Possible Infall in the Gas Disk around L1551 IRS 5

Abstract

We report observations of 13C0(J = 1-0) emission from L15551-IRS5 carried out with the Nobeyama Millimeter Array. We detected a strong and compact condensation associated with L15551-IRS5, and weaker extended components with "U "-like features with a spatial resolution of 5".1 x 3".9 (P.A. = 159°). The U-like features delineate the edges of the molecular outflow as well as the 2.2 μm 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 ˜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 C18O (J = 1-0). The13C0 elongated structure may be more extended than the actually measured size, suggesting that the compact13C0 gas is most probably the inner part of the gaseous disk around L15551-IRS5. The 13 C0 diskhas 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 Msun. We estimate the mass accretion rate at 600 AU in radius to be 1.3-2.6 x 10 Msun yr-1. The derived accretion rate might be larger than the accretion rate onto the star estimated from the bolometric luminosity of L15551-IRS5 on the assumption of steady accretion, which suggests that the accretion around L15551-IRS5 may also be nonsteady as was the case for HL Tau.