N2H+ and N15NH+ toward the prestellar core 16293E in L1689N

Context. Understanding the processes that could lead to an enrichment of molecules in 15N atoms is of particular interest because this may shed light on the relatively strong variations observed in the 14N/15N ratio in various solar system environments. Aims. The sample of molecular clouds where 14N/15N ratios have been measured currently is small and has to be enlarged to allow statistically significant studies. In particular, the N2H molecule currently shows the broadest spread of 14N/15N ratios in high-mass star-forming regions. However, the 14N/15N ratio in N2H was obtained in only two low-mass star-forming regions (L1544 and B1b). We here extend this sample to a third dark cloud. Methods. We targeted the 16293E prestellar core, where the N15NH+ J = 1−0 line was detected. Using a model previously developed for the physical structure of the source, we solved the molecular excitation with a nonlocal radiative transfer code. For this purpose, we computed specific collisional rate coefficients for the NNH-H2 collisional system. As a first step of the analysis, the N2H abundance profile was constrained by reproducing the N2H J = 1−0 and 3−2 maps. A scaling factor was then applied to this profile to match the N15NH+ J = 1−0 spectrum. Results. We derive a column density ratio N2H/NNH = 330+170 −100. Conclusions. We performed a detailed analysis of the excitation of N2H and N15NH+ in the direction of the 16293E core with modern models that solve the radiative transfer and with the most accurate collisional rate coefficients available to date. We obtained the third estimate of the N2H/NNH column density ratio in the direction of a cold prestellar core. The current estimate ∼330 agrees with the typical value of the elemental isotopic ratio in the local interstellar medium. It is lower than in some other cores, however, where values as high as 1300 have been reported.