A DYNAMICALLY ACCRETING GAS DISK AROUND HL TAURI

We present a new high-quality observation of HL Tau carried out with N obeyama Millimeter Array in the (CO)-C-13 (J = 1-0) emission. We have confirmed the presence of a gas disk elongated perpendicular to the op tical jet. The disk has a mass and radius of 0.03 M. and 1400 AU, resp ectively, and is located outside the inner compact disk observed in th e dust emission. Examination of the velocity field has revealed that a ccreting motion is dominated in the disk plane, although there is an i ndication of rotation at a speed of 0.2 km s-1 measured at 700 AU in r adius. The observed radial velocity of 1 km s-1 is consistent with tha t 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 centr ifugally supported viscous accretion disk. We estimate the mass accret ion rate of 0.9 x 10(-5) M. yr-1, which is an order of magnitude large r than that estimated from the luminosity of the inner disk. This sugg ests that the accretion in the entire HL Tau disk system is nonsteady.