Occurrence Frequency of CO Outflows in Massive Protostellar Candidates

We mapped 12 massive protostellar candidates in the CO J=2-1 line, which incombination with Zhang et al. (2005) completes an unbiased survey of outflowsfor all 48 sources with l>50^o in a sample of 101 massive protostellarcandidates. We detected outflows in 10 sources, implying 88% occurrencefrequency of outflows for the 48 sources. This supports the conclusion ofprevious studies that bipolar outflows are an integral component in theformation process of massive stars. The vast majority of the observed outflowsare much more massive (>10~Mo) and energetic (>100~Mo~km/s) than outflows fromlow-mass protostars. They also have large mass outflow rates(>2x10^{-4}~Mo/yr), suggesting large (~1x10^{-4}~Mo/yr) accretion ratessufficient to overcome radiation pressure of the central massive protostars. Wecompared the frequency distribution of collimation factors of 40 massiveoutflows including those of this study with that of 36 low-mass outflows fromthe literature, and found NO significant difference between the two. All theseresults are consistent with the suggestion that massive stars form throughaccretion as do low-mass stars but with much higher accretion rates.Comment: 15 pages, 3 figures, Accepted by Ap