Molecular outflows towards O-type young stellar objects

Context. The formation of massive stars is not well-understood and requires detailed observational studies in order to discriminate between the different proposed star formation models. Aims. We have searched for massive molecular outflows in a sample of high-mass star forming regions, and we have characterised both the outflow properties and those of their associated molecular clumps. With a sample composed largely of more luminous objects than previous ones, this work complements analogous surveys performed by other authors by adding the missing highest luminosity sources. Methods. The sample under study has been selected so as to favour the earliest evolutionary phases of star formation, and is composed of very luminous objects (Lbol > 2 × 104 L⊙ and up to ∼106 L⊙), possibly containing O-type stars. Each source has been mapped in 13CO(2−1) (an outflow tracer) and C18O(2−1) (an ambient gas tracer) with the IRAM-30m telescope on Pico Veleta (Spain). Results. The whole sample shows high-velocity wings in the 13CO(2−1) spectra, indicative of outflowing motions. In addition, we have obtained outflow maps in 9 of our 11 sources, which display well-defined blue and/or red lobes. For these sources, the outflow parameters have been derived from the line wing 13CO(2−1) emission. An estimate of the clump masses from the CO(2−1) emission is also provided and found to be comparable to the virial masses. From a comparison between our results and those found by other authors at lower masses, it is clear that the outflow mechanical force increases with the bolometric luminosity of the clump and with the ionising photon rate of the associated Hii regions, indicating that high-mass stars drive more powerful outflows. A tight correlation between outflow mass and clump mass is also found. Conclusions. Molecular outflows are found to be as common in massive star forming regions as in low-mass star forming regions. This, added to the detection of a few tentative large-scale rotating structures suggests that high-mass stars may generally form via accretion, as low-mass stars.