[Mesa-users] Surface rotation velocity, num_steps_to_relax_rotation

[Ebraheem Farag]

Hello Anuj,

Relaxation is a numerical procedure. In your case, the equations of stellar
structure are solved with an initial surface rotation velocity of 0 km/s
after which the velocity variable is slowly increased over '
num_steps_to_relax_rotation' to 20km/s or 'V_init'. This process allows the
MESA solver to find a converged initial hydrostatic solution to the
structure equations for your given initial conditions.

If you set num_steps_to_relax_rotation too low, you risk missing or
overshooting a stable solution, which can lead to an initial model with a
different structure and or in your case perhaps a different initial surface
rotation velocity that is too large or too small.

-EbF

On Mon, Apr 10, 2023 at 5:02 PM Anuj Gautam via Mesa-users <
mesa-users at lists.mesastar.org> wrote:

> Dear MESA devs and users,
>
> I am trying to evolve tracks for uniformly rotating intermediate mass
> (1.3-2 MSun) stars using MESA version r22.11.1.
> I initiate rotation at age = 0, with surface velocities
> (new_surface_rotation_v, call it V_init) in the range 0 to 20 km/s. I am
> observing a "jump" in how the surface velocity evolves with age after a
> particular V_init. This V_init "jump point" varies with the stellar mass
> and metallicity.
>
> Please see the attached plot of surf_avg_v_rot vs. age for a test star of
> 1.4 MSun and initial_z = 0.008. Please also see the corresponding inlist
> attached. (Kindly ignore the haphazard formatting, I am writing the inlists
> with a python automator so all added parameters are dumped at the end of
> each section.)
>
> To initiate rotation, I add the following to the inlist:
>
> change_rotation_flag = .true.
> new_rotation_flag = .true.
> change_initial_rotation_flag = .true.
> set_initial_surface_rotation_v = .true.
> set_surface_rotation_v = .true.
> new_surface_rotation_v = V_init
> relax_initial_surface_rotation_v = .true.
> num_steps_to_relax_rotation = 100
> set_uniform_am_nu_non_rot = .true.
>
>
> If I instead do num_steps_to_relax_rotation<10, I do not see this "jump"
> in surf_avg_v_rot. What could be happening here? Or am I doing something
> wrong?
> What exactly is the purpose of num_steps_to_relax_rotation? It is talked
> about very briefly in the docs...
>
> Any and all help is appreciated.
>
> Kind regards,
> Anuj
>
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