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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: vdtak@mpifr-bonn.mpg.de
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Laboratoire Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, France

Received: 31 January 2005
Accepted: 28 April 2005

Abstract

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 0.1 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.