Tracing the envelopes around embedded low-mass young stellar objects with HCO and millimeter-continuum observations

The envelopes and disks around embedded low-mass young stellar objects (YSOs) are investigated through millimeter continuum and HCO+ line emission. Nine sources, selected on the basis of their HCO+ 3–2 emission from an IRAS fluxand color-limited sample of 24 objects, are observed in λ = 3.4 and 2.7 mm continuum emission with the Owens Valley Millimeter Array, and in the HCO+ and H13CO+ 4–3, 3–2, and 1–0 transitions at the James Clerk Maxwell and IRAM 30m telescopes. All nine sources are detected at 3.4 and 2.7 mm in the interferometer beam, with total fluxes between 4 and 200 mJy. The visibilities can be fit with an unresolved (< 3′′) point source, and, in about half of the sources, with an extended envelope. The point sources, presumably thermal dust emission from circumstellar disks, typically contribute 30–75% of the continuum flux observed at 1.1 mm in a 19′′ beam, assuming a spectral slope of 2.5. The fact that at least two-thirds of our sources show point-source emission indicates that circumstellar disks are established early in the embedded phase. The remainder of the 1.1 mm single dish flux is attributed to an extended envelope, with a mass of 0.001–0.26 M within a 19 ′′ beam. In HCO+, the J=1–0 line is seen to trace the surrounding cloud, while the emission from J=3–2 and 4–3 is concentrated toward the sources. All sources look marginally resolved in these lines, indicative of a power-law brightness distribution. A beam-averaged HCO+ abundance of (1.2± 0.4)× 10−8 with respect to H2 is derived. The 1.1 mm continuum fluxes and HCO+ line intensities of the envelopes correlate well, and are modeled with the simple inside-out collapse model of Shu (1977) and with power-law density distributions of slopes p = 1–3. All models provide satisfactory fits to the observations, indicating that HCO+ is an excellent tracer of the envelopes. Of the 15 sources of the original sample that were either undetected in HCO+ 3–2