THE STRUCTURE OF G34.3+0.2 DEDUCED FROM MULTITRANSITION MOLECULAR LINE OBSERVATIONS OF HCO+

Extensive HCO+ and (HCO+)-C-13 observations of the molecular cloud ass ociated with the galactic H II region G 34.3 + 0.2 are compared to spe ctra produced by a radiative transfer model based on the method of Ryb icki and Stenholm. Initial model parameters, derived from both molecul ar line and sub-millimetre continuum observations, are refined until m odel spectra are in reasonable agreement with those observed, judged o n peak brightness temperature, linewidth, lineshape, and radial intens ity variation. The best fitting model with constant HCO+ abundance has a molecular hydrogen density which is constant out to a radius 0.1 pc but which falls as r-2.1 outside this out to r = 1.5 pc. There is a t urbulent velocity component of FWHM 5 km s-1 and a constant infall vel ocity 1.5 km s-1. The [HCO+]/[(HCO+)-C-13] abundance ratio is 20. The HCO+ observations are compared with NH3 and dust continuum observation s. A two component model for the cloud is suggested in which an outer halo in quasi-virial equilibrium is collapsing slowly onto an expandin g hot compact core. HCO+ is relatively under-abundant in the core.