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

Abstract

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.