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 _sun pc^-2, number density (0.2-30) × 10⁹ m^-3, mass 15-8000 M _sun, 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.