What to do on super-critical accretion-induced spin up

[ilyamandel]

The current treatment of accretion induced spin-up can cause a star to rotate at a super-critical velocity. That is unphysical. I would propose to cap the maximum rotation to Keplerian, with two options that both explicitly conserve angular momentum:
(a) the excess angular momentum is immediately deposited into the orbit (assuming an efficient feedback mechanism);
or
(b) accretion becomes non-conservative once the accretor reaches critical rotation.

Thoughts welcome; tagging folks for discussion rather than implementation.

[themikelau]

I think we should have both, with (b) being the default as it matches what is commonly assumed in detailed binary calculations. (a) could be justified by the Paczynski (1991) disk model, where angular momentum is transported from the star to the outer edge of an accretion disk, and returned to the orbit via tides.