On the frequency of N2H+ and N2D +

Abstract

Context. Dynamical studies of prestellar cores search for small velocity differences between different tracers. The highest radiation frequency precision is therefore required for each of these species. Aims. We want to adjust the frequency of the first three rotational transitions of N2H + and N2D+ and extrapolate to the next three transitions. Methods. We compare N2H+ and N 2D+ to NH3, the frequency of which, is more accurately known and which has the advantage of being spatially coexistent with N2H+ and N2D+ in dark cloud cores. With lines among the narrowest, and the N2H+ and NH 3 emitting region among the largest, L183 is a good candidate for comparing these species. Results. A collection of ∼10 kHz for the N 2H+ (J: 1-0) transition is found (∼0.03 km s -1). Similar corrections from a few ms-1 up to ∼0.05 km s-1 compared to previous astronomical determinations are reported for the other transitions (N2H+ (7: 3-2) and N 2D+(Z: 1-0), (J: 2-1), and (J: 3-2)). Einstein spontaneous-decay coefficients (Aul) are included. © ESO 2009.