VLT K-band spectroscopy of massive young stellar objects in(ultra-)compact HII regions

High-quality K-band spectra of strongly reddened point sources, deeply embedded in (ultra-)compact $\ion{H}{ii}$ regions, have revealed a population of 20 young massive stars showing no photospheric absorption lines, but sometimes strong Brγ emission. The Brγ equivalent widths occupy a wide range (from about 1 to over 100 Å); the line widths of 100–200 km s-1 indicate a circumstellar rather than a nebular origin. The K-band spectra exhibit one or more features commonly associated with massive young stellar objects (YSOs) surrounded by circumstellar material: a very red colour $(J-K) \ga 2$, CO bandhead emission, hydrogen emission lines (sometimes doubly peaked), and $\ion{Fe}{ii}$ and/or $\ion{Mg}{ii}$ emission lines. The large number of objects in our sample allows a more detailed definition and thorough investigation of the properties of the massive YSOs. In the ($K, J-K$) colour-magnitude diagram (CMD) the massive YSO candidates are located in a region delimited by the OB zero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e] supergiants. The massive YSO distribution in the CMD suggests that the majority of the objects are of similar spectral type as the Herbig Be stars, but some of them are young O stars. The spectral properties of the observed objects do not correlate with the location in the CMD. The CO emission must come from a relatively dense (${\sim} 10^{10}~ \mathrm{cm}^{-3}$) and hot ($T\sim 2000{-}5000$ K) region, sufficiently shielded from the intense UV radiation field of the young massive star. The hydrogen emission is produced in an ionised medium exposed to UV radiation. The best geometrical solution is a dense and neutral circumstellar disk causing the CO bandhead emission, and an ionised upper layer where the hydrogen lines are produced. We present arguments that the circumstellar disk is more likely a remnant of the accretion process than the result of rapid rotation and mass loss such as in Be/B[e] stars.