[Mesa-users] Star Mass Loss Question

[Josiah Schwab]

Hi Isaac,

> I was hoping to ask if anyone had a suggestion on how to modify the mass of
> a small star in a particular way so that the model loses hydrogen and
> helium in specific ways. I've written a hook in run_star_extras that
> determines how much mass for each element should be lost, but I'm unsure of
> how to modify the parameter in star_data.inc. Should I use something like
>
> s% star_mass_h1 = star_mass_h1 - escape_h1
>
> Similar to %s mstar_dot = blank_variable?
>
> I'd greatly appreciate any help/advice. This is all in the adjust_mdot and
> mass_loss subroutine.

Mass loss in MESA effectively removes material with the composition of
the outermost layers of the star.

Variables like star_mass_h1 are reported quantities derived from the
chemical profile of the star, so modifying them won't affect how the
model is evolving.

Can you say a little more about the application you have in mind?  I
assume this is modeling some process like Jeans escape?

Do you have a surface convection zone?  The first thing that comes to
mind is directly manipulating the model abundances within a well-mixed
region such that at the end of the step, the total mdot and the
forcibly-imposed abundance change conspire to give you the desired
change in h1 & he4 mass.

This would involve directly manipulating s% xa(:,:) -- the array that
stores the abundances.  So long as the changes needed are small (that is
say, the amount of h1 lost in a timestep is very small compared to the
total h1 in the outer convective zone) I bet one would be able to get
away with that.

In summary, I think this is doable, but not trivial, and I'm not aware
of it being done in MESA before.  (But I may be wrong on both counts!)

Josiah