[Mesa-users] Relaxing angular momentum

[Pablo Marchant]

Hi Roque,

a general thing about all the relaxation procedures is that there is no
guarantee about what is the thermal state of the star after the relaxation.
If you were to do this with a star in the main sequence, allowing for a
relaxation timescale much longer than the thermal timescale of the star,
then you would obtain a main sequence star in thermal equilibrium. This is
because during the relaxation procedure mixing and composition changes from
nuclear burning are turned off, but the energy generation from nuclear
reactions is kept on. If you were to do the same thing with a white dwarf,
it's much more uncertain since if you take a very long timescale the object
would not settle into thermal equilibrium but simply go down its cooling
track, making the final result sensitive to the timescale for the
relaxation. In this case using a very short timescale would resemble an
adiabatic process, happening much faster than energy can be transported
through the interior, and should give a meaningful post-relaxation state.

So it all depends on your application, this is just stellar engineering, a
method to get a specific outcome fro the code, so it depends entirely on
what outcome yu're looking for. In applications where 3d models are
imported for example, the entropy of the star can be relaxed as well to
provide a very specific thermal state for the star.

Probably if you specify in more detail what you're trying to achieve, and
what you've tried so far, I could provide additional help.

Cheers!

On Sun, Oct 20, 2019 at 3:53 AM Roque Caballero Navarro via Mesa-users <
mesa-users at lists.mesastar.org> wrote:

> Dear all,
>
> I'm aware of the existence of a number of "relax" options aimed to let
> some modifications be done gradually over several steps. In particular, I'm
> interesting in relaxing the angular momentum loss calculated in my
> other_torque routine.
>
> According to MESA documentation, the following parameters are the relevant
> for my purpose:
>
> relax_initial_angular_momentum = .false.
> max_steps_to_relax_angular_momentum = 1000
> timescale_for_relax_angular_momentum = 1d-10
> max_dt_for_relax_angular_momentum = 1d-9
> num_timescales_for_relax_angular_momentum = 1000
> relax_angular_momentum_filename = ''
>
> Additionally, it's mentioned that for relaxing angular momentum from
> current to specified over a certain amount of relaxation timescales. This
> is done by adding an extra torque term of the form and evolving the star
> without changing the composition for
> num_timescales_for_relax_angular_momentum times
> timescale_for_relax_angular_momentum.
>
> s% extra_jdot(k) =  &
>     (1d0 - exp_cr(-s% dt / (s% job%
> timescale_for_relax_angular_momentum*secyer))) * &
>     (desired_angular_momentum(k) - s% j_rot(k)) / s% dt
>
> I've some questions about how to modified the aforementioned parameters in
> a proper way, meaning that to make longer/shorter the period of time in
> which the angular momentum loss is applied:
>
>    1. If a want to extend the relaxing period, is it enough to assign to
>    max_steps_to_relax_angular_momentum a bigger number, let's say 2000. What
>    about num_timescales_for_relax_angular_momentum, should it be assigned the
>    same value used in max_steps_to_relax_angular_momentum?
>    2. Which is the effect of modifying
>    timescale_for_relax_angular_momentum and max_dt_for_relax_angular_momentum?
>
> Basically I don't understand how to modify those parameters in the right
> way. I don't want just to throw there some values and try to figure out
> what's going on, so any hint about that will be very appreciated.
>
>
> Kind regards,
>
> Roque
>
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
>

-- 
Pablo Marchant Campos
M.Sc on Astrophysics, Universidad Católica de Chile
PhD on Astrophysics, Argelander-Institut für Astronomie, Universität Bonn
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