[mesa-users] off-center Ne and O burning (again…)

[Bill Paxton]

On Apr 21, 2013, at 12:52 AM, Jakub Ostrowski wrote:

> Hi!
> 
> I'm working almost exclusively on 15 M_s models and I have found quite a big difference between the v4906 and previous ones too - but somewhere else than you.
> 
> I studied properties of models after depletion of central H, logTeff ~ 4.30, logL ~ 4.63. The behavior of Brunt-Vaisala frequency is much smoother in v4906 than before and finally now there is convection where it should be according to behavior of gradients.
> 
> And this is due to one change in controls.default. Now there is
> 
> prune_bad_cz_min_Hp_height = 0.1 ! lower limit on radial extent of cz
> 
> whereas previously the value was 1.0. I don't know if this is useful for you but for me this previously overlooked parameter made a huge change in case of my model. But it seems there is no dramatic influence on the shape of  track on HRD.
> 

Hi Jakub,

Great find!  I changed that parameter to fix a problem I encountered and thought it was a generally better value.
And perhaps it is!   But it certainly has an effect in the 15M_s case that I didn't expect.

Did you track down where the 'prune_bad_cz_min_Hp_height' control is used?  It is in star/private/mix_info.
As the name suggests, it determines the size in pressure scale height units that is considered to be too small
for a valid convection zone.  By reducing it from 1.0 to 0.1, we allow smaller convection zones to appear.   
You can turn this pruning off by setting the value to 0.  It would be interesting to try that.

I predict that Dave Arnett will chuckle and says this is yet another kludge that will be fixed by his new 321D.  ;-)

In general, this is a sobering illustration of the dependence of results on various choices in the code that are
typically never mentioned (at least not in public!).   In mesa, I've tried to expose those choices as explicit control
parameters so that at least they aren't hidden away in code that is inscrutable or completely unavailable because
it is not public.  

Perhaps I should randomly change one or two defaults in each new release!  It would be a crude sort of
Monte Carlo experiment to see which ones are important and force us all to be aware of their existence.

-Bill