[Mesa-users] tidal synchronization not decreasing orbital separation

[Ruggero Valli]

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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