High-quality K -band spectra of point sources , deeply embedded in massive star-forming regions , have revealed a population of 20 young massive stars showing no photospheric absorption lines , but only emission lines .
The K -band spectra exhibit one or more features commonly associated with massive Young Stellar Objects surrounded by circumstellar material : a very red color ( J - K ) = 2 , CO bandhead emission , hydrogen emission lines ( sometimes doubly peaked ) , and FeII and/or MgII emission lines .
The CO emission comes 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 .
Modeling of the CO-first overtone emission shows that the CO gas is located within 5 AU of the star .
The hydrogen emission is produced in an ionized medium exposed to UV radiation .
The best geometrical configuration is a dense and neutral circumstellar disk causing the CO bandhead emission , and an ionized upper layer where the hydrogen lines are produced .
We argue that the circumstellar disk is likely a remnant of the accretion via a circumstellar disk .