Outflows in dark clouds: their role in protostellar evolution

We present the results of a $$^{12}\text {CO}\enspace J = 2-1$$ survey of a sample of IRAS sources representative of low-mass young ($$\lesssim 2.5\times 10^5$$ yr) stellar objects embedded in dark molecular clouds. Several new outflows were identified and mapped. The survey reveals that $$\gtrsim70$$ per cent of sources have associated outflow activity, suggesting that most, if not all, low-mass stars undergo outflow at some stage during their early evolution. The statistically implied outflow durations are $$\sim 1.5-2\times 10^5$$ yr, i.e. an order of magnitude greater than flow dynamical time-scales as would be derived in the normal way from CO maps. The data suggest that outflows ‘switch-on’ early in the class I (protostellar) phase and that the outflow duration is similar to the ‘clearance time’ at the end of which the source appears as an optically visible pre-main-sequence star. We explore the possibility that outflows play a dominant role in driving the evolution of protostars, and we discuss their integration into the evolutionary sequence proposed by Lada.