N$_2$H$^+$ and N$^{15}$NH$^+$ towards the prestellar core 16293E in L1689N

Context. Understanding the processes that could lead to an enrichment of molecules in ^(15)N atoms is of particular interest because this may shed light on the relatively strong variations observed in the ^(14)N/^(15)N ratio in various solar system environments. Aims. The sample of molecular clouds where ^(14)N/^(15)N ratios have been measured currently is small and has to be enlarged to allow statistically significant studies. In particular, the N_2H^+ molecule currently shows the broadest spread of ^(14)N/^(15)N ratios in high-mass star-forming regions. However, the ^(14)N/^(15)N ratio in N_2H^+ 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 N^(15)NH^+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 N^(15)NH^+-H_2 collisional system. As a first step of the analysis, the N_2H^+ abundance profile was constrained by reproducing the N_2H^+J = 1−0 and 3−2 maps. A scaling factor was then applied to this profile to match the N^(15)NH^+J = 1−0 spectrum. Results. We derive a column density ratio N_2H^+/N^(15)NH^+ = 330^(+170)_(-100). Conclusions. We performed a detailed analysis of the excitation of N_2H^+ and N^(15)NH^+ 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 N_2H^+/N^(15)NH^+ 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.