[Mesa-users] Why does the orbital separation decrease

Fri Feb 11 08:52:27 UTC 2022

Hi Dave,

the current magnetic braking model built into MESA just follows that of
Rappaport, Verbunt & Joss 1983, which gives a value for the change in
orbital angular momentum associated with magnetic braking under the
assumption that the system is tidally coupled. So angular momentum is
removed directly from the orbit, not the star. This expression can be used
independently of whether or not rotation or tides are included. But it can
be somewhat inconsistent if you have rotation with a specific model for
tidal interaction, and this additional jdot term included.

The ideal situation would be to have an implemented model that removes
angular momentum to the stellar interior following a proper model for
magnetic braking, and then have a proper model of tidal interaction that
couples this to the orbit. But the magnetic braking part in particular is
quite uncertain, not just in terms of how much angular momentum is removed
(see for example Van & Ivanova (2019) for some recent work on that) but
also from which layers of the star it is removed. Owing to this we just
rely on the Rappaport model which has been the standard for this process,
and it's up to the user to decide if it is appropriate or not.

Cheers

On Fri, Feb 11, 2022 at 9:24 AM Dave Spiegel via Mesa-users <
mesa-users at lists.mesastar.org> wrote:

> 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.
> _______________________________________________
> 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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