[mesa-users] overshooting parameters

[Bill Paxton]

On Jun 26, 2013, at 5:49 AM, Ilka Petermann wrote:

> Hi Bill,
> thanks a lot for the explanations!
>  
> I have two more questions then:
> Is the value for 'overshoot_step_fraction' more than an on/off switch for the step-function, but also
> a 'real value' (that one can choose to calibrate some astrophysical properties)?

               ! overshooting extends a distance f*Hp0 from r0
               ! with constant diffusion coef D = overshoot_step_fraction*D0




> And for the two parameters mass_for_overshoot_full_on/off it says in the manual that
> overshooting is 'gradually enabled': does this mean by a certain percentage of D between the two masses?
> And if both values are 0 (default?) there is no overshooting at all?

Let's check the code.

1)
/Users/bpaxton/mesa/star/private: grep mass_for_overshoot_full_on *
ctrls_io.f:         mass_for_overshoot_full_on, &
ctrls_io.f:         s% mass_for_overshoot_full_on = mass_for_overshoot_full_on
ctrls_io.f:         mass_for_overshoot_full_on = s% mass_for_overshoot_full_on
overshoot.f:            if (s% star_mass <= s% mass_for_overshoot_full_on) then
overshoot.f:                      (s% mass_for_overshoot_full_on - s% mass_for_overshoot_full_off)

2)
in overshoot.f, it modifies the "f" parameter according to the star mass
            if (s% star_mass <= s% mass_for_overshoot_full_off) cycle
            if (s% star_mass <= s% mass_for_overshoot_full_on) then
               frac = (s% star_mass - s% mass_for_overshoot_full_off) / &
                      (s% mass_for_overshoot_full_on - s% mass_for_overshoot_full_off)
               frac = 0.5d0*(1 - cos(pi*frac))
               if (dbg) write(*,1) 'overshoot frac', frac
               f_above = f_above*frac
               f_below = f_below*frac
            end if
so f goes to 0 as mass drops to mass_for_overshoot_full_off,
and f goes to the full value from the inlist as mass goes up to mass_for_overshoot_full_on.


> 
> And one more question, because we just discussed it here: There is a parameter 'max_abar_for_burning' which, for a value > 55, should control a
> heavy element inert core. Does 'burning' refer here to e-captures and photodisintegrations only (for massive stars I see some energy generation
> in the core after Si burning and I was wondering what could happen there)?

"burning" here means any nuclear reaction.
I use this control with the neutron star envelope case
so that the code knows the the inert iron core really is inert.
It is possible that we don't really need to do anything
special -- sometimes controls outlive their purpose.
feel free to experiement with it.    here are the details:

1)
/Users/bpaxton/mesa/star/private: grep max_abar_for_burning *
ctrls_io.f:         max_abar_for_burning, &
ctrls_io.f:         s% max_abar_for_burning = max_abar_for_burning
ctrls_io.f:         max_abar_for_burning = s% max_abar_for_burning
net.f:            if (s% abar(k) > s% max_abar_for_burning) then

2) in net.f
            if (s% abar(k) > s% max_abar_for_burning) then
               category_factors = 0
            end if
the "category_factors" are multipliers for reaction rates,
so setting all category_factors to 0 sets all rates to 0.
You can set factors for particular categories using controls
such as "net_3alf_rate_factor" (see controls.defaults).

The default is max_abar_for_burning = 199, so we can safely
assuming it isn't coming into play unless you change it!

Cheers,
Bill





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