Yes, may be this reminds you the lyrics of “Ultraviolet”, a song from U2 that fits perfectly with our topic. Let’s sing some astrochemistry.

The world of science often says that space is hostile, that it is very difficult to find complex molecules (even if there are). The fact is that one of the culprits of breaking bonds between atoms and leaving everything broken is ultraviolet radiation from stars, and in this case, the distant ultraviolet. But also, like Ying and yang, it may be responsible for liven up certain organic molecules.

Astrochemistry looks for those molecules and study their endurance and how they react. In this particular work, scientists have studied the molecular gas in space that is being strongly irradiated by ultraviolet rays. The team, led by Sara Cuadrado (ICMM-CSIC), has performed (the words in bold are explained below) a complete spectral survey of lines in the millimeter range using the IRAM30m telescope. This spectral survey (a kind of thorough review of that entire range of the electromagnetic spectrum) has been carried out at the edge of the photodissociation region of the Orion Bar, which is being irradiated by a very intense field of distant ultraviolet radiation.

But let us take it one step at a time.

Maybe you already know the Orion Bar from other previous reports: it is located within the well-known Orion Nebula, about 1,300 light years from Earth, and is the closest massive stars formation region.

The energetic far ultraviolet reaches this area of the Orion Bar. This ultraviolet range of light comes from young and massive stars that are forming  nearby (the stars known by the name of the Trapezium) and emit much of their energy in this range of the electromagnetic spectrum. The far ultraviolet is responsible, in this case, for the photodissociation of the molecules.

The photodissociation region is the one in which ultraviolet light is dissociating, that is, separating the atoms of the molecules (although, at the same time, new bonds between atoms may be forming, creating new molecules). The photodissociation region of the Orion Bar is very special because, being “close”, we can study it in detail.

The millimeter range is the range of the electromagnetic spectrum that allows us to study cold areas of the cosmos. For an astronomer a “cold” environment is one in which the temperature does not allow us to observe it because it emits poorly and it is very difficult to detect objects. Infrared and millimeter help us “see” those cold objects. 

Finally, the lines we are talking about are like fingerprints of chemical species. We detect them in space and they are reflected in lines like the ones I show you:

Well, despite the fact that the Orion Bar is a hostile environment where it was only expected to  find very simple molecules, the observations show spectra with many lines (the team has detected more than 850!), of which about 250 correspond to complex organic molecules and related precursors  [1]: methanol, formaldehyde,  formic acid  (the ants one), acetaldehyde, etc.

What does this mean?

La zona de la Barra de Orión sufre el castigo constante de la radiación ultravioleta emitida por estrellas masivas jóvenes del entorno. Por eso se pensaba que no podía haber complejidad química. Pero la hay. Y, aunque se desconocen los procesos por los cuales se forman estas especies descubiertas en el borde de la Barra, se han planteado varios escenarios que explicarían cómo se forman las moléculas orgánicas complejas halladas:

The Orion Bar area suffers the constant punishment of ultraviolet radiation emitted by young massive stars in the environment. That’s why it was thought that there could be no chemical complexity. But there is. And, although the processes by which these species discovered at the edge of the Bar are formed are unknown, several scenarios have been proposed that would explain the formation of the complex organic molecules found:

The first scenario would take into account new chemical reactions that only occur in the hottest gas and that have not yet been included in current theoretical chemistry models that try to reproduce the processes that occur in the interstellar medium.

In the second, complex organic molecules would be produced on the hot surfaces of the nearly bare grains (without ice sheets [2]).

And, in the third scenario, the dynamics of the photodissociation regions (something like currents of motion within the cloud) would cause complex organic molecules or their precursors, which have formed in the icy mantles of dust grains inside the molecular cloud, to sublimate and reach the edge of the Bar.

In short: the presence of complex organic molecules in the interstellar medium is more ubiquitous than initially expected. It includes environments as adverse as gas in the process of colliding at high speeds and, now, gas strongly illuminated by distant ultraviolet radiation. The formation of complex organic molecules reflects the complicated interaction between the chemical processes that occur in the gas phase and on the surface of the dust grains, leaving us with the question of what do all these molecules do in the Bar?

For Sara Cuadrado, “The formation routes of these species are not entirely clear and may not even be the same in different environments. More theoretical studies and laboratory experiments are needed to investigate the different chemical processes that take place on the surface of grains. The next step is, thanks to the new and increasingly powerful telescopes, to study regions similar to the Orion Bar to learn more about the different mechanisms taking place in these chemically surprising regions.”

Going back to the title of this report, you will understand that we sing the chorus of the “Ultraviolet” song from U2, “Baby, baby, baby, light (photodissociate, photoionize and photodesorb) my way”.

Notes:

[1] H₂CO, CH₃OH, HCO, H₂CCO, CH₃CHO, H₂CS, HCOOH, CH₃CN, CH₂NH, HNCO, H₂¹³ CO, and HC₃N (in decreasing order of abundance). The inferred column densities are in the range 10¹¹— 10¹³ cm^-2. The work also provides the upper limit of abundance for some organic molecules that have not been detected in spectral scanning, but are present in other star formation regions: HDCO, CH₃O, CH₃NC, CH₃CCH, CH₃OCH₃, HCOOCH₃, CH₃CH₂OH, CH₃CH₂CN, and CH₂CHCN.

[2] The desorption of complex organic molecules from the icy mantles that coat the dust grains by the action of UV radiation is one of the main mechanisms of formation of these species in the interstellar medium, a mechanism known as photodesorption. But this process does not occur on the illuminated and warmer edge of the Orion Bar, as the dust grains are no longer coated by ice.

More information:

This work has been published in the paper “Complex organic molecules in strongly UV-irradiated gas”, by S. Cuadrado (Molecular Astrophysics Group, Institute of Materials Science of Madrid –CSIC, Spain); J. R. Goicoechea (Molecular Astrophysics Group, ICMM-CSIC, Spain); J. Cernicharo (Molecular Astrophysics Group, ICMM-CSIC, Spain); A. Fuente (Observatorio Astronómico Nacional – IGN, Spain); J. Pety (Institute of Millimeter Radio Astronomy (IRAM); LERMA, Paris Observatory, CNRS/PSL Research University, France); and B. Third (Molecular Astrophysics Group, ICMM-CSIC, Spain).

IMAGES

Image 1: Orion Nebula: The Orion Nebula, an immense stellar nursery about 1,500 light-years away. This stunning false-color view has been based on infrared data obtained with the Spitzer Space Telescope.  Credits: NASA/JPL-Caltech

Image 2: Orion Bar spectrum: Part of the spectral survey in the Orion Bar photodissociation region obtained with the IRAM-30m radio telescope. Credit: Sara Cuadrado.

Originally published in Spanish on the Naukas website: “¿Qué tiene el ultravioleta que a todas horas…?” (2017/10/23).