[Mesa-users] Surface rotation velocity, num_steps_to_relax_rotation

[Anuj Gautam]

Hi,

Thanks a lot for your response. Very insightful!

I tried increasing num_steps_to_relax_rotation from 100 to higher values. However, I am finding that for slow rotators this convergence does not happen even for tens of thousands of steps. I have attached two plots showing the difference between using num_steps_to_relax_rotation=100 and 1000.
Note: Each line in the plot is for a star initiated with a certain initial velocity.

Please guide me. I appreciate all the help.
Thank you.

Regards,
Anuj

________________________________
From: Ebraheem Farag <ekfarag at asu.edu>
Sent: 11 April 2023 06:07
To: Anuj Gautam <Anuj.Gautam at usq.edu.au>
Cc: mesa-users at lists.mesastar.org <mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] Surface rotation velocity, num_steps_to_relax_rotation

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<mailto: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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