MODELING RADIATIVE-TRANSFER IN MOLECULAR CLOUDS .1. HCO+ IN THE STAR-FORMING REGION W49A NORTH

A new general multilevel, non-LTE radiative transfer code, valid for any velocity field, is applied to HCO+ observations of W49A North. Three classes of collapse models are considered: free-fall collapse (v proportional to 1/root r), homologous collapse (v proportional to r), and ''inside-out'' collapse (Shu 1977). A simple free-fall collapse model, with rho proportional to r(-3/2) throughout the molecular cloud, successfully reproduces the features of the observations and gives peer the best fit to the J = 1-0 and J = 3-2 profiles both toward the prominent H II component G of W49A North and off the center. In addition to a slow radial fall-off of density, the theoretical modeling implies the following for the molecular cloud: the large line widths result from motions occurring within the inner 1 pc, and there are probably one or more fragments with peculiar velocities within this same region.