Richardson, K. J.; White, G. J.; Monteiro, T. S. and Hayashi, Saeko S.
Submillimetre HCO+ observations of warm cloud cores: the excitation of molecular lines in dense star formation regions.
Astronomy and Astrophysics, 198(1-2) pp. 237–248.
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The authors have made submillimetre wavelength observations towards the cores of the molecular clouds S 255, DR21, DR21(OH), OMC-2 and Mon R2, in the J = 4-3 transitions of HCO+ and H13CO+, in order to investigate their spatial and velocity structure. In general, the observed line profiles towards these sources suggest the presence of small scale fragmentation (clumping) in the densest regions on scales of ~0.1 pc. These data are analyzed in conjunction with submillimetre wavelength continuum measurements at λ = 370 μm, which are used to estimate average gas densities. Hence, combined spatial and velocity dilution factors are derived, together with values for the relative abundance of HCO+. The dilution factors have a fairly invariant value of ~0.1 for all sources. The relative molecular abundances estimated for Mon R2 and DR21 are an order of magnitude higher than those for the other sources observed. The radiative transfer conditions and possible alternative mechanisms for excitation of the HCO+ J = 4-3 transition are discussed.
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