[mesa-users] semi-convection

Tue Apr 2 22:22:13 EDT 2013

HI Nataf and David,

I did some work on the luminosity of the RGB bump ~7 years ago and found 
good agreement between my models and the  observations

`Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant 
Branch Bump' Bjork & Chaboyer 2006 
http://adsabs.harvard.edu/abs/2006ApJ...641.1102B

I took a quick look at the more recent papers which are finding a 
discrepancy, and believe the biggest difference is that I adopted a 
distance scale to globular clusters motivated by a variety of 
measurements (including main sequence fitting).  The more recent papers 
are effectively using their theoretical luminosity of the horizontal 
branch (HB; core helium burning) stars to set the distance scale to the 
globular clusters.  Thus, the problem they are finding could either be 
due to the RGB clump luminosity, or the HB luminosities.

HB models have their own uncertainties, including the fact that 
semiconvection  occurs in these stars. How one treats semiconvection in 
HB stars has a strong impact on their lifetimes, and quite a long time 
ago (~1970's) people used this fact to constrain how they treat 
semiconvection in HB stars.  I ran into this issue when I implemented a 
version of nonlocal mixing-length theory that Dave worked on in the 
early 1990s
`A theory of nonlocal mixing-length convection. I - The moment 
formalism' 1993 Grossman, Narayan & Arnett.

I worked with Scott Grossman on this, and we found that using this 
approach led to much longer lifetimes in HB stars (though we never 
published a refereed paper on this work).  Using number
counts of HB  & RGB stars in globular clusters is a good way to test a 
new treatment of convection which avoids having to know the distance to 
globular clusters.

cheers,
Brian

----------------------
Brian Chaboyer, Professor
Department of Physics and Astronomy
Dartmouth College
Hanover, NH USA 03755

On 4/2/13 6:53 PM, David Nataf wrote:
> 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
> Office Phone: +61-02-6125-0225
>
>
>
>
> On 03/04/2013, at 9:42 AM, David Arnett <wdarnett at gmail.com 
> <mailto: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
>>
>> Facts are stubborn, but statistics are more pliable.  Mark Twain
>> Facts do not cease to exist because they are ignored.  Aldous Huxley
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