A surprisingly abundant radical

Astronomers discover the presence of interstellar ketenyl (HCCO)

Stars use to form in dark and cold clouds. But everything has a beginning. When the process of stellar formation in these clouds has not yet begun, recent research reveals that they could undergo chemical processes other than those we imagined, taking on a relevant role the grains of dust, whose action, until now, had essentially been confined to the clouds where the existence of a nascent star was already known.

Lupus 1 molecular cloud region, where the presence of the ketenyl radical has been discovered.

The molecular clouds we find in the interstellar environment are dim, seemingly empty spaces in which stars are born. It is a process by which the gas and dust contained in these clouds begin to condense and fragment at some points, collapsing with gravity and leading to protostars. There are still many unknowns related to the reasons that cause this process to trigger. Therefore, molecular clouds are highly studied environments in the field of Astrochemistry.

Recently, a team of researchers has studied nine of these dark clouds using data obtained with the IRAM 30m radio telescope. Among them were both dark and cold clouds without stellar nuclei, as well as molecular clouds in which the birth of a protostar has already taken place.

Detailed analysis of these observations has revealed, for the first time, the presence of ketenyl radical (HCCO) in both the dark and cold cloud core Lupus-1A [1], in which there is still no indication of the presence of a protostar, and in the molecular cloud L483, which has a protostar inside.

In addition, ketene (H2CCO) and acetaldehyde (CH3CHO) molecules have been found in these two sources and in three more dark clouds. Finally, formyl radical (HCO) has been detected in the nine sources (which has been a surprise) and propylene (CH2CHCH3)[2] in four of the observed clouds, significantly expanding the number of dark clouds where these molecules are known to be present. This work has therefore presented two rather eye-catching new study fronts.

HCO, you used to be cool

The formyl radical (HCO) is usually a tracer of photodissociation regions, that is, it is found in areas where there is an intense activity of stellar formation and radiation tends to break the bonds of molecules, making the existence of complex molecules difficult. But it now seems to be confirmed that it is not only present in regions of photodissociation, but are also found in the dark and cold clouds: it has been detected in the nine clouds studied in this work. [3] So, from being a radical present only in active areas, now it has become present in both environments, losing his exclusivity.

Cold dark cloud gas phase chemical models can reproduce the observed abundances of HCO, but they cannot explain the presence of ketenyl radical (HCCO) in Lupus-1A and L483 and the high abundance derived from propylene.

HCCO, insistent ketenyl

The real discovery has been to know the abundance of ketenyl radical. Normally a radical is expected to be less abundant than its stable counterpart, in our case ketene. This is because radicals are chemically more unstable and reactive.  

But in both Lupus-1A and L483, ketenyl radical is only about 10 times less abundant than ketene, indicating that there must be an effective mechanism for forming this radical. And one more fact: the ketenyl radical (HCCO) found in Lupus-1A and L483 is a missing link in the HxC2O series. [4]

Cloud with star, cloud without star

Organic molecules are ubiquitous in interstellar clouds. The most complex and saturated are found in clouds inside which stellar objects are forming. The energy released by the forming star is the engine that causes the chemical machinery to become operational, allowing the thermal evaporation of  molecules frozen on the surface of the dust grains. 

But, in dark cold clouds where there are no stellar nuclei yet, the chemical composition is characterized by being simpler and more unsaturated: highly unsaturated carbon chains are found in the polyyne and cyanopolyyne families, as well as relatively simple oxygen-carrying organic molecules, whose synthesis depends to a large extent on the chemical processes that take place in the gas phase. As there is no star, there is no evaporation of ice in the dust grains.

However, the frequent presence of methanol (CH3OH) in cold dark clouds and the most recent detections of other complex and saturated organic molecules such as propylene (CH2CHCH3), methyl formicate (CH3OCOH), dimethyl ether (CH3OCH3), methoxyl (CH3O) and formic acid (HCOOH), have put on the table the role of reactions on the surface of dust grains and non-thermal desorption processes in these cold and seemingly calm environments.  

In short, the dark and cold clouds, in which stars are not yet forming, could undergo chemical processes other than those we imagined, taking on a relevant role the grains of dust, whose action, until now, had essentially been confined to the clouds where the existence of a nascent star was already known.

It is clear that, in light of these new observational results, it will be necessary to review the chemistry of cold dark clouds. We need in-depth observational studies capable of expanding both the number of chemically characterized sources and the inventory of identified molecules. All because, until now, we thought the ices slept peacefully on the dust grains if there was no heat to wake them up.

More information:

This work has been published in the scientific paper Discovery of interstellar ketenyl (HCCO), a surprisingly abundant radical, whose authors are Marcelino Agúndez (Institute of Materials Science of Madrid-CSIC, Spain);  José Cernicharo (Institute of Materials Science of Madrid-CSIC, Spain); and Michel Guélin (Institute de Millimetric Radioastronomy, IRAM, France).


[1] Located in the constellation Lupus (the wolf), this area was discovered by Sakai et al. (2010). This starless core in the molecular cloud of Lupus has a chemical richness comparable to that of the widely studied TMC-1 cloud and provides an excellent opportunity for the study of dark cloud chemistry.

[2] Formyl radical (HCO) had only previously been detected in L1448, B1 and TMC-1, and propylene (CH2CHCH3), only in TMC-1. 

[3] Perhaps it was because until now the searches had been carried out at other wavelengths. 

[4] Many of the organic oxygen-carrying molecules observed in the dark clouds can be described by the general formula HxCnO, with n = 1 (CO, HCO, H2CO, CH3O, y CH3OH), n = 2 (C2O, H2CCO, and CH3CHO), andn = 3 (C3O y HCCCHO). The formation of most of them is reasonably explained either by chemical reactions of the gas phase (with notable exceptions as in the case of CH3OH). That’s why ketenyl radical (HCCO) is considered to be a missing link in the HxC2O series.


Lupus 1 molecular cloud region, where the presence of the cetenyl radical has been discovered. Credits: Apo TEC140 (140/f7.2) – FLI Proline 16803 – L (340m) R (120m) G (120m) B (120m) – Warrumbungle Observatory, Coonabarabran, NSW, Australia.

Originally published in Spanish on the Naukas website: Un radical sorprendentemente abundante (2015/05/22).

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