[mesa-users] semi-convection

[David Nataf]

Hi Brian,

I remember the paper you wrote with Bjork, I liked it a lot and I probably read it once a year :-) It was good to see a lot of those derivatives completed in a separate stellar models; I didn't mention your paper in the previous emails because I didn't want to saturate the email. 

I agree that HB stars can lead to uncertain distances, but isn't ~0.4 magnitudes at [Fe/H]~-2.0 totally absurd? See Di Cecco et al. (2011).

Also, it's not just about the horizontal branch luminosity. 

Troisi et al. 
http://adsabs.harvard.edu/abs/2011PASP..123..879T
compared the bump luminosity to the position on the main-sequence at the same colour as the bump, which is distance and reddening independent, and is also "nearly" independent of age. They showed that it doesn't work, you need a helium abundance of Y~0.20 to match their models with data.

Also, Cassisi et al. (2011) 
http://adsabs.harvard.edu/abs/2011A%26A...527A..59C
compared the bump magnitudes to those of the main-sequence turnoff, which is again distance and reddening-independent (I think????). For that to work they found that you needed ages of ~8 Gyr for the globular clusters, which is substantially lower than all other estimates. 

So, given the comparisons to non-HB calibrators, I don't think the discrepancy in the bump luminosity between models and data is due to uncertainties in horizontal branch models, not anymore. 

I think it's mostly due to uncertainties in the position of the convective envelope, that is my semi-educated guess. Another option is to rescale the globular metallicity scale such that [Fe/H]_{reality} =~ 0.8*[Fe/H]_{beliefs}, making the low metallicity clusters more metal-rich.

 It's also partly due to diffusion -- as you and Bjork pointed out in 2006 turning on diffusion makes the bump ~0.08 mag fainter, reducing the discrepancy by one third for [Fe/H]=-1.0. But that still leaves two thirds of a discrepancy. 

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 1:22 PM, Brian Chaboyer <Brian.Chaboyer at Dartmouth.edu> wrote:

> y 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. 

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