I asked this on the physics SE but it received little attention:
Consider a high-mass zero-age main-sequence (ZAMS) (e.g., $m_{\rm ZAMS} \gtrsim 30\,$M$_{\odot}$) star.
I understand that the core-envelope boundary is only (semi-) well defined for Main Sequence stars that do have a core, and that in principle the final spin (angular velocity) of the core should depend on the spin of the star prior to losing its envelope due to strong wind mass-loss in late-stage evolution if the star is in isolation (or due to a mass transfer event if the star is in a binary). I've read, here for instance, that the core should be nearly completely uncoupled from the envelope, however in this recent paper they assume that the coupling is sufficient enough to determine the natal spin of the stellar core (see figure 1). Here, they treat core-envelope coupling of high mass stars as a great uncertainty. I understand that this coupling is uncertain (observationally poorly constrained) and is theoretically uncertain due to many complicated processes involved in sustaining the transfer of angular momentum through the star.
Is there a (preferably) simple way to understand the strength of core-envelope coupling in terms of the ZAMS quantities of the star, without performing full stellar population synthesis?... and without introducing another uncertain parameter, such as a timescale? Is there a known parameterization of the effects of this process on single-star evolution, i.e. with the ZAMS mass and metallicity?
