Theoretical models of star formation make predictions about the density and velocity structure of the envelopes surrounding isolated, low-mass young stars. This paper tests such models through high quality submillimeter continuum imaging of four embedded young stellar objects in Taurus and previously obtained molecular-line data. Observations carried out with the Submillimeter Continuum Bolometer Array on the James Clerk Maxwell Telescope at 850 and 450 µm of L1489 IRS, L1535 IRS, L1527 IRS, and TMC 1 reveal ∼ 2000 AU elongated structures embedded in extended envelopes. The density distribution in these envelopes is equally well fit by a radial power-law of index p = 1.0-2.0 or with a collapse model such as that of Shu (1997: ApJ, 214, 488). This inside-out collapse model predicts 13 CO, C 18 O, HCO + , and H 13 CO + line profiles which closely match observed spectra toward three of our four sources. This shows that the inside-out collapse model offers a good description of YSO envelopes, but also that reliable constraints on its parameters require independent measurements of the density and the velocity structure, e.g., through continuum and line observations. For the remaining source, L1489 IRS, we find that a model consisting of a 2000 AU radius, rotating, disklike structure better describes the data. Possibly, this source is in transition between the embedded Class I and the optically revealed T Tauri phases. The spectral index of the dust emissivity decreases from β = 1.5-2.0 in the extended envelope to 1.0 ± 0.2 in the central peaks, indicating grain growth or high optical depth on small scales. The

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