Radiative transfer and starless cores

We develop a method of analyzing radio-frequency spectral line observations to derive data on the temperature, density, velocity, and molecular abundance of the emitting gas. The method incorporates a radiative transfer code with a new technique for handling overlapping hyperfine emission lines within the accelerated Lambda-iteration algorithm and a heuristic search algorithm based on simulated annealing. We apply this method to new observations of N2H+ in three Lynds clouds thought to be starless cores in the first stages of star formation and determine their density structure. A comparison of the gas densities derived from the molecular line emission and the millimeter dust emission suggests that the required dust mass opacity is about k(1.3) (mm) = 0.04 cm(2) g(-1), consistent with models of dust grains that have opacities enhanced by ice mantles and fluffy aggregrates.