IMAGING OF AN INFALLING DISK-LIKE ENVELOPE AROUND L1551 IRS-5

Aperture synthesis observations of (HCO+)-C-13 (J = 1-0) emission from L1551 IRS 5 were made using the Nobeyama Millimeter Array. It is foun d that the emission has a disklike structure with a size of 5600 x 280 0 AU at P.A. = 160 degrees. If we assume a geometrically thin disk, it s radius and inclination angle are estimated to be 2800 AU and 60 degr ees [=cos(-1) (2800/5600)], and the disk major axis is almost perpendi cular to the molecular outflow. It is noted that there exists radial m otion, i.e., velocity gradient along the disk minor axis in the inner region (r < 1000 AU). The motion can be interpreted as infalling motio n because the blueshifted emission is located on the far side of the d isk and the redshifted emission on the near side by considering the ge ometrical relation between the (HCO+)-C-13 feature and the bipolar out flow. The infalling velocity corrected for the inclination is estimate d to be 0.6 km s(-1) at r = 800 AU and is smaller than the free-fall v elocity (similar to 1.5 km s(-1) at r = 800 AU) with a stellar mass of 1 M.. The H-2 mass of the disk and the mass infall rate are estimated to be 0.27 M. and similar to 1.1 x 10(-5) M. yr(-1) respectively. In addition to the infalling motion, the molecular emission also has a ve locity structure along the major axis, suggesting rotating motion. The rotational velocity corrected for the inclination is estimated to be 0.23 km s(-1) at r = 900 AU, which is smaller than Keplerian rotationa l velocity, suggesting that the envelope is not rotationally supported . The (HCO+)-C-13 disklike structure would be a disklike infalling env elope around L1551 IRS 5.