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.


[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).

L483 trilogy (Part One) – In Search of Lost Sulphur

First identification of two species in space, HCS and HSC

Image of the L483 region captured by NASA’s Spitzer Space Telescope

Over the past few decades, various research teams working to study different areas of space have had the same question: where is the missing sulfur? This is what happens, for example, with the study of some protoplanetary disks  and interstellar clouds. That is because sulfur chemistry outside the Earth, especially in the dark and cold clouds, on which we will focus today, presents some unknowns such as the detection of less sulfur than expected.

Sulfur in the gas phase can be found as part of different molecules (called sulfur “carrier” species).  However, these species are less than 0.1% of the estimated cosmic abundance of sulfur for dark clouds, that is, only a tiny amount of what is supposed to be there has been detected.

The case of HCS and HSC in the cold cloud L483

The team led by Marcelino Agúndez, Ramón y Cajal researcher at the Institute of Fundamental Physics of the CSIC, who is among those looking for “lost sulfur”, detected two new molecules carrying sulfur: the HCS radical and its less stable isomer, HSC. They identified them in the dense cloud L483, a source with a very rich chemistry and where new molecules from other families (such as  HCCO, NCCNH+ and  NS+) have also been discovered.  

L483 is a molecular cloud, located in the Aquila Rift, which houses the IRAS 18148-0440 protostar. This protostar is in full transition, going from being a protostar of class 0 to class I, that is, the dust and gas around it are taking the form of a disk and begin to distinguish its layers (although it does not yet have reactions in the core, that will come after going through phases II and III, after which it will end up being a full-right star).

The presence of the protostar in L483 involves a great deal of activity, with material from the cloud falling by gravity on it, fattening it, as well as a powerful jet of matter emanating from it, taking away much of the angular momentum and favoring the process of star formation. Given the environment, there should be a lot of sulfur, so it must be identified among the observations’ data.

The sulfur molecules discovered, HCS and HSC, are not abundant enough to explain the problem of lost sulfur, but their detection has revealed several peculiarities about how sulfur chemistry works in interstellar clouds. Since sulfur is in the same column of the periodic table as oxygen, both elements are considered to have similar chemical properties. However, the detection of HCS and HSC has revealed that sulfur and oxygen chemistry behave significantly differently than expected. Let’s see why.

To understand this we can compare the abundances of sulfur molecules with those of their oxygen analogues. That is, if we talk about the molecule with hydrogen, carbon and sulfur (HCS), its analog with oxygen is HCO (hydrogen, carbon and oxygen). In this way we can compare the relative abundances of the species carrying sulfur H2CS /HCS (being H2CS the stable form and HCS the unstable radical) with the relative amounts of H2CO /HCO.

The result is that in L483, H2CO (formaldehyde) is ten times more abundant than HCO (formyl radical). But in the case of its sulfur analogue the same does not happen, since H2CS (Thioformaldehyde) is as abundant as HCS. In addition, in L483 HCS is even more abundant than HCO, which is surprising, given that oxygen is more abundant than sulfur in the cosmos.

On the other hand, the metastable isomer HSC is found with a low abundance, while its oxygen analogue HOC has not yet been observed in space, mainly because there is a total lack of experimental information about this species (it has not been possible to characterize it in laboratory experiments).

Delving a little into the title of this report, we wonder, again, where is the sulfur that we don’t see. It is interesting to note a recent study, by Vidal et al., “On the reservoir of sulphur in dark clouds: chemistry and elemental abundance reconciled“, which    concludes that most of the sulfur in dark cold clouds should be in the form of H2S and SH ice on the surface of dust grains.

This study also indicated that, to detect HCS in gas phase, it would be necessary more than a thousand hours of observation with the IRAM 30 m telescope, concluding that it was difficult to locate HCS in gas phase in dark and cold clouds. However, this work, in the words of Agúndez, “shows that, although there are many unknowns to be solved, we have managed to detect the presence of HCS with much fewer hours of observation“.

Then, where is the sulfur missing from the dark, cold clouds? It is possible that sulfur is deposited only on dust grains, although it is not clear how.  Part of it could get trapped in the core of the grains as refractory compounds and another part could be in the form of ice. Although it is also possible that molecules of the gas phase that have not yet been identified may contain a significant amount of sulfur. It may even be in atomic form.

So, we assume that sulfur is there, but we haven’t been able to detect it yet with our instruments. On the other hand, studies of this region have not only revealed its “lack” of sulfur… but that will be in the next chapter of this trilogy about L483, the dark cloud that has so many things to reveal about the chemical complexity of the universe.

More information:

This work has been published in the paper “Detection of interstellar HCS and its metastable isomer HSC: new pieces in the puzzle of sulfur chemistry”,  A&A 611, L1 (2018). By M. Agúndez, N. Marcelino, J. Cernicharo  (Institute of Fundamental Physics, CSIC, Spain)  and M. Tafalla, (Observatorio Astronómico Nacional, IGN, Spain). Based on observations carried out with the IRAM 30m Telescope.

IMAGE: Image of the L483 region captured by NASA’s Spitzer Space Telescope. http://www.spitzer.caltech.edu/images/3132-sig10-006e-Protostellar-Envelope-and-Jet-L483

Originally published in Spanish on the Naukas website: “En busca del azufre perdido. Trilogía de L483 (Primera parte)” (2019/06/18).

Broad-band high-resolution rotational spectroscopy for laboratory astrophysics

Authors: Cernicharo, J.; Gallego, J. D.; Lopez-Perez, J. A.; Tercero, F.; Tanarro, I; Beltran, F.; de Vicente, P.; Lauwaet, K.; Aleman, B.; Moreno, E.; Herrero, V. J.; Domenech, J. L.; Ramirez, S., I; Bermudez, C.; Pelaez, R. J.; Patino-Esteban, M.; Lopez-Fernandez, I; Garcia-Alvaro, S.; Garcia-Carreno, P.; Cabezas, C.; Malo, I; Amils, R.; Sobrado, J.; Diez-Gonzalez, C.; Hernandez, J. M.; Tercero, B.; Santoro, G.; Martinez, L.; Castellanos, M.; Vaquero Jimenez, B.; Pardo, J. R.; Barbas, L.; Lopez-Fernandez, J. A.; Aja, B.; Leuther, A.; Martin-Gago, J. A.

Contribution: Article


Publication date: 2019/06/07

DOI: 10.1051/0004-6361/201935197

Oxygen fractionation in dense molecular clouds

Authors: Loison, Jean-Christophe; Wakelam, Valentine; Gratier, Pierre; Hickson, Kevin M.; Bacmann, Aurore; Agundez, Marcelino; Marcelino, Nuria; Cernicharo, Jose; Guzman, Viviana; Gerin, Maryvonne; Goicoechea, Javier R.; Roueff, Evelyne; Le Petit, Franck; Pety, Jerome; Fuente, Asuncion; Riviere-Marichalar, Pablo

Contribution: Article


Publication date: 2019/06/01

DOI: 10.1093/mnras/stz560