[mesa-users] semi-convection

[David Nataf]

Hi David,

If you're writing this up, please evolve your stars up the red giant branch and see what changing the stellar model in this manner does to the brightness of the red giant branch bump

The red giant branch bump (a 30 million year phase of stellar evolution for a sun-like star) happens when the hydrogen burning shell encounters the inner regions of the convective envelope, because the mean molecular weight drops leading to less efficient hydrogen burning. As such, whatever you do to convection in your models should affect the RGBB. 

The brightness of the red giant branch bump in Galactic globular clusters is systematically 0.20-0.40 mag fainter than predictions by models, with some recent papers arguing that the deficit is even worse at lower metallicities. This suggests that there are imperfections (but obviously doesn't demonstrate) 't how stellar codes treat convection.

For references, this is the classic theory paper in the field:
A critical investigation on the discrepancy between the observational and the theoretical red giant luminosity function `bump'
Cassisi & Salaris
http://adsabs.harvard.edu/abs/1997MNRAS.285..593C

And this is the most voluminous and precise observational paper:
Red Giant Branch Bump Brightness and Number Counts in 72 Galactic Globular Clusters Observed with the Hubble Space Telescope
Nataf, Gould, Pinsonneault, Udalski
http://adsabs.harvard.edu/abs/2013ApJ...766...77N

Cheers,
 - David

-------------
David M. Nataf
Postdoctoral Research Fellow
Australian National University
Research School of Astronomy and Astrophysics
Cotter Road
Weston Creek, ACT 2611
Australia

Mobile Phone: +61-04-5073-2843
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On 03/04/2013, at 9:42 AM, David Arnett <wdarnett at gmail.com> wrote:

> 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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