2004ApJ...617.1167B

High-resolution (λ/Δλ=50,000) K-band spectra of massive, embedded, young stellar objects are presented. The present sample consists of four massive young stars located in nascent clusters powering Galactic giant H II regions. Emission in the 2.3 µm 2-0 vibrational-rotational band head of CO is observed. A range of velocity-broadened profiles seen in three of the objects is consistent with the emission arising from a circumstellar disk seen at various inclination angles. Brγ spectra that support an accretion disk or torus model for massive stars of the same spectral and spatial resolution are also presented. In the fourth object, Br emission suggesting a rotating torus is observed, but the CO profile is narrow, indicating that there may be different CO emission mechanisms in massive stars; this is consistent with earlier observations of the BN object and MWC 349. To date, only young massive stars of late O or early B types have been identified with clear accretion disk signatures in such embedded clusters. Often such stars are found in the presence of other, more massive stars that are revealed by their photospheric spectra but exhibit no disk signatures. This suggests that the timescale for dissipating their disks is much faster than that of the less massive OB stars or that the most massive stars do not form with accretion disks.