2006ApJ...645.1198V

Prestellar cores are unique laboratories for studying the chemical and physical conditions preceding star formation. We observed the prestellar core L1544 in the fundamental transition of ortho-H₂D⁺ (1_1,0-1_1,1) at different positions over 100" and found a strong correlation between its abundance and the CO depletion factor. We also present a tentative detection of the fundamental transition of para-D₂H⁺ (1_1,0-1_0,1) at the dust emission peak. Maps in N₂H⁺, N₂D⁺, HCO⁺, and DCO⁺ are used and interpreted with the aid of a spherically symmetric chemical model that predicts the column densities and abundances of these species as a function of radius. The correlation between the observed deuterium fractionation of H⁺₃, N₂H⁺, and HCO⁺ and the observed integrated CO depletion factor across the core can be reproduced by this chemical model. In addition, a simpler model is used to study the H₂D⁺ ortho-to-para ratio. We conclude that, in order to reproduce the observed ortho-H₂D⁺ observations, the grain radius should be larger than 0.3 µm.