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 Br \gamma ( 2.1661 \mu m ) and He I ( 2.1126 \mu m ) are seen in absorption in S0-2 with equivalent widths ( 2.8 \pm 0.3 Å ~ { } & 1.7 \pm 0.4 Å ) and an inferred stellar rotational velocity ( 220 \pm 40 km/s ) that are consistent with that of an O8-B0 dwarf , which suggests that it is a massive ( \sim 15 M _ { \odot } ) , young ( < 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 further out where tidal forces should no longer preclude star formation .
The radial velocity measurements ( < v _ { z } > = -510 \pm 40 km/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 ( \pm 0.6 ) \times 10 ^ { 6 } ( ( \frac { R _ { o } } { 8 kpc } ) ^ { 3 } M _ { \odot } .
The Keplerian orbit parameters have uncertainities that are reduced by a factor of 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 ( \pm 2 milli-arcsec ) .
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 counter-revolving 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 .