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 lambda = 850 mum. The survey covers a total of 0.5 deg(2) 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 rho 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 similar to2 x 10(5) yr. It also suggests there is no rapid initial accretion phase in Class 0 objects. A burst of triggered star formation some similar to10(5) yr ago can explain the earlier results for rho Ophiuchus. Second, the number of starless cores is approximately twice that of the total number of protostars, indicating a starless core lifetime of similar to8 x 10(5) 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 similar to10(4) AU at least, the dynamical evolution of starless cores is probably not controlled by magnetic processes.