[Mesa-users] Why does the orbital separation decrease

[M Sun]

Hi Ali,

Magnetic braking is the main mechanism that shrinks the orbit in your case.
In MESA binary, although you don't have the magnetic braking setting in
your attached inlist_project file, magnetic braking turns on by default.
And I suggest keeping the MESA default setting, since magnetic braking is
important for solar-type stars. The physics was found from the single star
evolution, from observation, generally young stars rotate faster and old
stars rotate slower. The magnetized corona wind following along the
magnetic field line near the surface of stars reduces the star's angular
momentum so the star spin rate slows down. When we apply the magnetic
braking physics to binary evolution, usually, we assume the system is
tidally locked. If the primary star slows down in its spin rate due to
magnetic braking, to keep the synchronization, the system shrinks in
separation and spins up the star again. This is how magnetic braking changes
the binary separation.

If you would like to see what's going on when magnetic braking (or
gravitational radiation etc) turns off for binary evolution, please make
sure to check the namelists below in binary_controls
do_jdot_gr = .true.
do_jdot_ml = .true.
do_jdot_mb = .true.
do_jdot_ls = .true.
The detailed explanation for those settings is at
$MESA_DIR/binary/defaults/binary_controls.defaults

Hope it helps.

cheers,
Meng


On Wed, Feb 9, 2022 at 9:29 PM Ali Pourmand <pourmand at ualberta.ca> wrote:

> Hello,
>
> I have been trying to evolve a 1Msun star in a binary system with a
> companion of 0.625, and an initial separation of 11.2Rsun.
> I have shut off the tides in the inlists that I have attached below.
>
> Up to this time that I evolve this star, no mass transfer happens in the
> binary. I was wondering why the separation is decreasing when tidal
> evolution is off. I have also attached the binary_history.data of this run.
> I am not sure if I am switching off the tidal evolution correctly, or is
> there something else going on that could cause this that I am missing?
>
> I attached the plots of binary separation and Jdot versus time in case it
> helps.
> There isn't any eccentricity so no circulization should have happened as
> well.
> [image: separation of mesa binaries.png]
> [image: Js_vs_age.png]
> best
> Ali Pourmand
>
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
>
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