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Samples of pristine Solar System material found in meteorites andinterplanetary dust particles are highly enriched in (15)N. Conspicuousnitrogen isotopic anomalies have also been measured in comets, and the(14)N/(15)N abundance ratio of the Earth is itself larger than the recognisedpre-solar value by almost a factor of two. Ion--molecules, low-temperaturechemical reactions in the proto-solar nebula have been repeatedly indicated asresponsible for these (15)N-enhancements. We have searched for (15)N variantsof the N2H+ ion in L1544, a prototypical starless cloud core which is one ofthe best candidate sources for detection owing to its low central coretemperature and high CO depletion. The goal is the evaluation of accurate andreliable (14)N/(15)N ratio values for this species in the interstellar gas. Adeep integration of the (15)NNH+ (1-0) line at 90.4 GHz has been obtained withthe IRAM 30 m telescope. Non-LTE radiative transfer modelling has beenperformed on the J=1-0 emissions of the parent and (15)N-containing dyazeniliumions, using a Bonnor--Ebert sphere as a model for the source. A high-qualityfit of the N2H+ (1--0) hyperfine spectrum has allowed us to derive a revisedvalue of the N2H+ column density in L1544. Analysis of the observed N(15)NH+and (15)NNH+ spectra yielded an abundance ratio N[N(15)NH+]/N[(15)NNH+] = 1.1+/- 0.3. The obtained (14)N/(15)N ratio is ~ 1000 +/- 200, suggestive of asizeable (15)N depletion in this molecular ion. Such a result is not consistentwith the prediction of present nitrogen chemical models. As chemical modelspredict large (15)N fractionation of N2H+, we suggest that (15)N(14)N, or (15)Nin some other molecular form, is preferentially depleted onto dust grains.

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