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 ^ { \circ } > l > 280 ^ { \circ } ) in the HCO ^ { + } J =1 \rightarrow 0 line , which is usually thought to trace gas at densities up to 10 ^ { 11 } m ^ { -3 } .
In this paper we introduce the survey and its strategy , describe the observational and data reduction procedures , and give a complete catalogue of moment maps of the HCO ^ { + } J =1 \rightarrow 0 emission from the ensemble of 301 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 } pc ^ { 2 } , FWHM size 0.2–2.5 pc , mean projected axial ratio 2 , optical depth 0.08–2 , total surface density 30–3000 M _ { \odot } pc ^ { -2 } , number density 0.2–30 \times 10 ^ { 9 } m ^ { -3 } , mass 15–8000 M _ { \odot } , 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 ( 68 ) .
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 pressurised , 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 this phase lasts < 1 % of the clumps ’ lifetime .