Imaging of an Infalling Disklike Envelope around L1551 IRS 5

Abstract

Aperture synthesis observations of H13CO+ (J = 1-0) emission from L1551 IRS 5 were made using the Nobeyama Millimeter Array. It is found that the emission has a disklike structure with a size of 5600 × 2800 AU at P.A. = 160°. If we assume a geometrically thin disk, its radius and inclination angle are estimated to be 2800 AU and 60° [= cos–1 (2800/5600)], and the disk major axis is almost perpendicular to the molecular outflow. It is noted that there exists radial motion, i.e., velocity gradient along the disk minor axis in the inner region (r < 1000 AU). The motion can be interpreted as infalling motion because the blueshifted emission is located on the far side of the disk and the redshifted emission on the near side by considering the geometrical relation between the H13CO+ feature and the bipolar outflow. The infalling velocity corrected for the inclination is estimated to be 0.6 km s–1 at r = 800 AU and is smaller than the free-fall velocity (∼ 1. 5 km s−1 at r = 800 AU ) with a stellar mass of 1 M☉. The H2 mass of the disk and the mass infall rate are estimated to be 0.27 M☉ and ~1.1 × 10-5 M☉ yr–1, respectively. In addition to the infalling motion, the molecular emission also has a velocity 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 rotational velocity, suggesting that the envelope is not rotationally supported. The H13CO+ disklike structure would be a disklike infalling envelope around L1551 IRS 5.