A DYNAMICALLY ACCRETING GAS DISK AROUND HL TAURI

We present a new high-quality observation of HL Tau carried out with Nobeyama Millimeter Array in the (CO)-C-13 (J = 1-0) emission. We have confirmed the presence of a gas disk elongated perpendicular to the optical jet. The disk has a mass and radius of 0.03 M. and 1400 AU, respectively, and is located outside the inner compact disk observed in the dust emission. Examination of the velocity field has revealed that accreting motion is dominated in the disk plane, although there is an indication of rotation at a speed of 0.2 km s-1 measured at 700 AU in radius. The observed radial velocity of 1 km s-1 is consistent with that obtained when the gas is dynamically accelerated by the central star. This means that the outer gas disk is undergoing dynamical accretion toward the central star/inner disk system, and that it is not a centrifugally supported viscous accretion disk. We estimate the mass accretion rate of 0.9 x 10(-5) M. yr-1, which is an order of magnitude larger than that estimated from the luminosity of the inner disk. This suggests that the accretion in the entire HL Tau disk system is nonsteady.