Accretion Signatures from Massive Young Stellar Objects

High resolution (lambda / Delta-lambda = 50,000) K-band spectra of massive,embedded, young stellar objects are presented. The present sample consists offour massive young stars located in nascent clusters powering Galactic giant HII regions. Emission in the 2.3 micron 2--0 vibrational--rotational bandhead ofCO is observed. A range of velocity broadened profiles seen in three of theobjects is consistent with the emission arising from a circumstellar disk seenat various inclination angles. Br gamma spectra of the same spectral andspatial resolution are also presented which support an accretion disk or torusmodel for massive stars. In the fourth object, Br emission suggesting arotating torus is observed, but the CO profile is narrow, indicating that theremay be different CO emission mechanisms in massive stars and this is consistentwith earlier observations of the BN object and MWC 349. To--date, only youngmassive stars of late O or early B types have been identified with clearaccretion disk signatures in such embedded clusters. Often such stars are foundin the presence of other more massive stars which are revealed by theirphotospheric spectra but which exhibit no disk signatures. This suggests thetimescale for dissipating their disks is much faster than the less massive OBstars or that the most massive stars do not form with accretion disks.Comment: 28 pages, 10 Figures, accepted for publication in the Astrophysical Journa