[kbelder]

When I was younger, I read that stars tended to have a bimodal distribution of rotational periods... many have a period measured in hours, and then another large group (like our sun) have a period of many days. The thought at the time was that the slower-rotating stars lost their angular momentum to planetary systems.

That makes sense to me, but that was before we had discovered exoplanets. I wonder if that theory still holds up?

[vikingerik]

https://en.wikipedia.org/wiki/Stellar_rotation doesn't say anything about bimodal. There's a formula with a relation between size and rotation speed, including the fact that smaller stars live longer and have more time to spin down, but it's nearly monotonic. Stars spin down because of their magnetic field exerting torque on their stellar wind, an effect that's much larger than any transfer of angular momentum to planets.