An H13CO+Survey for Dense Envelopes around Low‐Mass Embedded Sources in Taurus

Abstract

A dense gas survey was carried out toward 10 low-mass embedded young sources in Taurus with the Nobeyama Millimeter Array (NMA) to investigate protostellar evolution. All the sources were observed in the H13CO+(J = 1-0) line, a high-density tracer. Significant H13CO+ emission (≥4.5 σ) was detected toward six of them. The H13CO+ emission is distributed roughly perpendicular to the molecular outflow axes, indicating that the H13CO+ line traces the dense envelopes associated with the central stars. The sizes and masses of the dense envelopes are estimated to be (1-7) × 103 AU and 0.01-0.2 M☉, respectively. The 10 sources are divided into the following three classes based on their H13CO+ intensities of the NMA maps, and their properties are studied using our own and other available data. Class A sources have H13CO+ emission centered on the star with its elongation perpendicular to the molecular outflow axes. These sources also have dense outflowing gas and centrally condensed parent cores. Class B sources have H13CO+ emission near the source positions and dense outflowing gas. The parent cores around class B sources, however, have a shallower density profile. Class C sources have neither H13CO+ emission nor wing emission in dense gas tracers. From these properties, we conclude that low-mass protostars evolve from class A, B to C sources by dissipating their parent cloud cores, which is consistent with the widely accepted ideas of star formation. In addition, these observational data suggest that significant dispersion of a parent core by a molecular outflow and main accretion phase ends at the early protostellar phase. Six of the 10 sources are detected in continuum emission at 87 GHz. The intensities of the H13CO+ emission do not correlate with the flux densities at 87 GHz. This is because our continuum maps trace compact disks on a 102 AU scale and not dense gas on a 103-4 AU scale.