2017A&A...604A..26D

Context. Isotopic fractionation in dense molecular cores has been suggested as a possible origin of large ¹⁴N/¹⁵N ratio variations in solar system materials. While chemical models can explain some observed variations with different fractionation patterns for molecules with -NH or -CN functional groups, they fail to reproduce the observed ratios in diazenylium (N₂H⁺).
Aims. Observations of doubly ¹⁵N-substituted species could provide important constraints and insights for theoretical chemical models of isotopic fractionation. However, spectroscopic data are very scarce.
Methods. The rotational spectra of the fully ¹⁵N-substituted isopologues of the diazenylium ion, ¹⁵N₂H⁺ and ¹⁵N₂D⁺, have been investigated in the laboratory well into the THz region by using a source-modulation microwave spectrometer equipped with a negative glow discharge cell. An extended chemical reaction network has been used to estimate what ranges of ¹⁵N fractionation in doubly ¹⁵N-substituted species could be expected in the interstellar medium (ISM).
Results. For each isotopologue of the H- and D-containing pair, nine rotational transitions were accurately measured in the frequency region 88 GHz-1.2 THz. The analysis of the spectrum provided very precise rest frequencies at millimeter and sub-millimeter wavelengths, useful for the radioastronomical identification of the rotational lines of ¹⁵N₂H⁺ and ¹⁵N₂D⁺ in the ISM.