Detection of 15NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine line emission and accurate14N/15N values

Context. Samples of pristine solar system material found in meteorites and interplanetary dust particles are highly enriched in 15N. Conspicuous nitrogen isotopic anomalies have also been measured in comets, and the 14N/15N 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 15N -enhancements. Aims. We have searched for 15N 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 14N/15N ratio values for this species in the interstellar gas. Methods. A deep integration of the 15NNH+ (1−0) line at 90.4GHz was obtained with the IRAM 30m telescope. Non-LTE radiative transfer modelling was performed on the J = 1−0 emissions of the parent and 15N-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 N15NH+ and 15NNH+ spectra yielded an abundance ratio N(N15NH+)/N(15NNH+) = 1.1 ± 0.3. The obtained 14N/15N isotopic ratio is ∼1000 ± 200, suggestive of a sizeable 15N 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 15N fractionation of N2H+, we suggest that 15N14N, or 15N in some other molecular form, tends to deplete onto dust grains.