Aperture synthesis (CO)-O-18 J=1-0 observations of L1551 IRS 5: Detailed structure of the infalling envelope

We report aperture synthesis (CO)-O-18 J = 1-0 observations of L1551 IRS 5 with a spatial resolution of 2." 8 x 2." 5 using the Nobeyama Millimeter Ar ray. We have detected an emission component centrally condensed around IRS 5, as well as a diffuse component extending in the north-south direction fr om the centrally condensed component. The centrally condensed component, 23 80 x 1050 AU in size, is elongated in the direction perpendicular to the ou tflow axis, indicating the existence of a flattened circumstellar envelope around L1551 IRS 5. The mass of the centrally condensed component is estima ted to be 0.062 M.. The position-velocity (P-V) diagrams reveal that the ve locity held 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 similar to 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 w hich the matter falls under the gravity and eventually settles down in Kepl erian rotation inside the centrifugal radius, and compare them with the obs erved P-V diagrams of the centrally condensed component. The main character istics of the observed P-V diagrams are reproduced by either (1) an envelop e with a moderately flattened density distribution, or (2) a spherical enve lope with a bipolar cavity whose half-opening angle is about 50 degrees. De tailed comparison of the observed and model P-V diagrams suggests that the (CO)-O-18 J = 1-0 emission from the outer part of the centrally condensed c omponent is well reproduced with the models with the central mass similar t o 0.15 M. and the mass infall rate similar to 6 x 10(-6) M. yr(-1). However , the higher velocity features of the emission near the star cannot be repr oduced unless the central mass is taken to be similar to 0.5 M.. These fact s suggest either that the gas pressure and/or magnetic force dilute the eff ect of the gravity in the outer part of the envelope, or that the velocity structure inside the centrifugal radius deviates significantly from the Kep lerian rotation.