A submillimetre map of the W51 region

Submillimetre mapping of a section of the molecular cloud/H II region complex W51 with a resolution of 40 arcsec reveals an extended distribution of dust encompassing two separate density enhancements. The total mass of the cloud is up to one thousand times that of previously mapped objects such as OMC1, NGC 6334 and W3 and, like these other sources, it exhibits star formation sites which are coincident with the density peaks. The secondary density peak is centred on the strong, compact infrared source IRS2 and H2O maser W51N, while the main submillimetre peak is positioned about 1 arcmin (2.2 pc) to the south-east, adjacent to IRS1 and coincident with the very powerful H2O maser W51 (Main). This massive dust peak is both cooler and much denser (nH2∼4×105cm−3) than the peak associated with the high-luminosity source IRS2 and does not feature on a far-infrared map of W51. It contains about half the mass of the cloud and can provide an optical extinction (AV) of about 1700, possibly explaining the puzzling lack of 20-μm emission and peculiar maser kinematics noted in this candidate star-formation site