Adaptive Optics Near-Infrared Spectroscopy of the Sagittarius A* Cluster

We present K-band λ/Δλ ~ 2600 spectroscopy of five stars (K ~ 14-16 mag) within 0.″5 of Sgr A*, the radio source associated with the compact massive object suspected to be a 2.6 × 106 M☉ black hole at the center of our Galaxy. High spatial resolution of ~0.″09 and good Strehl ratios of ~0.2 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 rproj < 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.