Tidal spin-up of stars in dense stellar cusps around massive black holes

We show that main-sequence stars in dense stellar cusps around massive blac k holes are likely to rotate at a significant fraction of the centrifugal b reakup velocity as a result of spin-up by hyperbolic tidal encounters. We u se realistic stellar structure models to calculate analytically the tidal s pin-up in soft encounters and extend these results to close and penetrating collisions using smoothed particle hydrodynamics simulations. We find that the spin-up effect falls off only slowly with distance from the black hole because the increased tidal coupling in slower collisions at larger distan ces compensates for the decrease in the stellar density. We apply our resul ts to the stars near the massive black hole in the Galactic center. Over th eir lifetime, similar to1 M. main-sequence stars in the inner 0.3 pc of the Galactic center are spun-up on average to similar to 10%-30% of the centri fugal breakup limit. Such rotation is similar to 20-60 times higher than is usual for such stars and may affect their subsequent evolution and their o bserved properties.