A Keplerian Gaseous Disk around the B0 Star R Monocerotis

We present high angular resolution observations of the circumstellar disk around the massive Herbig Be star R Mon (M*~8 Msolar) in the continuum at 2.7 and 1.3 mm and the 12CO 1-->0 and 2-->1 rotational lines. On the basis of the new 1.3 mm continuum image, we estimate a disk mass (gas+dust) of 0.007 Msolar and an outer radius of <150 AU. Our CO images are consistent with the existence of a Keplerian rotating gaseous disk around this star. Up to our knowledge, this is the most clear evidence for the existence of Keplerian disks around massive stars reported thus far. The mass and physical characteristics of this disk are similar to those of the more evolved T Tauri stars and indicate a shorter timescale for the evolution and dispersal of circumstellar disks around massive stars which lose most of their mass before the star becomes visible.