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 associated with the galactic H II region G 34.3 + 0.2 are compared to spectra produced by a radiative transfer model based on the method of Rybicki and Stenholm. Initial model parameters, derived from both molecular line and sub-millimetre continuum observations, are refined until model spectra are in reasonable agreement with those observed, judged on peak brightness temperature, linewidth, lineshape, and radial intensity 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 turbulent velocity component of FWHM 5 km s-1 and a constant infall velocity 1.5 km s-1. The [HCO+]/[(HCO+)-C-13] abundance ratio is 20. The HCO+ observations are compared with NH3 and dust continuum observations. A two component model for the cloud is suggested in which an outer halo in quasi-virial equilibrium is collapsing slowly onto an expanding hot compact core. HCO+ is relatively under-abundant in the core.