Orbits and origins of the young stars in the central parsec of the Galaxy

We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within a parsec of the supermassive black hole at the Galactic Center. Our proper motion measurements have uncertainties of only 0.07 mas yr−1 (3 km s−1), which is ! 7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas yr−2 (7 km s−1 yr−1). These measurements, along with stellar line-of-sight velocities from the literature, constrain the true orbit of each individual star and allow us to directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one disk of young stars using a method that is capable of detecting disks containing at least 7 stars. The detected disk contains 50% (38 of 73) of the young stars, is inclined by ∼ 115◦ from the plane of the sky, and is oriented at a position angle of ∼ 100◦ East of North. The on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger radii as ∝ r−2. The disk has an out-of-the-disk velocity dispersion of 28 ± 6 km s−1, which corresponds to a half-opening angle of 7◦ ± 2◦, and several candidate disk members have eccentricities greater than 0.2. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple thin circular disk.