14N/15N ratio measurements in prestellar cores with N2H+:new evidence of 15N-antifractionation

Context. The N-15 fractionation has been observed to show large variations among astrophysical sources, depending both on the type of target and on the molecular tracer used. These variations cannot be reproduced by the current chemical models. Aims. Until now, the N-14/N-15 ratio in N2H+ has been accurately measured in only one prestellar source, L1544, where strong levels of fractionation, with depletion in N-15, are found (N-14/N-15 approximate to 1000). In this paper, we extend the sample to three more bona fide prestellar cores, in order to understand if the antifractionation in N2H+ is a common feature of this kind of source. Methods. We observed N2H+, (NNH+)-N-15, and (NNH+)-N-15 in L183, L429, and L694-2 with the IRAM 30m telescope. We modelled the emission with a non-local radiative transfer code in order to obtain accurate estimates of the molecular column densities, including the one for the optically thick N2H+. We used the most recent collisional rate coefficients available, and with these we also re-analysed the L1544 spectra previously published. Results. The obtained isotopic ratios are in the range 580-770 and significantly differ with the value, predicted by the most recent chemical models, of approximate to 440, close to the protosolar value. Our prestellar core sample shows a high level of depletion of N-15 in diazenylium, as previously found in L1544. A revision of the N chemical networks is needed in order to explain these results.