Line profiles of molecular ions toward the pre-stellar core LDN 1544

Velocity profiles of ground state lines of H2D+, (HCO+)-O-18 and N2H+, observed previously with the CSO and IRAM 30 m 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 (HCO+)-O-18 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 (CO)-O-18 emission indicates a chemical age of less than or similar to 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.