2013A&A...555A.109B

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. 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. 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. 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. 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.