350 micron images of massive star formation regions

We present 350 mu m continuum images of 24 massive star formation regions o btained with the Caltech Submillimeter Observatory equipped with the SHARC focal-plane array. At this wavelength the spatial resolution is 11 ". The 3 50 mu m emission is compared with thermal radio continuum emission and OH, H2O, and CH3OH masers. Emission at 350 mu m is believed to be thermal emission from dust heated by embedded or nearby stars. Compact radio continuum sources are usually pres ent in the mapped 350 mu m fields, and more than 60% of the 350 mu m peaks coincide with radio continuum peaks. This association lends strong support to the notion that the dust is heated primarily by hot stars. Masers are al so a common property of massive star formation regions. Usually OH, H2O, an d/or CH3OH masers are found near, but generally not coincident with, the 35 0 mu m peaks. Less than 25% of the 350 Irm peaks have no reported masers lo cated close by. In most of the observed regions, the 350 mu m dust maps sho w one or more components surrounded by fainter extended emission. In total, we identify 28 separate 350 mu m components. Ten of the 28 components do n ot have radio continuum counterparts. These are luminous sources and should produce detectable H II regions. It is postulated that these sources may b e undergoing such rapid accretion that the infalling matter quenches the H II region very close to the protostar, thereby making the H II region undet ectable in free-free emission. Objects in this evolutionary state may well represent the long-sought precursors to ultracompact H II regions and shoul d have the properties of accreting massive protostars (e.g., accretion disk s, bipolar molecular outflows, hot shocked gas, infall with spin-up toward the protostar). We suggest that the 10 sources in this category in our samp le merit further observational study for these properties. Two-temperature graybody models constrained by the observed infrared spectr al energy distributions were calculated to estimate the total mass, luminos ity, average dust temperature, and hydrogen column and number densities for each source. The graybody models do not account for possible low-level emi ssion from the coolest dust (<25 K) and therefore may underestimate the tot al mass. There is, however, no evidence for emission in excess of the model s at 1.3 mm, and we conclude that the mass contribution of dust colder than similar to 25 K cannot be large. Thus, the graybody models shown provide u seful average global properties required to understand massive star formati on environments.