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 \mu m. The survey covers a total of 0.5 square degrees and is unbiased with reference to cloud size , star formation activity , or 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 \sim 2 \times 10 ^ { 5 } yr .
It also suggests there is no rapid initial accretion phase in Class 0 objects .
A burst of triggered star formation some \sim 10 ^ { 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 \sim 8 \times 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 \sim 10 ^ { 4 } AU at least , the dynamical evolution of starless cores is probably not controlled by magnetic processes .