[mesa-users] implementation changes of the overshooting

[Jean-Claude Passy]

Hi all,

I have a few questions regarding how the implementation of overshooting has changed in MESA. I am seeing some inconsistency between different versions so I would like to make sure that I did not miss anything.

I am starting a run with an ~ 3 Msun ZAMS (’start.mod'), evolve it for one timestep, and look at the overshooting region of the output model. I do that with two versions, v6208 and v7503.

In version 6208, D_0 was calculated by interpolation using the nearest cells to the convective boundary. One only had to supply the f factor, so my inlist would contain something like:

#---------------------------------------------------------------
      overshoot_f_above_nonburn = 0.030
      overshoot_f_below_nonburn = 0.030
      overshoot_f_above_burn_h = 0.030
      overshoot_f_below_burn_h = 0.030
#---------------------------------------------------------------

In version 7503, D_0 is now calculated at a distance f0*Hp inside the convective zone, location which then becomes the new convective boundary. We thus need to add “f0” to the “new f”. So to be consistent with version 6208, I would have in my inlist something like:

#---------------------------------------------------------------
overshoot_f_above_burn_h_core = 0.040
overshoot_f_above_burn_h_shell = 0.040
overshoot_f_below_burn_h_shell = 0.040

overshoot_f0_above_burn_h_core = 0.010
overshoot_f0_above_burn_h_shell = 0.010
overshoot_f0_below_burn_h_shell = 0.010
#---------------------------------------------------------------

The results I get are shown on the figure attached. The overshooting region extends much further with the new version (green) in comparison with the old one (blue). If I compare the different slopes, they correspond to a difference of about 30% in f. If I then change f from 0.040 to 0.030, I get the red curve which seems to better agree with the blue line. 

I looked at overshoot.f90 and could not find any mistake regarding how Dov, D_0, or even f were called/calculated. However two things puzzle me:
	- I expected the overshooting region to go deeper in the new code (there is no difference in radius either), since some of the convective region is effectively ‘transformed’ into overshooting;
	- that said, it would be fine if we used a (f-f0) under the exponential to calculate Dov. But that is not the case (at least from what I can see).

All the inlists are attached. You will need the run_star_extra.f for the version 7503 to set up f and f0 more rapidly.

Hope that someone will help me to solve this mystery!

Cheers,

JC


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