Completion of a SCUBA Survey of Lynds Dark Clouds and Implications for Low-mass Star Formation

We have carried out a survey of optically selected dark clouds using the bolometer array SCUBA on the James Clerk Maxwell Telescope at λ = 850 μm. The survey covers a total of 0.5 deg2 and is unbiased with reference to cloud size, star formation activity, and the presence of infrared emission. Several new protostars and starless cores have been discovered; the protostars are confirmed through the detection of their accompanying outflows in CO (2–1) emission. The survey is believed to be complete for Class 0 and Class I protostars, and yields two important results regarding the lifetimes of these phases. First, the ratio of Class 0 to Class I protostars in the sample is roughly unity, very different from the 1 : 10 ratio that has previously been observed for the ρ Ophiuchi star-forming region. Assuming star formation to be a homogeneous process in the dark clouds, this implies that the Class 0 lifetime is similar to the Class I phase, which from infrared surveys has been established to be ∼2 × 105 yr. It also suggests there is no rapid initial accretion phase in Class 0 objects. A burst of triggered star formation some ∼105 yr ago can explain the earlier results for ρ Ophiuchus. Second, the number of starless cores is approximately twice that of the total number of protostars, indicating a starless core lifetime of ∼8 × 105 yr. These starless cores are therefore very short lived, surviving only two or three free-fall times. This result suggests that, on size scales of ∼104 AU at least, the dynamical evolution of starless cores is probably not controlled by magnetic processes.