  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 Brgamma (2.1661 micron) and He I (2.1126 micron) are seen in
absorption in S0-2 with equivalent widths (2.8+-0.3 Ang & 1.7+-0.4 Ang) and an
inferred stellar rotational velocity (220+-40 km/s) that are consistent with
that 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 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 (-510+-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(+-0.6)x10^6(Ro/8kpc)^3
Msun. 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 (+-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.
