paper

The Hills Mechanism and the Galactic Center S-stars

arXiv:2002.10547 · doi:10.3847/1538-4357/ab94bc

Abstract

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.

arXiv comment21 pages, 16 figures + Appendix. Accepted for publication in ApJ. The new version is substantially improved with a more detailed treatment of resonant and nonresonant relaxation from the observed isotropic cluster in the Galactic center. In contrast to the first version, we now conclude that these processes alone can reproduce the S-stars' eccentricity distribution in less than 10 Myr

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