Discovery of the Ubiquitous Cation NS⁺ in Space Confirmed by Laboratory Spectroscopy

We report the detection in space of a new molecular species which has been characterized spectroscopically and fully identified from astrophysical data. The observations were carried out with the 30m IRAM telescopea. The molecule is ubiquitous as its J=2→1 transition has been found in cold molecular clouds, prestellar cores, and shocks. However, it is not found in the hot cores of Orion-KL and in the carbon-rich evolved star IRC+10216. Three rotational transitions in perfect harmonic relation J' = 2/3/5 have been identified in the prestellar core B1b. The molecule has a ¹Σ electronic ground state and its J=2→1 transition presents the hyperfine structure characteristic of a molecule containing a nucleus with spin 1. A careful analysis of possible carriers shows that the best candidate is NS⁺. The derived rotational constant agrees within 0.3-0.7% with ab initio calculations. NS⁺ was also produced in the laboratory to unambiguously validate the astrophysical assignment. The observed rotational frequencies and determined molecular constants confirm the discovery of the nitrogen sulfide cation in space. The chemistry of NS⁺ and related nitrogen-bearing species has been analyzed by means of a time-dependent gas phase model. The model reproduces well the observed NS/NS⁺ abundance ratio, in the range 30-50, and indicates that NS⁺ is formed by reactions of the neutral atoms N and S with the cations SH⁺ and NH⁺, respectively.