mesa-users] a question about MESA

[Bill Paxton]

Hi,

One small item to add concerning overshooting in MESA star
There is an option not mentioned in the paper to do a step function instead of the usual exponential.

Here are the relevant lines from star_defaults.dek

         
         ! optional step function for overshooting to be used in place of exponential
         overshoot_step_fraction = 0 ! use this if > 0
         
            ! as above, f0*Hp determines r0 where switch from convection to overshooting.
            ! D0 = diffusion coefficient D at point r0
            ! Hp0 = the scale height at r0.
            ! overshooting extends a distance f*Hp0 from r0
            ! with constant diffusion coef D = overshoot_step_fraction*D0
         


-Bill





On Sep 16, 2010, at 6:33 AM, Aaron Dotter wrote:

> Dear Masanobu,
> 
> Thanks for your question.  
> 
> Today I am writing to ask about adequate parameters of Overshooting like " overshoot_f_..." and "mass_for_overshoot_...".  I'm using version 2258 of MESA, but I don't know and couldn't find the parameters I should use.  Would you tell me generally adequate parameters for Overshooting?
> 
> My first suggestion would be that you read the MESA "instrument paper" that recently appeared on astro-ph!  
> 
> http://arxiv.org/abs/1009.1622
> 
> It contains a description of the overshoot mixing in MESA/star and plenty of references on how the method is implemented and what values of the overshoot_f_* are recommended for different boundaries (and evolutionary phases).
>  
> Moreover, I want to use same treatment for Overshooting with Girardi. If you know the parameters to do so, would you tell me also those?
> 
> There isn't a simple answer to this question.  MESA/star treats overshoot mixing in a very different manner from the Padova group in a couple of ways: (1) MESA/star treats convective mixing as a time-dependent, diffusive process and (2) the overshoot diffusion coefficient is modeled as an exponential decay of the diffusion coefficient at the convective boundary.  Many (but not all) other codes treat convective regions as instantaneously mixed and overshoot as an "all or nothing" approach, where the extent of the overshoot region is fully mixed with the adjoining convective region.
> 
> Best wishes,
> Aaron
> 
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