Detection of 15NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine line emission and accurate 14N/15N values

Context. Samples of pristine solar system material found in meteorites and interplanetary dust particles are highly enriched in N-15. Conspicuous nitrogen isotopic anomalies have also been measured in comets, and the N-14/N-15 abundance ratio of the Earth is itself higher than the recognised presolar value by almost a factor of two. Low-temperature ion/molecule reactions in the proto-solar nebula have been repeatedly indicated as being responsible for these N-15-enhancements. Aims. We have searched for N-15 variants of the N2H+ ion in L1544, a prototypical starless cloud core that is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. The goal is to evaluate accurate and reliable N-14/N-15 ratio values for this species in the interstellar gas. Methods. A deep integration of the (NNH+)-N-15 (1-0) line at 90.4 GHz was obtained with the IRAM 30 m telescope. Non-LTE radiative transfer modelling was performed on the J = 1-0 emissions of the parent and N-15-containing dyazenilium ions, using a Bonnor-Ebert sphere as a model for the source. Results. A high-quality fit of the N2H+ (1-0) hyperfine spectrum has allowed us to derive a revised value of the N2H+ column density in L1544. Analysis of the observed (NNH+)-N-15 and (NNH+)-N-15 spectra yielded an abundance ratio N((NNH+)-N-15)/N((NNH+)-N-15) = 1.1 +/- 0.3. The obtained N-14/N-15 isotopic ratio is similar to 1000 +/- 200, suggestive of a sizeable N-15 depletion in this molecular ion. Such a result is not consistent with the prediction of the current nitrogen chemical models. Conclusions. Since chemical models predict high N-15 fractionation of N2H+, we suggest that (NN)-N-15-N-14, or N-15 in some other molecular form, tends to deplete onto dust grains.