Possible infall in the gas disk around L1551-IRS 5

We report observations of (CO)-C-13(J = 1-0) emission from L1551-IRS5 carried out with the Nobeyama Millimeter Array. We detected a strong and compact condensation associated with L1551-IRS5, and weaker extended components with ''U''-like features with a spatial resolution of 5.'' 1 x 3.'' 9 (P.A. = 159 degrees). The U-like features delineate the edges of the molecular outflow as well as the 2.2 mu infrared reflection nebula. This suggests that the extended component may be a dense shell swept up by the molecular outflow. The compact component was marginally resolved with the present angular resolution. The estimated deconvolved size is similar to 1200 x 670 AU with a position angle perpendicular to the optical jet. This elongated structure is very similar to the compact gaseous disk observed in (CO)-O-18 (J = 1-0). The (CO)-C-13 elongated structure may be more extended than the actually measured size, suggesting that the compact (CO)-C-13 gas is most probably the inner part of the gaseous disk around L1551-IRS5. The (CO)-C-13 disk has a velocity gradient along its minor axis, which can be explained in terms of infalling motion in the plane of the disk with a central mass of 0.5 M.. We estimate the mass accretion rate at 600 AU in radius to be 1.3-2.6 x 10(-5) M. yr(-1). The derived accretion rate might be larger than the accretion rate onto the star estimated from the bolometric luminosity of L1551-IRS5 on the assumption of steady accretion, which suggests that the accretion around L1551-IRS5 may also be nonsteady as was the case for HL Tau.