Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?
  We present new observations and analysis of G2 - the intriguing red
emission-line object which is quickly approaching the Galaxy's central black
hole. The observations were obtained with the laser guide star adaptive optics
systems on the W. M. Keck I and II telescopes and include spectroscopy (R ~
3600) centered on the Hydrogen Br-gamma line as well as K' (2.1 micrometer) and
L' (3.8 micrometer) imaging. Analysis of these observations shows the Br-gamma
line emission has a positional offset from the L' continuum. This offset is
likely due to background source confusion at L'. We therefore present the first
orbital solution derived from Br-gamma line astrometry, which when coupled with
radial velocity measurements, results in a later time of closest approach
(2014.21 +/- 0.14), closer periastron (130 AU, 1900Rs), and higher eccentricity
(0.9814 +/- 0.0060) compared to a solution using L' astrometry. The new orbit
casts doubt on previous associations of G2 and a low surface brightness "tail".
It is shown that G2 has no K' counterpart down to K' ~ 20 mag. G2's L'
continuum and the Br-gamma line-emission is unresolved in almost all epochs;
however it is marginally extended in our highest quality Br-gamma data set from
2006 and exhibits a clear velocity gradient at that time. While the
observations altogether suggest that G2 has a gaseous component which is
tidally interacting with the central black hole, there is likely a central star
providing the self-gravity necessary to sustain the compact nature of this
object.
