2005A&A...439..195V

Velocity profiles of ground state lines of H₂D⁺, HC¹⁸O⁺ 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 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 ≲0.1Myr. 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.