Centre for Astrochemical Studies, Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85749 Garching bei München, Germany
e-mail: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
2 Department of Physics, University of Helsinki, PO Box 64, 00014, Finland
3 Istituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
4 Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via F. Selmi 2, 40126 Bologna, Italy
Accepted: 31 May 2018
Context. The 15N 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 14N/15N ratio in N2H+ has been accurately measured in only one prestellar source, L1544, where strong levels of fractionation, with depletion in 15N, are found (14N/15N ≈ 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+, N15NH+, and 15NNH+ in L183, L429, and L694-2 with the IRAM 30 m 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 ≈440, close to the protosolar value. Our prestellar core sample shows a high level of depletion of 15N in diazenylium, as previously found in L1544. A revision of the N chemical networks is needed in order to explain these results.
Key words: ISM: clouds / ISM: molecules / ISM: abundances / radio lines: ISM / stars: formation
This work is based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
The reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/617/A7
© ESO 2018