利用 HCO+ 和毫米continuum 观测追踪嵌入式低质量年轻恒星体的包裡层

presented of nine embedded low-mass young stellar objects (YSOs) in Taurus. All nine objects are detected at 3.4 and 2.7 mm, with fluxes of 4-200 mJy, and consist of unresolved (<3 arcsec) point sources, plus, toward about half of the objects, an extended envelope. The point sources likely are circumstellar disks, showing that these are established early in the embedded phase. Literature values of 1.1 mm continuum emission are used to trace the envelopes, carrying 0.001-0.26 M(sol). In HCO+, the 1-0 lines trace the surrounding clouds, while the 3-2 and 4-3 are concentrated toward the sources with intensities well correlated with the envelope flux. An HCO+/H2 abundance of 1.2e-8 is derived. The HCO+ line strengths and envelope fluxes can be fit simultaneously with the simple collapse model of Shu (1977), and related density power laws with slopes p=1-3. As an indicator of the relative evolutionary phase of a YSO, the ratio of HCO+ 3-2 line intensity over bolometric luminosity is proposed, which is roughly proportional to the current ratio of envelope over stellar mass. It is concluded that HCO+ 3-2 and 4-3 are excellent tracers of the early embedded phase of star formation.