[mesa-users] semi-convection

[Arlette Grotsch]

Hi all,

Thanks David! This is not only coming from simulations but also from stability analyses. A situation where the T-gradient is in between Schwartzschid's and Ledoux's leads to a possible vibrational instability but not a dynamical one (which is the case for convection). That means that apart from some partial mixing a radiative layer cannot become convective unless the Ledoux criterion is violated. 
Now, what asteroseismology can give us is a hint about the extent of the extra-mixed zone (overshooting, rotation, whatever...) during MS for low mass as well as for massive stars but no general conclusions up to now.
I would like to point out the importance of red giants in that respect since detailed asteroseismic analyses of populations of red giants (not individual cases) can lead (or will...) lead to an estimation of the extent if overshooting during MS as well as during helium burning (a very different situation).  See arKiv:1302.3173 for more info.

Cheers
Arlette 


Le 3 avr. 2013 à 00:42, David Arnett <wdarnett at gmail.com> a écrit :

> What Arlette says is related to what the simulations show: there is a boundary layer between the convectively unstable region according to Schwarzschild and the convectively stable region which can support a composition gradient. It does not appear in MLT because the adiabatic excess is negative there (imaginary convective velocity in MLT, in reality it gives negative buoyancy to keep convection inside the mixing region); the point of zero buoyancy and zero mixing velocity occur at different radii. In this context, "semi-convective mixing" near the boundary is "entrainment". I am writing this up, so comments, questions, and complaints are welcome! 
> 
> -- 
> David Arnett
> Regents Professor
> Steward Observatory
> University of Arizona
> 
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