Radiative Transfer and Starless Cores

We develop a method of analyzing radio frequency spectral line observationsto derive data on the temperature, density, velocity, and molecular abundanceof the emitting gas. The method incorporates a radiative transfer code with anew technique for handling overlapping hyperfine emission lines within theaccelerated lambda iteration algorithm and a heuristic search algorithm basedon simulated annnealing. We apply this method to new observations of N_2H^+ inthree Lynds clouds thought to be starless cores in the first stages of starformation and determine their density structure. A comparison of the gasdensities derived from the molecular line emission and the millimeter dustemission suggests that the required dust mass opacity is aboutkappa_{1.3mm}=0.04 cm^2/g, consistent with models of dust grains that haveopacities enhanced by ice mantles and fluffy aggregrates.Comment: 42 pages, 17 figures, to appear in Ap