Does accretion cease when a star approaches breakup?

Abstract

A steady-state thin accretion disk is modeled around a uniformlyrotating unmagnetized star in order to investigate whether diskaccretion can continue as the accreted angular momentum spins thecentral star to near breakup. A mapping between the specific angularmomentum (SAM) added to the star and the stellar rotation rate (SRR) isobtained. When SRR is somewhat less than the breakup rotation rate ofthe star, a class of solutions is found where the angular velocity ofthe disk attains a maximum close to the star and then decreases rapidlyin a boundary layer to match SRR. If SRR is near breakup, a second classof solutions is found where the disk angular velocity has no maximum butincreases monotonically all the way down to the stellar surface. SAMdecreases very rapidly with increasing SRR and even takes on fairlylarge negative values. The spin-up of an accreting star slows down andeventually stops at a rotation rate near breakup. Beyond this point, thestar can continue to accrete any amount of matter without actuallybreaking up.