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Context. The study of pre-stellar cores (PSCs) suffers from a lack ofundepleted species to trace the gas physical properties in their very denseinner parts. Aims. We want to carry out detailed modelling of N2H+ and N2D+cuts across the L183 main core to evaluate the depletion of these species andtheir usefulness as a probe of physical conditions in PSCs. Methods. We havedeveloped a non-LTE (NLTE) Monte-Carlo code treating the 1D radiative transferof both N2H+ and N2D+, making use of recently published collisionalcoefficients with He between individual hyperfine levels. The code includesline overlap between hyperfine transitions. An extensive set of core models iscalculated and compared with observations. Special attention is paid to theissue of source coupling to the antenna beam. Results. The best fitting modelsindicate that i) gas in the core center is very cold (7$\pm$ 1 K) andthermalized with dust, ii) depletion of N2H+ does occur, starting at densities5-7E5 cm&#8722;3 and reaching a factor of 6 (+13/&#8722;3) in abundance, iii)deuterium fractionation reaches &#8764;70% at the core center, and iv) thedensity profile is proportional to r^-1 out to &#8764;4000 AU, and tor^&#8722;2 beyond. Conclusions. Our NLTE code could be used to (re-)interpretrecent and upcoming observations of N2H+ and N2D+ in many pre-stellar cores ofinterest, to obtain better temperature and abundance profiles.

Depletion and low gas temperature in the L183 (=L134N) prestellar core: the N2H+–N2D+ tool

Depletion and low gas temperature in the L183 (=L134N) prestellar core: the N₂H⁺–N₂D⁺ tool