A Photoevaporating Rotating Disk in the Cepheus A HW2 Star Cluster

We present VLA and PdBI subarcsecond images (0.15"-0.6") of the radiocontinuum emission at 7 mm and of the SO2 J=19_{2,18}-18_{3,15} and J=27_{8,20}-28_{7,21} lines toward the Cepheus A HW2 region. The SO2 images reveal the presence of a hot core internally heated by an intermediate mass protostar, and a circumstellar rotating disk around the HW2 radio jet with size 600AUx100AU and mass of 1M_sun. Keplerian rotation for the disk velocity gradient of 5 kms-1 requires a 9 M_sun central star, which cannot explain the total luminosity observed in the region. This may indicate that the disk does not rotate with a Keplerian law due to the extreme youth of this object. Our high sensitivity radiocontinuum image at 7 mm shows in addition to the ionized jet, an extended emission to the west (and marginally to the south) of the HW2 jet, filling the south-west cavity of the HW2 disk. From the morphology and location of this free-free continuum emission at centimeter and millimeter wavelengths (spectral index of 0.4-1.5), we propose that the disk is photoevaporating due to the UV radiation from the central star. All this indicates that the Cepheus A HW2 region harbors a cluster of massive stars. Disk accretion seems to be the most plausible way to form massive stars in moderate density/luminosity clusters.
Comment: 4 pages, 2 figures, accepted for publication in ApJL