[mesa-users] Thermal timescale

[Rene Breton]

Dear Bill,

That's brilliant. Pretty much exactly what I needed! Obviously (cp dm T /
L) is also a very nice way of calculating it and I could even do it by hand
using a given profile. Any case it's great to have it implemented.

I was looking at the various default lists (in $MESA/star/defaults/) but
perhaps my version is too outdated (r6794) and it wasn't implemented back
then. In any case, I now know it exists and so will proceed to update to
the latest release. Thank to Ehsan as well for the pointer.

Rene

On Sun, Jul 26, 2015 at 9:10 PM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:

> Try these in your profile_columns.list
>
> thermal_time_to_surface
> log_thermal_time_to_surface
>
> It is given by this expression (in star/private/profile_getval.f90)
>
> sum(s% dm(1:k)*s% cp(1:k)*s% T(1:k))/s% L(1)
>
> i.e., the sum of dm*cp*T from surface inward divided by the surface
> luminosity.
>
> -Bill
>
>
>
> On Jul 26, 2015, at 12:44 PM, Rene Breton wrote:
>
> > Hi,
> >
> > I'm looking into calculating the thermal timescale in the envelope of a
> star and I was wondering how to go about it. The goal is basically to
> investigate possible thermal instabilities. As far as I know, MESA does not
> store any variable directly related to timescales pertaining to various
> shells of the star, either in the history file or in the profile.
> >
> > I know that for the star as a whole, the Kelvin-Holmholtz timescale
> would be:
> > t_{KH} = \Omega / (2L) \approx GM^2 / (2RL)
> >
> > with \Omega the gravitational binding energy.
> >
> > It is less clear to me how I would calculate it for, say, the envelope
> of a star (or a certain layer within it) in terms of whatever variables are
> available from MESA. Presumably the timescale would go like:
> > t_{KH,s} = \Omega / (2L) = - \int_{R_in}^{R_out} G m(r) dm(r) / r / (2L)
> >
> > i.e. the integral of the binding energy from R_in to R_out divided by 2L.
> >
> > This integral could always be recast in terms of pressure I suppose but
> perhaps I'm just overcomplicating it.
> >
> > Would anyone have a clue?
> >
> > Thanks,
> >
> > Rene
> >
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>


-- 
Rene Breton
Marie Curie Fellow & Lecturer in Astrophysics
Jodrell Bank Centre for Astrophysics
The University of Manchester
Manchester
M13 9PL, UK

Office room: Alan Turing Building, 3.119
Office: +44-(0)-161-275-4195
Mobile: +44-(0)-742-934-8036
Web: http://www.renebreton.org
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