Nitrogen chemistry and depletion in starless cores

Aims. We investigated the chemistry of nitrogen-containing species, principally isotopologues of CN, HCN, and HNC, in a sample of pre-protostellar cores. Methods. We used the IRAM 30 m telescope to measure the emission in rotational and hyperfine transitions of CN, HCN, (CN)-C-13, (HCN)-C-13, (HNC)-C-13, and (HCN)-N-15 in L 1544, L 183, Oph D, L 1517B, L 310. The observations were made along axial cuts through the dust emission peak, at a number of regularly-spaced offset positions. The observations were reduced and analyzed to obtain the column densities, using the measurements of the less abundant isotopic variants in order to minimize the consequences of finite optical depths in the lines. The observations were compared with the predictions of a free-fall gravitational collapse model, which incorporates a non-equilibrium treatment of the relevant chemistry. Results. We found that CN, HCN, and HNC remain present in the gas phase at densities well above that at which CO depletes on to grains. The CN: HCN and the HNC: HCN abundance ratios are larger than unity in all the objects of our sample. Furthermore, there is no observational evidence for large variations of these ratios with increasing offset from the dust emission peak and hence with density. Whilst the differential freeze-out of CN and CO can be understood in terms of the current chemistry, the behaviour of the CN: HCN ratio is more difficult to explain. Models suggest that most nitrogen is not in the gas phase but may be locked in ices. Unambiguous conclusions require measurements of the rate coefficients of the key neutral-neutral reactions at low temperatures.