Tag: L483

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L483 Trilogy (Part Three) – Where do you hide, Dicyanopolyyne?

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).

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L483 trilogy (Part Two) – Put a radical in your dark cloud

Detection of the NCO radical in space

This is what the L483 dark cloud looks like if we don’t use radio astronomy

In the first part of this series we talked about the sulfur lost in L483. But in the studies carried out in this dark cloud, much more has been discovered. Among them, the first detection in space of the isocyanate radical (NCO) with a significant abundance.

Studying the observations of L483 carried out with the IRAM 30m radio telescope, it was seen that there were bright carbon chains such as C4H, which suggested that the region could host a propitious environment to carbon chain chemistry [1].

Why are carbon chains important? Most molecules observed in space can be formed only with atoms of hydrogen (H), carbon (C), nitrogen (N) and oxygen (O). These atoms are the pieces for building organic and prebiotic molecules and, together, constitute the backbone of the peptide bond that binds two amino acids and allows the construction of long proteins. Therefore, the observation of simple molecules with the C(=O)–N group in space can provide important clues about the first chemical steps in amino acid synthesis, considered key in all biological processes.

The isocyanate radical precisely consists of a C(=O)–N structure and is therefore the simplest molecule that houses the base scheme of the peptide bond. It has efficient formation mechanisms but, although it is predicted (from what the models tell us) that it must be abundant in dark clouds… its abundance is small, which complicates its detection. In addition, it has a low polarity (the higher the polarity, the more intense the lines of the molecule) [2] so the observed lines are weak. To this must be added the “noise” (which will depend on the observation time and the sensitivity of the instrument).

We usually use the metaphor of the field of grass that does not let you see the flowers: our field of herbs (the noise) will be reduced the higher the quality and sensitivity of our observations, letting us distinguish the “flowers”, which would be the lines of the molecules.

Technological advances are enabling us to make progress in this direction. Increasingly sensitive detectors are being built, which makes the noise less. In addition, in this work a deep survey has been carried out that has allowed us to observe in more detail, providing many unexpected results (which we will continue to talk about in the third part of this trilogy, what did you think, that we were going to tell you at once? Well, no).

How the NCO is formed

When talking about “zones” or regions in a certain environment of space, we must clarify that there is no uniformity in the conditions that give rise to the chemistry of these places. In fact, recent observations carried out with the ALMA interferometer  have demonstrated a chemical differentiation in L483, which has carbon chains such as C2H that trace the envelope, and more complex organic compounds distributed around the protostar, that is, in these two areas different physical and chemical phenomena are occurring that give rise to a different chemical richness.

The detection of NCO (carried out with IRAM 30m) has taken place in the envelope of the low mass protostar in L483, and with this information it follows that the chemical processes for the formation of NCO are mainly two: one is from the reaction between CN and O2 and another would be by the recombination of the ion H2NCO+, which has also been detected in this work, thus supporting the formation of NCO by this route.

One of the important aspects of taking steps in the discoveries of new molecules is that the chemical models are updated, in this case, those related to NCO: taking into account the uncertainties in the model, the observed abundances are reproduced relatively well, which indicates that we are on the right track.

But there is still much to study. Although the survey has been of incredible sensitivity, “The next step -says Nuria Marcelino, lead author of this paper- would be to carry out NCO observations on sources that are at different stages of the star formation process. This could help us understand its role in the prebiotic chemistry of space.”

“With this survey –she continues– we have revealed the chemical richness of L483, discovering several species that had not been detected before and confirming others that had been detected tentatively. Finally, we have been able to see the flowers among the grass.”

But, friends, there are still many flowers to be revealed. We’ll look at some of them in the next part of this L483 trilogy.

Notes:

[1] Apart from carbon chains, L483 is also rich in oxygen-carrying organic molecules    such as HCO, HCCO, H2CCO, CH3CHO, HCCCHO and c-C3H2O.

[2] Polarity has to do with the distribution of the electric charge in the molecule. The more asymmetric the charge distribution, the more polar the molecule. The main implication of this is that, the more polar a molecule is, the more intense the lines. Therefore, as far as the NCO is concerned, the low polarity makes the lines weak, making it difficult to detect them.

More information:

This work has been published in the paper “Discovery of the elusive radical NCO and confirmation of H2NCO+ in space“, A&A 612,L10  (2018). By N. Marcelino, M. Agúndez, J. Cernicharo (Instituteof FundamentalPhysics, CSIC, Spain), E. Roueff (Sorbonne University, Paris Observatory, CNRS, France) and M. Tafalla, (National Astronomical Observatory, IGN, Spain). Based on observations carried out with the IRAM 30m radio telescope.

Image: This is what the L483 dark cloud looks like if we don’t use radio astronomy. Credit:  NRAO/Gary Fuller. https://www.cv.nrao.edu/~awootten/l483/l483.html

Originally published in Spanish on the Naukas website: “Pon un radical en tu nube oscura. Trilogía de L483 (Segunda parte)” (2019/06/26).