The Hills Mechanism and the Galactic Center S-stars
  Our Galactic center contains young stars, including the few million year old
clockwise disk between 0.05 and 0.5 pc from the Galactic center, and the S-star
cluster of B-type stars at a galactocentric distance of ~0.01 pc. Recent
observations suggest the S-stars are remnants of tidally disrupted binaries
from the clockwise disk. In particular, Koposov et al. 2020 discovered a
hypervelocity star that was ejected from the Galactic center 5 Myr ago, with a
velocity vector consistent with the disk. We perform a detailed study of this
binary disruption scenario. First, we quantify the plausible range of binary
semimajor axes in the disk. Dynamical evaporation of such binaries is dominated
by other disk stars rather than by the isotropic stellar population. For the
expected range of semimajor axes in the disk, binary tidal disruptions can
reproduce the observed S-star semimajor axis distribution. Reproducing the
observed thermal eccentricity distribution of the S-stars requires an
additional relaxation process. The flight time of the Koposov star suggests
that this process must be effective within 10 Myr. We consider three
possibilities: (i) scalar resonant relaxation from the observed isotropic star
cluster, (ii) torques from the clockwise disk, and (iii) an intermediate-mass
black hole. We conclude that the first and third mechanisms are fast enough to
reproduce the observed S-star eccentricity distribution. Finally, we show that
the primary star from an unequal-mass binary would be deposited at larger
semimajor axes than the secondary, possibly explaining the dearth of O stars
among the S-stars.
