High Spectral Resolution Observations of the Massive Stars in the Galactic Center

We present high-resolution near-infrared spectra, obtained with the NIRSPECspectrograph on the W. M. Keck II Telescope, of a collection of hot, massivestars within the central 25 arcseconds of the Galactic center. We haveidentified a total of twenty-one emission-line stars, seven of which are newradial velocity detections with five of those being classified as He Iemission-line stars for the first time. These stars fall into two categoriesbased on their spectral properties: 1) those with narrow 2.112, 2.113 micron HeI doublet absorption lines, and 2) those with broad 2.058 micron He I emissionlines. These data have the highest spectral resolution ever obtained for thesesources and, as a result, both components of the absorption doublet areseparately resolved for the first time. We use these spectral features tomeasure radial velocities. The majority of the measured radial velocities haverelative errors of 20 kms, smaller than those previously obtained withproper-motion or radial velocity measurements for similar stellar samples inthe Galactic center. The radial velocities estimated from the He I absorptiondoublet are more robust than those previously estimated from the 2.058 micronemission line, since they do not suffer from confusion due to emission from thesurrounding ISM. Using this velocity information, we agree that the stars areorbiting in a somewhat coherent manner but are not as defined into a disk ordisks as previously thought. Finally, multi-epoch radial velocity measurementsfor IRS 16NE show a change in its velocity presumably due to an unseen stellarcompanion.Comment: ApJ accepted, 42 pages, 16 figure