The Initial Conditions for Formation of Low‐Mass Stars: Kinematics and Density Structure of the Protostellar Envelope in B335

Abstract

We have observed dense molecular gas toward a deeply embedded protostar in B335 using the Nobeyama 45 m telescope and the Nobeyama Millimeter Array. The H13CO+ and C18O maps taken by the 45 m telescope show elongated features perpendicular to the axis of molecular outflow, suggesting that these emission lines arise from a dense disklike envelope surrounding the protostar. The size and mass of the H13CO+ disklike envelope are 0.17 × 0.15 pc and 2.4 M☉, respectively. The C18O envelope gas has a linear velocity gradient along its major axis indicative of a rigid rotation with an angular velocity of 1.1 × 10-14 radians s-1. The density profile derived from the C18O and H13CO+ data shows a power law of ρ(r)~ρ0r-1.95~(a2/2πG)r-2 over the radius range between 0.03 and 0.2 pc. In addition, the coefficient of the density profile is consistent with Shu's solution rather than Larson's, though there is uncertainty particularly in the fractional abundance of the H13CO+ molecule. Our results suggest that the protostar in B335 was formed in an isothermal core with a rigid rotation. The interferometric observations of the H13CO+ line reveal a dense compact feature centered on the protostar. This compact feature has a size of 2000 AU, and its elongation is roughly perpendicular to the outflow axis. We thus consider that this compact feature is an inner part of the disklike envelope. There is a velocity gradient along the minor axis of the feature which might be interpreted as a disk infalling motion. The previous observations also suggested the existence of infalling motion toward the protostar B335 IRS. In addition, the inner envelope shows a rotating motion of Vθ=0.14 km s-1 at r=490 AU. This rotational velocity is smaller than the corresponding Keplerian velocity of ~0.42 km s-1, indicating that the inner envelope is not rotationally supported.