[mesa-users] Fwd: [mesa:code] [r7407] - bill_paxton: now require user to set f0 > 0 for overshooting whenever have f > 0; both step and exponential

[Radek Smolec]

Hi Bill and all,

I am still confused about correct setting of the overshoot parameters. In
the literature there are (at least) two formulations of step overshooting:
ABOVE the border of the convective core (Geneva group, more common I think)
and ACROSS the border of the convective core (Padova group). See for
example here http://adsabs.harvard.edu/abs/2012MNRAS.427..127B, section
2.6.1. Quoting from the paper, 0.5Hp in the Padova group (across) roughly
corresponds to 0.25Hp in the Geneva group (above).

My belief was that setting f=0.25 and f0=0 in MESA I get the Geneva
formulation and when I set f=0.25 and f0=0.25 I get the Padova formulation.
As I prefer the Geneva formulation I was using f0=0 so far and in fact
didn't notice anything suspicious (see below).

Am I right that now:
- to get the Padova formulation I should set f0=0.25 and f=0.5
- setting f0=f=0.25 means that effectively there is no overshooting
- to get Geneva formulation I should set f0 to something very small, but
non-zero, say f0=0.01, f=0.26 ?

I understand that Dmix at the edge of the convective core may be ill
defined, but MESA was apparently dealing with it without problem when f0=0
- see the attached plots (mesa7184) with profiles of the diffusion
coefficients for a model along the main sequence (model 300) . I used f0=0
as you can see in the attached inlist. Everything seems fine, D is
continuous and equal to the value od Dmix in the last convective zone. The
model behaves as expected (this is mesa 7184). Also, when I gradually
increase f (with f0=0) the tracks behave as expected (e.g. luminosity is
smoothly and gradually increasing; helium burning loops behave as expected).

My understanding is that since Dmix vary rapidly at the edge of the
convective core its value in the last zone (and so the value of D_ovr if
f0=0) may be somehow random, depending on the grid details at the edge of
the core. To get rid of this it is now preferred to take the value from the
deeper zones, right?

If non-zero f0 is now necessary, what happens then the convective zone just
appears and its extent is smaller than f0*Hp?
Radek




2015-02-05 1:13 GMT+01:00 Bill Paxton <paxton at kitp.ucsb.edu>:

> Hi Falk,
>
> On Feb 4, 2015, at 3:25 PM, Falk Herwig wrote:
>
> Hi Bill, just saw this change on the repo. Can you elaborate a bit. Sounds
> as if you are not allowing f0=0, which makes sense. Or did you
> remove the implicit setting of f0 = f.
>
>
> Both.  When f is > 0, then the corresponding f0 must now be explicitly set
> to something > 0 as well.  No defaults for f0.
>
> There has been confusion about f0 and f for exponential and step
> overshooting.   By getting rid of the default option I hope to at least
> bring the confusion out into the open where it can be resolved rather than
> have the code quietly do something that the user might not expect.
>
> Here are the current lines from controls.defaults concerning
> overshooting.  Please take a look and feel free to edit it into something
> better.
>
> - Bill
>
> from star/defaults/controls.defaults
>
>             ! The switch from convective mixing to overshooting happens
>             ! at a distance f0*Hp into the convection zone
>             ! from the estimated location where `grad_ad == grad_rad`,
>             ! where Hp is the pressure scale height at that location.
>             ! A value <= 0 for f0 is a mistake -- you are required to set
> f0 as well as f.
>             ! take a look at the following from an email concerning this:
>
>                ! Overshooting works by taking the diffusion mixing
> coefficient at the edge
>                ! of the convection zone and extending it beyond the zone.
> But -- and here's the issue --
>                ! at the exact edge of the zone the mixing coefficient goes
> to 0.  So we don't want that.
>                ! Instead we want the value of the mixing coeff NEAR the
> edge, but not AT the edge.
>                ! The "f0" parameter determines the exact meaning of "near"
> for this.  It tells the code
>                ! how far back into the zone to go in terms of scale
> height. The overshooting actually
>                ! begins at the location determined by f0 back into the
> convection zone rather than at
>                ! the edge where the diffusion coeff is ill-defined.  So,
> for example, if you want
>                ! overshooting of 0.2 scale heights beyond the normal edge,
> you might want to back up
>                ! 0.05 scale heights to get the diffusion coeff from near
> the edge and then go out
>                ! by 0.25 scale heights from there to reach 0.2 Hp beyond
> the old boundary.  In the
>                ! inlist this would mean setting the "f0" to 0.05 and the
> "f" to 0.25.
>
>             ! There is no default value for f0; if you set f > 0 then you
> must get f0 > 0 as well.
>
> ----
>
>             ! Optional step function for overshooting.
>             ! This can be used simultaneously with exponential
> overshooting.
>             ! When using step overshoot, you must set f0 as well as f.
>
>             ! As above, `f0*Hp` determines r0 where switch from convection
> to overshooting.
>             ! Overshooting extends a distance `step_f*Hp0` from r0
>             ! with constant diffusion coeff `D = step_D + step_D0_coeff*D0`
>             ! where D0 = diffusion coefficient D at point r0.
>
>
>
>
>
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