Occurrence frequency of CO outflows in massive protostellar candidates

We mapped 12 massive protostellar candidates in the CO J = 2-1 line, which, in combination with Zhang et al., completes an unbiased survey of outflows for all 48 sources with l > 50 degrees in a sample of 101 massive protostellar candidates. We detected outflows in 10 sources, implying 88% occurrence frequency of outflows for the 48 sources. This supports the conclusion of previous studies that bipolar outflows are an integral component in the formation process of massive stars. The vast majority of the observed outflows are much more massive (>10 M-circle dot) and energetic (>100 M-circle dot km s(-1)) than outflows from low-mass protostars. They also have large mass outflow rates (>2 x 10(-4) M-circle dot yr(-1)), suggesting large (similar to 1 x 10(-4) M-circle dot yr(-1)) accretion rates sufficient to overcome the radiation pressure of the central massive protostars. We compared the frequency distribution of collimation factors of 40 massive outflows including those of this study with that of 36 low-mass outflows from the literature and found no significant difference between the two. All these results are consistent with the suggestion that massive stars form through accretion as do low-mass stars but with much higher accretion rates.