Young massive stars in ultra-compact H II regions

Lex Kaper

Using ISAAC mounted at ESO's Very Large Telescope, we have obtained medium-resolution K-band spectra of newly formed massive stars, which are deeply embedded in ultra-compact H II regions (UCHIIs). Candidate young massive stars were selected on the basis of their near-infrared luminosity and colour measured from narrow-band images obtained in a survey of 45 southern UCHIIs. This strategy turned out to be very successful: follow-up spectroscopy confirmed the OB-star nature of 36 embedded stars, among them O stars of very early spectral type. The K-band spectra of over a dozen stars do not show photospheric absorption lines, but include a strong and broad Br-gamma emission line. These stars might represent an early phase in the evolution of massive stars, when they are still surrounded by a circumstellar disk. Our ultimate goal is to better understand the formation process of the most massive stars.

Manuscript: jad8_8f.ps