OUTFLOWS IN DARK CLOUDS - THEIR ROLE IN PROTOSTELLAR EVOLUTION

We present the results of a (CO)-C-12 J = 2-1 survey of a sample of IRAS sources representative of low-mass young (less-than-or-similar-to 2.5 x 10(5) yr) stellar objects embedded in dark molecular clouds. Several new outflows were identified and mapped. The survey reveals that greater-than-or-similar-to 70 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 greater-than-or-similar-to 1.5-2 x 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.