[hinkley]

How do you capture a PBH unless there’s substantial, fast mass (momentum) transfer from the star to the black hole? Otherwise the ballistic trajectory would carry it out and through. Wouldn’t you be more likely to find a PBH orbiting a main sequence star? Or gone altogether?

[hinkley]

It came to me while doing some housework that this feels like “what if the moon crashed into the earth?” The moon cannot crash into the earth. Any alien that could make that happen would be so terrifyingly powerful they wouldn’t have to crash the moon into the earth. It would be the twentieth most interesting way to doom us. Tidal waves would be easier.

[pdonis]

While your statement about the ballistic trajectory is true in the short term, over longer time scales (my off the cuff guess is thousands of years, much shorter than the time scale covered by the numerical simulations in the paper), there will be momentum transfer due to the PBH perturbing the star's matter as it passes through, and the PBH will settle into the center of the star if there are no other perturbations (i.e., no other massive bodies near enough to affect the process). The paper doesn't discuss the capture process in any detail, but I suspect that something like that is what they have in mind.