Month: July 2019

Interstellar isocyanogen (CNCN) discovered

This is the third and final part of a trilogy in which we have unveiled several new molecules discovered in our L483 dark cloud. In the first part, it was the HSC and HCS isomers; in the second part, NCO; and today we close with the CNCN: that is, carbon everywhere.

It is true that interstellar chemistry is essentially organic. About three-quarters of the nearly 200 molecules detected to date in the interstellar and circumstellar medium contain at least one carbon atom. Among them there are alcohols, aldehydes, acids,  ethers and amines, but the most frequent functional group is that of nitriles, which contain a group of cyanide –C≡N. In fact, the strong bond of this group is present in more than 30 interstellar molecules, although, until recently, no molecule containing two cyano groups (dinitrile) had been observed in interstellar space.

We have already talked before about cyanogen (NCCN), that lethal gas for humans indirectly detected in our protagonist dark cloud, L483, in 2015 (see the article “Cyanogen: a poison, a comet and a jedi story”, where we talked about the protonated cyanogen, NCCNH⁺.) In this new study, the isocyanogen CNCN, which is a polar and metastable isomer of the cyanogen [1] has been detected for the first time in space (in the same cloud).  

The detection of CNCN in the interstellar medium reinforces the long-held idea that cyanogen is the main precursor to cyanide (CN) that has been observed for decades in many comets. In fact, recently the Rosetta mission  has detected cyanogen in comet 67P.

Cyanogen is the simplest member of the family of dicyanopolyynes, consisting of a highly unsaturated linear skeleton of carbon atoms topped by a cyano group at each end, i.e., N≡C−(C≡C)_n−C≡N. They are stable molecules and the authors of this work that closes our L483 trilogy have deduced that, in interstellar clouds, these molecules with two cyano groups (such as NCCN) are probably as abundant as molecules with a single group –C≡N  (as HCN)  [2].

And why do we have to go around deducting? Can’t we see them directly? Well, that’s the point. The problem in detecting certain species in the interstellar medium (among them, the dicyanopolyynes) is that they do not leave “footprint” because they are not  polar.

As we said in the second part of this trilogy, the more polar a molecule is, the more intense the lines. Therefore, if a molecule has low polarity the lines will become weaker and detecting them will be more complicated.

In this particular case everything is even more complicated, since there is no way to detect the dicyanopolyynes because they are totally apolar. Therefore, having no rotational spectrum, it cannot be observed by radioastronomical techniques. But there are other ways to deduce their presence.

For example: in the carbon-rich envelope IRC+10216 (which we have also talked about a lot for its diva complex), the presence of NCCP – a chemical cousin of the cyanogen in which an atom of N is replaced by an atom of P – was tentatively identified. Therefore, it is reasonable to think (although we cannot observe them) that the dicyanopolyynes can be abundant in molecular clouds.

To investigate the plausibility of this hypothesis, it was proposed that the presence of NCCN in interstellar and circumstellar clouds can be indirectly tested through the observation of chemically related polar molecules, a hypothesis that has been confirmed with the detection a few years ago of protonated cyanogen (NCCNH⁺) and the discovery of the new member of the family, CNCN.

The isocyanogen CNCN

At this point in our trilogy, we see that, to deduce the presence of a molecule indirectly, we have to use a multitude of tools: chemical models that we are perfecting, better detectors and instruments in our radio telescopes and, therefore, more sensitive observations of the areas we are studying.

While the presence of NCCN in interstellar clouds seems undoubted due to the detection of NCCNH+ and CNCN, their abundance remains difficult to define due to the little knowledge about the chemistry that relates to these species. To further know the chemistry of dicyanopolyynes in space it will be necessary to carry out experiments and theoretical studies of some key reactions, in addition to astronomical observations of high sensitivity. It seems we have to keep looking at and interpreting the data from these regions.

Concerning the dark cloud L483, it has revealed some of its secrets in the survey that has given rise to this trilogy, based on several scientific publications with associated discoveries that are cementing a path on which to continue asking us, if it does, “Where do you hide, dicyanopolyyne?”.

Notes:

[1] It has also been tentatively discovered in TMC-1, the taurus molecular cloud.

[2] It is estimated that the abundance of NCCN in relationto H₂ may be of the order of between 10⁻⁹–10⁻⁷, similar to that of HCN.

More information:

“Discovery of Interstellar Isocyanogen (CNCN): Further Evidence that Dicyanopolyynes Are Abundant in Space“. M. Agúndez, N. Marcelino and J. Cernicharo (Institute of Fundamental Physics, CSIC, Spain).

“A sensitive λ 3 mm line survey of L483. A broad view of the chemical composition of a core around a Class 0 object“. M. Agúndez, N. Marcelino and J. Cernicharo (Institute of Fundamental Physics, CSIC, Spain), E. Roueff (Paris Observatory, Sorbonne University, PSL University, CNRS, LERMA 2), and M. Tafalla (National Astronomical Observatory, OAN-IGN, Spain).

Based on observations carried out with the IRAM 30 m radio antenna.

Image: Image of the L483 region captured by NASA’s Spitzer Space Telescope. The circle points the area studied with the IRAM 30m radio telescope and published in the article “A sensitive λ 3 mm line survey of L483. A broad view of the chemical composition of a core around a Class 0 object’.

Originally published in Spanish on the Naukas website: “¿Dónde te escondes, dicianopoliino? Trilogía de L483 (Tercera parte)“. (2019/07/22).