A VLT/ISAAC Study of Massive YSOs in UCHII Regions

Abstract

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 strong Brγ emission. About half the objects exhibit features commonly associated with massive young stellar objects (YSOs) surrounded by a circumstellar disk: a very red colour (J–K)≥2, CO bandhead emission, hydrogen emission lines (sometimes doubly peaked), and Fe ii and/or Mg ii emission lines. The CO emission must come from a relatively dense (~10¹⁰cm^–3) and hot (T~2000–5000 K) region, sufficiently shielded from the intense UV radiation field of the young massive star. The best geometrical solution is a dense and neutral circumstellar disk causing the CO bandhead emission. This conclusion is supported by detailed model fits.