Full Three Dimensional Orbits for Multiple Stars on Close Approaches to the Central Supermassive Black Hole

PACS 04A25 With the advent of adaptive optics on the W. M. Keck 10m telescope, two significant steps forward have been taken in building the case for a supermassive black hole at the center of the Milky Way and understanding the black hole’s effect on its environment. Using adaptive o ptics and speckle imaging to study the motions of stars in the plane of sky with ±�2 mas precision over the past 7 years, we have obtained the firs t simultaneous orbital solution for multiple stars. Among the included stars, three are newly identified (S016, S0-19, S0-20). The most dramatic orbit is that of the newly identified star S0-16, which passed a mere 60 AU from the central dark mass at a velocity of 9,000 km/s in 1999. The orbital analysis results in a new central dark mass estimate of 3.6(±0.4) × 10 6 (( R o 8kpc ) 3 M ⊙. This dramatically strengthens the case for a black hole at the center of our Galaxy, by confining the dark ma tter to within a radius of 0.0003 pc or 1,000 Rsh and thereby increasing the inferred dark mass density by four orders of magnitude compared to earlier estimates. With the introduction of an adaptive-optics-fed spectrometer, we have obtained the first detection of spectral absorption lines in one of the high-velocity stars , S0-2, one month after its closest approach to the Galaxy’s central supermassive black hole. Both Br (2.1661 µm) and He I (2.1126 µm) are seen in absorption with equivalent widths and an inferred stellar r otational velocity that are consistent with that of an O8-B0 dwarf, which suggests that S0-2 is a massive (�15 M⊙), young (<10 Myr) main sequence star. Similarly, the lack of CO detected in our first AO spectra sugg est that several other of the high-velocity stars are also young. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by these stars in the ir current orbits and the difficulty in migrating these stars inward during their lifetime from further out where tidal forces should no longer preclude star formation.