SIMBAD references

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 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.
Results. We derive a column density ratio N₂H⁺/N¹⁵NH⁺=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⁺/N¹⁵NH⁺ 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.