[Mesa-users] tidal synchronization not decreasing orbital separation

[Ali Pourmand]

Hi Ruggero,

Thanks for the explanation, I thought jdot_ls was due to something else.

Best
Ali

On Wed, May 10, 2023 at 11:59 PM Ruggero Valli <ruvalli at mpa-garching.mpg.de>
wrote:

> Hi Ali,
>
> I think the problem is that you need to enable
>     do_jdot_ls = .true.
>
> If this option is true, whenever a star feels a torque that changes its
> angular momentum (e.g. due to tides), then the orbital angular momentum
> is adjusted in such a way to keep the total angular momentum constant.
> See equation 7 of the third MESA instrument paper for reference
> https://ui.adsabs.harvard.edu/abs/2015ApJS..220...15P
>
> I see that you have enabled
>      do_jdot_ml = .true.
> This option tells MESA that every time some mass is lost from the system
> (e.g. winds or inefficient mass transfer) the angular momentum of the
> orbit should be reduced to account for the loss of angular momentum
> carried away with the mas. See equation 5 of the third MESA instrument
> paper for reference.
> This could be the reason why you see the orbit changing for other masses
> and final evolutionary stages: the wind mass loss might become larger
> for larger masses or for giant stars, and that could result in a orbital
> change. Although it seems to me that you have not enabled the winds in
> the inlists you sent..
>
> As a final remark, if you plan to continue the evolution even after the
> primary fills the Roche lobe, I suggest you also enable
>
>      do_jdot_missing_wind = .true.
>
> This option is needed to correctly compute the orbital angular momentum
> change when there is mass transfer involved and the net mass change of
> the accretor (mdot_winds + mdot_transfer) is positive. Quoting from the
> documentation:
>
> ! Usually MESA computes stellar AM loss due to winds by taking the
> angular momentum from
> ! the removed layers of the star. However, when mass transfer is
> included, wind mass
> ! loss and mass accretion are added up, and only the remainder, if
> corresponding to
> ! net mass loss, contributes to stellar AM loss. ``jdot_missing_wind``
> compensates for this,
> ! by removing from the orbit an amount of angular momentum equal to the
> mass lost
> ! that does not contribute to stellar AM loss, times the specific
> angular momentum
> ! at the surface.
>
> I hope this was helpful,
>
> Ruggero
>
> On 5/10/23 21:51, Ali Pourmand via Mesa-users wrote:
> > Hello,
> >
> > I am using the attached inlists to evolve a star in a binary system up
> > to its Roche Lobe size. As you can see, the tidal synchronization
> > options are switched on.
> >
> > What I notice is that the orbital separation doesn't change throughout
> > the evolution. (I have attached the binary_history.data file) I wanted
> > to ask why this is the case, as I would have expected the orbital
> > separation to change at least to some extent throughout this lengthy
> > evolution (this seems to be the case for other masses and final
> > evolutionary stages).
> >
> > best
> > Ali
> >
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