Aperture Synthesis C18OJ= 1–0 Observations of L1551 IRS 5: Detailed Structure of the Infalling Envelope

Abstract

We report aperture synthesis C18O J = 1-0 observations of L1551 IRS 5 with a spatial resolution of 28 × 25 using the Nobeyama Millimeter Array. We have detected an emission component centrally condensed around IRS 5, as well as a diffuse component extending in the north-south direction from the centrally condensed component. The centrally condensed component, 2380 × 1050 AU in size, is elongated in the direction perpendicular to the outflow axis, indicating the existence of a flattened circumstellar envelope around L1551 IRS 5. The mass of the centrally condensed component is estimated to be 0.062 M☉. The position-velocity (P-V) diagrams reveal that the velocity field in the centrally condensed component is composed of infall and slight rotation. The infall velocity in the outer part is equal to the free-fall velocity around a central mass of ~0.1 M☉, e.g., 0.5 km s-1 at r = 700 AU, whereas the rotation velocity, 0.24 km s-1 at the same radius, gets prominent at inner radii with a radial dependence of r-1. We make up P-V diagrams for the model envelopes with vertical structure, in which the matter falls under the gravity and eventually settles down in Keplerian rotation inside the centrifugal radius, and compare them with the observed P-V diagrams of the centrally condensed component. The main characteristics of the observed P-V diagrams are reproduced by either (1) an envelope with a moderately flattened density distribution, or (2) a spherical envelope with a bipolar cavity whose half-opening angle is about 50°. Detailed comparison of the observed and model P-V diagrams suggests that the C18O J = 1-0 emission from the outer part of the centrally condensed component is well reproduced with the models with the central mass ~0.15 M☉ and the mass infall rate ~6 × 10-6 M☉ yr-1. However, the higher velocity features of the emission near the star cannot be reproduced unless the central mass is taken to be ~0.5 M☉. These facts suggest either that the gas pressure and/or magnetic force dilute the effect of the gravity in the outer part of the envelope, or that the velocity structure inside the centrifugal radius deviates significantly from the Keplerian rotation.