N2H+ and N15NH+ towards the prestellar core 16293E in L1689N
Abstract
Understanding the processes that could lead to enrichment of molecules in 15N atoms is of particular interest in order to shed light on the relatively large variations observed in the 14N/15N ratio in various solar system environments. Currently, the sample of molecular clouds where 14N/15N ratios have been measured is small and has to be enlarged in order to allow statistically significant studies. In particular, the N2H+ molecule currently shows the largest 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). The current work extends this sample to a third dark cloud. 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 non-local radiative transfer code. For that purpose, we computed specific collisional rate coefficients for the N15NH+-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. We derive a column density ratio N2H+ / N15NH+ = 330+170-100. The current estimate 330 agrees with the value typical of the elemental isotopic ratio in the local ISM. It is however lower than in some other cores, where values as high as 1300 have been reported.
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