[mesa-users] overshoot

[Falk Herwig]

check the comments in star_defaults.dek for the f0 parameters:
            ! the switch from convective mixing to overshooting happens
            ! at a distance f0*Hp from the estimated location where grad_ad == grad_rad;
            ! where Hp is the pressure scale height at that location.
            ! a value < 0 tells the system to use the value of f for f0.

It depends on your particular setup, resolution near the boundary. You want to be inside of the convection zone enough so that D does not fluctuate due to mesh zoning effects.

For the parameters 
            overshoot_f_above_nonburn = 0
            overshoot_f_below_nonburn = 0
            overshoot_f_above_burn_h = 0
            overshoot_f_below_burn_h = 0
            overshoot_f_above_burn_he = 0
            overshoot_f_below_burn_he = 0
            overshoot_f_above_burn_z = 0
            overshoot_f_below_burn_z = 0
The idea is that you have burning zones of Z (not H, He), H, He and no burn (like envelope), and your can set different values for above and below the convection zone. What the exact value is would depend on the convective boundary, and what you think is appropriate. Remember that there is not much physics in this simple recipy, which is just a simple envelope to take into account whatever convective boundary mixing physics may take place there. That insight or decision you have to put in yourself. I can say that for stellar interior shells without burning at the convective boundary f=0.008, for example at the bottom of the He-shell flash convection zone, seems to be a reasonable  value. However, if there is simultaneous burning going on at the boundary (which effectively means you have a flame), then much smaller values may be adequate. In that case this convective boundary mixing takes the place of a proper flame model …. hmm, maybe good enough for tests, and in the absence of something smarter.

Falk.



On 2012-03-12, at 12:19 AM, chenhl wrote:

> Dear Falk:
> 
>        I am using MESA to study the evolution of x-ray binaries. But I am a little confused about the parameters setting.
> There are several  parameters about the overshoot such as overshoot_f_above/below, overshoot_f0_above/below in the inlist file.
>  What the value of these parameters should I take when I am using MESA? Can you give me some advice?
> 
>       All the best!
> 
>      Chen
>  

--
Falk Herwig
Dept of Physics & Astronomy, U of Victoria
fherwig at uvic.ca, tel: +1 (250) 721-7743