An (HCO+)-C-13 survey for dense envelopes around low-mass embedded sourcesin Taurus

A dense gas survey was carried out toward 10 low-mass embedded young source s in Taurus with the Nobeyama Millimeter Array (NMA) to investigate protost ellar evolution. All the sources were observed in the (HCO+)-C-13 (J = 1-0) line, a high-density tracer. Significant (HCO+)-C-13 emission (greater tha n or equal to4.5 sigma) was detected toward six of them. The (HCO+)-C-13 em ission is distributed roughly perpendicular to the molecular outflow axes, indicating that the (HCO+)-C-13 line traces the dense envelopes associated with the central stars. The sizes and masses of the dense envelopes are est imated to be (1-7) x 10(3) AU and 0.01-0.2 M-., respectively. The 10 source s are divided into the following three classes based on their (HCO+)-C-13 i ntensities of the NMA maps, and their properties are studied using our own and other available data. Class A sources have (HCO+)-C-13 emission centere d on the star with its elongation perpendicular to the molecular outflow ax es. These sources also have dense outflowing gas and centrally condensed pa rent cores. Class B sources have (HCO+)-C-13 emission near the source posit ions and dense outflowing gas. The parent cores around class B sources, how ever, have a shallower density profile. Class C sources have neither (HCO+) -C-13 emission nor wing emission in dense gas tracers. From these propertie s, 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 wide ly 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 (HCO+)-C-13 emission do not correlate with the flux densities at 87 GH z. This is because our continuum maps trace compact disks on a 10(2) AU sca le and not dense gas on a 10(3-4) AU scale.