High-quality K -band spectra of strongly reddened point sources , deeply embedded in ( ultra- ) compact H ii regions , have revealed a population of 20 young massive stars showing no photospheric absorption lines , but sometimes strong Br \gamma emission .
The Br \gamma 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 Fe ii and/or 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 .