Line profiles of molecular ions toward the pre-stellar core LDN 1544
  Velocity profiles of ground state lines of H2D+, HC18O+ and N2H+, observed previously with the CSO and IRAM 30m telescopes, are modeled with a Monte Carlo radiative transfer program to study the temperature, density and velocity structure of the pre-stellar core LDN 1544. The H2D+ line is double-peaked like that of the other ions, but previous models that fit the HC18O+ and N2H+ profiles are found not to fit the H2D+ data. Matching the H2D+ observations requires at least three modifications to the model at small radii: (1) the density profile must continue to rise inward and not flatten off toward the center; (2) the gas temperature must be nearly constant and not drop inwards significantly; (3) the infall velocity must increase inward, in a fashion intermediate between `quasi-static' (ambipolar diffusion) and `fully dynamic' (Larson-Penston) collapse. The C18O emission indicates a chemical age of <~0.1 Myr. The effects of a flattened structure and rotation on the line profiles are shown to be unimportant, at least on the scales probed by single-dish telescopes. Alternatively, the H2D+ profile is affected by absorption in the outer layers of the core, if gas motions in these layers are sufficiently small. 
F. F. S. van der Tak (MPIfR, Bonn), P. Caselli (Osservatorio di   Arcetri), C. Ceccarelli (Observatoire de Grenoble)