[Mesa-users] other_torque question

[Pablo Marchant]

Hi Arend,

One important thing you have to be aware of is that your angular momentum
removal does not live in isolation. I assume you are doing this for the
accretor in your binary system. The angular momentum of the outer layers
will be given by a variety of things, which include tides, angular momentum
transport from other layers of the star, as well as angular momentum coming
from accretion. other_torque, which is really an explicit method to modify
the angular momentum, will not know about this. You might want to try to
achieve the same thing with the other_torque_implicit hook.

I would also suggest you start simple. Work first with a single star
(rather than a binary) that evolves to critical during the main sequence
just with its single star evolution and see if your method does what you
expect there. If it does you can try to extend it to binaries.

Cheers

On Mon, Mar 8, 2021 at 7:17 PM AJ Poelarends <aj.poelarends at wheaton.edu>
wrote:

> Hello everyone,
>
>
>
> My two research students are experimenting with a very simple toy model to
> see whether torques in an accretor can keep the angular momentum of that
> accretor at (or below) critical rotation (such as in an accretion disk
> boundary layer).
>
> In this simple model, we use the other_torque routine, and “shave off” any
> angular momentum that is above the critical angular momentum, by defining
>
>
>
> omega_cr = sqrt(gamma_factor*cgrav*mmid/rmid**3)
>
> j_cr = s% i_rot(k) * omega_cr
>
> j_extra= max(s% j_rot(k)-j_cr,0.0d0)
>
>
>
> if (s% j_rot(k) > j_cr) then
>
>     s% extra_jdot(k) = -j_extra/s% dt
>
> end if
>
>
>
> We’re aware that we don’t conserve angular momentum, and that you can’t
> really “shave off” excess angular momentum, but it’s a first iteration that
> we hope to improve in the future by redistributing the angular momentum.
> However, for whatever reason, this simple model does not work, and the
> accretor keeps spinning up above critical rotation, and we’re not really
> sure why that is. Does anyone have any insight in what we might be doing
> wrong?
>
>
>
> A couple of observations that I can’t explain:
>
>    - surf_avg_omega_div_omega_crit (first figure) rises higher than
>    omega_div_omega_crit. I would have expected them to be close, or
>    omega_div_omega_crit higher than surf_avg_omega_div_omega_crit as the
>    latter one is averaged over several surface cells.
>    - The second figure plots j_rot, j_crit and extra_jdot. extra_jdot is
>    calculated as j_rot-j_crit (in run_star_extras), but it does not have the
>    same magnitude as the gap between j_rot and j_crit, as plotted from the
>    profiles data.
>    - The third graph shows the time evolution of omega_div_omega_crit,
>    and it seems that there is no effect extra_jdot at all once the outer
>    layers of the star become super-critical.
>
>
>
> I’ve attached our inlists and run_star_extras.f files and some plots
> showing the evolution of the star. We use mesa-r12778.
>
> Any help would be appreciated.  Thanks!
>
>
>
>
>
> [image: Vnqx6PzwAAAABJRU5ErkJggg==]
>
> Evolution of surf_avg_omega_div_omega_crit.
>
>
>
> Profiles of j_rot, j_crit and extra_jdot for model number 430.
>
>
>
> Evolution of the omega_div_omega_crit profile for model numbers 300 – 433.
>
>
>
>
>
> *Arend J. Poelarends, PhD*
>
> *Associate Professor of Physics and Astronomy, Wheaton College*
>
>
>
> (630) 752-5894  |   www.wheaton.edu
>
> 501 College Ave  |  Wheaton, IL 60187
> _______________________________________________
> 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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