K-band spectroscopy of luminous young stellar objects

We present spectroscopy from 2.0 to 2.33 mum of 32 luminous young stellar o bjects (YSOs), which are presumed to be precursors of Herbig Ae/Be stars. F rom these stars, Br gamma, H-2, CO, He I, and Fe II were found in emission with detection rates of 97%, 34%, 22%, 9%, and 3%, respectively. We compare the spectral features with those of Herbig Ae/Be stars in the literature t o investigate the spectral behavior of intermediate- to high-mass YSOs and to search for their relations to the spectral energy distributions (SEDs). H-2 emission is detected only in Class I SEDs with particularly large spect ral indices. The detection of H-2 emission is related to the degree of the dispersal of circumstellar envelopes, where H-2 molecules are probably exci ted by shocks from outflows. On the other hand, Br gamma emission, which is generally thought to occur in stellar winds close to the stars, does not d epend on the SEDs. This indicates that stellar wind from luminous YSOs does not change much from the embedded phase to the optically visible phase. CO emission is also independent of the SEDs, but the detection rate is much l ower than that of Br gamma emission. Probably, more specific physical condi tions regarding circumstellar disks and stellar radiation are necessary for CO emission to take place.