HCN Hyperfine Ratio Analysis of Massive Molecular Clumps

Abstract The CHaMP project has identified a uniform sample of 303 massive (20–8000 M ⊙ ), dense (200–30,000 cm −3 ) molecular clumps in a large sector of the southern Milky Way that includes much of the Carina Arm. These are the kinds of clumps that are likely to be the precursors to IRDCs, large stellar clusters, and massive stars. We report new results of the physical conditions in these clouds based on the J =1 → 0 emission at 3mm from the HCN molecule. Analysis of the HCN emission from these clumps reveals that the physical conditions in the gas (i.e., the excitation temperature, optical depth, and column density) do not follow the molecular line emissivity in a straightforward way. This means that large fractions of the molecular material involved in massive cluster formation, while not completely“dark”, are under-luminous and easily missed in certain studies.