[Mesa-users] Why does the orbital separation decrease

Ali Pourmand pourmand at ualberta.ca
Thu Feb 10 18:53:05 UTC 2022

Thanks a lot Meng! This helped a lot.


On Wed, Feb 9, 2022 at 9:06 PM M Sun <sunmeng1118 at gmail.com> wrote:

> 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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