Evidence for an inner molecular disk around massive Young Stellar objects

Abstract

We present observations of CO overtone bandhead emission toward four massive Young Stellar Objects (spectral type 06-B5). The high signal-to-noise ratio K-band spectra were obtained with VLT-ISAAC at a resolution Δu = 30 km s-1, sufficient to resolve the bandheads, but not the individual .J-lines. We are able to explain the shape of the lines by assuming a simple isothermal Keplerian disk model seen at different inclinations. The gas temperature ranges from 1500 to 4500 K and the CO column density is between 0.1 and 4 × 1021 cm-2. The emission probably arises within the first few astronomical units of the disk, consistent with the high gas temperature. Our results indicate that molecules can survive close to a hot star and suggest that dense (nH > 1010 cm-3) inner disks may be relatively common at an advanced stage of high-mass star formation.