Molecular detectives

The discovery of the new NS+ cation, present in numerous astrophysical environments, confirmed with laboratory spectroscopy

NS+. Credits: KIDA, Kinetic Database for Astrochemistry.

A team of researchers, led by José Cernicharo (IFF-CSIC) has announced the detection in space of a new molecular species, identified from astrophysical data obtained with observations carried out with the IRAM 30m telescope.

Although nitrogen sulfide  (NS) was first detected in space in 1975, the presence of its NS+ cation had not been discovered until now. A cation is an atom or molecule with a positive electric charge because it has lost electrons from its original endowment. In the case of  NS+, the chemical models applied in this work indicate that it is formed by the reactions of the neutral atom N with the cation SH+ and that of the neutral atom S with the cation NH+.

The interesting thing about this work is not only the first detection of NS+ [1], but the discovery of its ubiquity and its presence in most astrophysical environments: it  has been detected in cold and faint molecular clouds where there is still no activity of star formation, in somewhat denser clouds where matter begins to collapse and prestellar nuclei begin to be seen, and in clouds that are authentic stellar nurseries, where violent processes due to ultraviolet radiation from young stars and to jets of material ejected by the protostars  [2] are already taking place.

A detective work confirmed in the laboratory

The gaseous phase chemistry of cold and dark clouds is mainly based on the reactions between ions (electrically charged molecules) and neutral molecules. However, ions (positively charged cations, or negatively charged anions) represent only a small percentage (about 15%) of the molecular species detected.

But if NS+ is present in so many astrophysical environments, why wasn’t it identified before?

Since astrochemistry has increasingly precise tools, we often talk about spectra with “forests of lines”, a deep collection of overlapping data that is difficult to discriminate. Where does one molecule end and another begin? If they have never been characterized before, if they have not been studied and checked against laboratory data or if their possible presence has not been theorized, it is very likely that they will remain anonymous, sometimes before our eyes.

In the words of José Cernicharo (Institute of Fundamental Physics, CSIC),“The detection of NS+ was a real work of “molecular detectives”. When we realized that in that observational data there was a pattern that was repeated, we started a search in which, first, we discarded various candidates.”  

An in-depth understanding of how molecular spectroscopy works and interstellar chemistry were the tools that led the team to determine that the NS+ cation is the species most likely responsible for producing the lines found.

On the other hand, in astrochemistry there is a lot of laboratory effort whose purpose is to validate the results of the theoretical models and the data observed with the telescopes (in this case the IRAM 30m antenna, in Pico Veleta, Granada). To confirm that it was possible for NS+ to be in so many different environments, the “Laboratory of Atoms, Molecules and Lasers Physics” (CNRS and  University of  Lille, France) carried out several experiments to reproduce NS+.

The information obtained, applying spectroscopy techniques, corroborated full coincidences with both observational data and theoretical models: they had found a new molecular species in space.


[1] The NS+ cation has been completely characterized through three rotational transitions, one of them with hyperfine structure, distinctive of a molecule with an atom with spin 1.

[2] Although present in a wide variety of environments, it is apparently not in others such as, for example, the hots cores of  Orion-KL or the evolved star IRC+10216.

More information:

This work has been presented in the paper Discovery of the ubiquitous cation NS+ in space confirmed by laboratory spectroscopy and its authors are  J. Cernicharo  (Molecular Astrophysics Group (ICMM-CSIC)/Molecular Astrophysics Group, Department of atomic, molecular and surface processes (IFF-CSIC), Spain);  B. Lefloch  (University Grenoble Alpes, France); M. Agundez  (Molecular Astrophysics Group (ICMM-CSIC)/Molecular Astrophysics Group, Department of atomic, molecular and surface processes (IFF-CSIC), Spain);  S. Bailleux (Laboratory of Atoms, Molecules and Lasers Physics, CNRS, University of  Lille, France); L. Margulès  (Laboratory of Atoms, Molecules and Lasers Physics, CNRS, University of  Lille, France); E. Roueff (LERMA, Paris Observatory, PSL Research University, CNRS, University of The Sorbona, France);   R. Bachiller  (National Astronomical Observatory (OAN, IGN), Spain); N. Marcelino  (Molecular Astrophysics Group (ICMM-CSIC)/Molecular Astrophysics Group, Department of atomic, molecular and surface processes (IFF-CSIC), Spain); B. Tercero  (Molecular Astrophysics Group (ICMM-CSIC)/ National Astronomical Observatory (OAN, IGN), Spain); C.  Vastel (IRAP, University of Toulouse, CNRS, UPS, CNES, France); E. Caux (IRAP, University of Toulouse, CNRS, UPS, CNES, France).

Originally published in Spanish on the Naukas website: “Detectives moleculares” (2018/02/12).