The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy’s Central Black Hole: A Paradox of Youth

We have obtained the first detection of spectral absorption lines in one ofthe high-velocity stars in the vicinity of the Galaxy's central supermassiveblack hole. Both Brgamma (2.1661 micron) and He I (2.1126 micron) are seen inabsorption in S0-2 with equivalent widths (2.8+-0.3 Ang & 1.7+-0.4 Ang) and aninferred stellar rotational velocity (220+-40 km/s) that are consistent withthat of an O8-B0 dwarf, which suggests that it is a massive (~15 Msun), young(<10 Myr) main sequence star. This presents a major challenge to star formationtheories, given the strong tidal forces that prevail over all distances reachedby S0-2 in its current orbit (130 - 1900 AU) and the difficulty in migratingthis star inward during its lifetime from further out where tidal forces shouldno longer preclude star formation. The radial velocity measurements (-510+-40km/s) and our reported proper motions for S0-2 strongly constrain its orbit,providing a direct measure of the black hole mass of 4.1(+-0.6)x10^6(Ro/8kpc)^3Msun. The Keplerian orbit parameters have uncertainities that are reduced by afactor of 2-3 compared to previously reported values and include, for the firsttime, an independent solution for the dynamical center; this location, whileconsistent with the nominal infrared position of Sgr A*, is localized to afactor of 5 more precisely (+-2 milli-arcsec). Furthermore, the ambiguity inthe inclination of the orbit is resolved with the addition of the radialvelocity measurement, indicating that the star is behind the black hole at thetime of closest approach and counter-revolving against the Galaxy. With furtherradial velocity measurements in the next few years, the orbit of S0-2 willprovide the most robust estimate of the distance to the Galactic Center.Comment: 14 pages, Latex, Accepted for Publication in ApJ Letter