International audienceWe present new diffraction-limited images of the Galactic center, obtained with the W. M. Keck I 10 m telescope. Within 0.4" of the Galaxy's central dark mass, 17 proper-motion stars, with K magnitudes ranging from 14.0 to 16.8, are identified, and 10 of these are new detections (six were also independently discovered by others). In this sample, three newly identified (S0-16, S0-19, and S0-20) and four previously known (S0-1, S0-2, S0-4, and S0-5) sources have measured proper motions that reveal orbital solutions. Orbits are derived simultaneously so that they jointly constrain the central dark object's properties: its mass, its position, and, for the first time using orbits, its motion on the plane of the sky. This analysis pinpoints the Galaxy's central dark mass to within 1.3 mas (10 AU) and limits its proper motion to 1.5+/-0.5 mas yr-1 (or equivalently 60+/-20 km s-1) with respect to the central stellar cluster. This localization of the central dark mass is consistent with our derivation of the position of the radio source Sgr A* in the infrared reference frame (+/-10 mas) but with an uncertainty that is a factor of 8 times smaller, which greatly facilitates searches for near-infrared counterparts to the central black hole. Consequently, one previous claim for such a counterpart can now be ascribed to a close stellar passage in 1996. Furthermore, we can place a conservative upper limit of 15.5 mag on any steady state counterpart emission. The estimated central dark mass from orbital motions is 3.7(+/-0.2)×106[R0/(8kpc)]3Msolar this is a more direct measure of mass than those obtained from velocity dispersion measurements, which are as much as a factor of 2 smaller. The Galactic center's distance, which adds an additional 19% uncertainty in the estimated mass, is now the limiting source of uncertainty in the absolute mass. For stars in this sample, the closest approach is achieved by S0-16, which came within a mere 45 AU (=0.0002pc=600Rs) at a velocity of 12,000 km s-1. This increases the inferred dark mass density by 4 orders of magnitude compared to earlier analyses based on velocity and acceleration vectors, making the Milky Way the strongest existing case for a supermassive black hole at the center of a normal-type galaxy. Well-determined orbital parameters for these seven Sgr A* cluster stars also provide new constraints on how these apparently massive, young (<10 Myr) stars formed in a region that seems to be hostile to star formation. Unlike the more distant He I emission line stars-another population of young stars in the Galactic center-that appear to have coplanar orbits, the Sgr A* cluster stars have orbital properties (eccentricities, angular momentum vectors, and apoapse directions) that are consistent with an isotropic distribution. Therefore, many of the mechanisms proposed for the formation of the He I stars, such as formation from a preexisting disk, are unlikely solutions for the Sgr A* cluster stars. Unfortunately, alternative theories for producing young stars, or old...

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxy's central supermassive black hole. Both Brg (2.1661 mm) and He i (2.1126 mm) are seen in absorption in S0-2 with equivalent widths ( and Å ) and an inferred stellar rotational velocity 2.8 ‫ע‬ 0.3 1.7 ‫ע‬ 0.4 ( k ms Ϫ1 ) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive 220 ‫ע‬ 40 (∼15 M , ) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements ( km s Ϫ1 ) and our reported Av S p Ϫ510 ‫ע‬ 40 z proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of . The Keplerian orbit parameters have uncertainties that are reduced by a factorof 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A*, is localized to a factor of 5 more precisely ‫2ע(‬ mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within 3. 5 (0.14 pc) 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 ). Using these measurements, line-of-sight velocities from the literature, and three-dimensional velocities for additional young stars in the central parsec, we constrain the true orbit of each individual star and 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 of the previously proposed disks of young stars using a method that is capable of detecting disks containing at least seven stars. The detected disk contains 50% 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. Additionally, 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.