Adaptive optics near-infrared spectroscopy of the Sagittarius A* cluster

We present K-band lambda/Deltalambda similar to 2600 spectroscopy of five stars (K similar to 14-16 mag) within 0.5 of Sgr A*, the radio source associated with the compact massive object suspected to be a 2 : 6 10 6 M black hole at the center of our Galaxy. High spatial resolution of similar to0.09 and good Strehl ratios of similar to0.2 achieved with adaptive optics on the 10 m Keck telescope make it possible to measure moderate-resolution spectra of these stars individually for the first time. Two stars (S0-17 and S0-18) are identified as late-type stars by the detection of CO band head absorption in their spectra. Their absolute K magnitudes and CO band head absorption strengths are consistent with early K giants. Three stars (S0-1, S0-2, and S0-16) with r(proj) < 0.0075 pc (∼0.2) from Sgr A* lack CO band head absorption, confirming the results of earlier lower spectral and lower spatial resolution observations that the majority of the stars in the Sgr A* cluster are early-type stars. The absolute K magnitudes of the early-type stars suggest that they are late O, early B main-sequence stars of ages less than 20 Myr. The presence of young stars in the Sgr A* cluster so close to the central supermassive black hole poses the intriguing problem of how these stars could have formed or could have been brought within its strong tidal field.