[Mesa-users] Why does the orbital separation decrease

[Dave Spiegel]

Hi Meng, I'd like to follow up on Ali's question —

You write,

When we apply the magnetic braking physics to binary evolution, usually, we
> assume the system is tidally locked.


Perhaps I'm misunderstanding, but it seems to me that there's some tension
between this comment and Ali's original comment that

I have shut off the tides in the inlists that I have attached below.


Concretely, have tides actually been turned off?  If so, shouldn't the
primary be able to spin down due to magnetic braking without having any
impact on the companion's orbit?  If, on the contrary, the companion's
orbit shrinks in response to the primary's magnetic spindown, doesn't that
imply that there's a tidal interaction occurring despite Ali having tried
to turn off tides?  Do you see why I'm confused?  I'd appreciate any
guidance/feedback to help resolve the apparent tension.

Thanks!
-Dave


On Thu, Feb 10, 2022 at 10:54 AM Ali Pourmand <pourmand at ualberta.ca> wrote:

> Thanks a lot Meng! This helped a lot.
>
> Best
> Ali
>
> 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
>>>
>>> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
>

-- 
Dave Spiegel, Ph.D.
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