Discovery of a rotating protoplanetary gas disk around the young star GG Tauri

We have made aperture synthesis (C-12)O (J = 1-0) observations of a T Tauri star, GG Tau, in Taurus with 8-arcsec resolution using the Nobeyama Millimeter Array. We have discovered a rotating gas disk having a radius of about 500 AU around GG Tau. The rotation velocity is 0.8 km/s/sin i at the the radius, which is roughly consistent with a Kepler rotation around the central star. The inclination angle of the disk, i, is estimated to be about 60 deg, and the gas kinetic temperature is estimated to be about 8 K at that radius. The lower limit of the molecular gas mass in the disk is obtained to be 0.0001 solar mass by assuming that the CO line is optically thin, and the upper limit is estimated to be about 0.4 solar mass from kinematical consideration. This would be the first example of the resolved protoplanetary gas disk with an AU scale of several 100.