approximately the same place, tens of trillions of  
years apart, maybe theres a local maximum in degeneracy in that region?  
I'd expect degeneracy to strictly increase with depth, though, so it seems  
a bit implausible.

-- 
Jon Brase
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