Massive young stellar objects in high-mass star-forming regions

Abstract

High-quality K-band spectra of point sources, deeply embedded in massive starforming 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 Fell and/or MgII emission lines. The CO emission comes from a relatively dense (∼1010 cm-3 ) and hot (T ∼ 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. © 2005 International Astronomical Union.