THE GALACTIC CENSUS OF HIGH- AND MEDIUM-MASS PROTOSTARS. I. CATALOGS AND FIRST RESULTS FROM MOPRA HCO+ MAPS

The Census of High- and Medium-mass Protostars (CHaMP) is the first large-scale, unbiased, uniform mapping survey at sub-parsec-scale resolution of 90?GHz line emission from massive molecular clumps in the Milky Way. We present the first Mopra (ATNF) maps of the CHaMP survey region (300? > l > 280?) in the HCO+ J = 1 ? 0 line, which is usually thought to trace gas at densities up to 1011?m?3. In this paper, we introduce the survey and its strategy, describe the observational and data reduction procedures, and give a complete catalog of moment maps of the HCO+ J = 1?0 emission from the ensemble of 303 massive molecular clumps. From these maps we also derive the physical parameters of the clumps, using standard molecular spectral-line analysis techniques. This analysis yields the following range of properties: integrated line intensity 1-30?K??km?s?1, peak line brightness 1-7?K, linewidth 1-10?km?s?1, integrated line luminosity 0.5-200?K?km?s?1?pc2, FWHM size 0.2-2.5?pc, mean projected axial ratio 2, optical depth 0.08-2, total surface density 30-3000?M ??pc?2, number density (0.2-30) ? 109?m?3, mass 15-8000?M ?, virial parameter 1-55, and total gas pressure 0.3-700?pPa. We find that the CHaMP clumps do not obey a Larson-type size-linewidth relation. Among the clumps, there exists a large population of subthermally excited, weakly emitting (but easily detectable) dense molecular clumps, confirming the prediction of Narayanan et?al. These weakly emitting clumps comprise 95% of all massive clumps by number, and 87% of the molecular mass, in this portion of the Galaxy; their properties are distinct from the brighter massive star-forming regions that are more typically studied. If the clumps evolve by slow contraction, the 95% of fainter clumps may represent a long-lived stage of pressure-confined, gravitationally stable massive clump evolution, while the CHaMP clump population may not engage in vigorous massive star formation until the last 5% of their lifetimes. The brighter sources are smaller, denser, more highly pressurized, and closer to gravitational instability than the less bright sources. Our data suggest that massive clumps approach critical Bonnor-Ebert-like states at constant density, while others' suggest that lower-mass clumps reach such states at constant pressure. Evidence of global gravitational collapse of massive clumps is rare, suggesting that this phase lasts <1% of the clumps' lifetime.