Using the IRAM-30m radio telescope and sophisticated interstellar chemistry models, an ASTROMOL team has studied the composition and spatial distribution of small hydrocarbons in the “Orion Bar”, a clear example of molecular cloud radiated by ultraviolet light
Hydrocarbons are the simplest organic molecules, formed only by hydrogen and carbon. They are one of the main sources of energy in the modern world, as they are part of oil, natural gas, gasoline; they are also found in many materials that we usually use, such as plastics, fibers or paints, and we even walk on them every day as they are the main component of asphalt.
But not only can we find hydrocarbons on our planet: since the 1970s it is known that hydrocarbons are present in much of the interstellar environment, and one of the issues that astrochemistry has been looking to clear since then is how they form and what their chemical behavior is in that environment.
In order to study them, one of the most appropriate environments are Photodissociation Regions (PDRs), transition zones between cold and neutral gas (mostly molecules) protected from ultraviolet radiation, and atomic and ionized gas, illuminated by intense ultraviolet fields mostly coming from massive stars .
Photodissociation regions are found in many astrophysical environments and on many spatial scales, from the nuclei of star-forming galaxies to the illuminated surfaces of protoplanetary disks. All of them show a chemistry whose common characteristic is the photodissociation of molecules caused by ultraviolet radiation.
The most spectacular and close example of this type of photodissociation region is the so-called Orion Bar, which is located within the well-known Orion Nebula, located about 1,300 light-years from Earth. The Orion Nebula is one of the most studied astronomical objects of all time: it is an immense cloud of gas and dust lovingly regarded as a stellar nursery, as thousands of stars begin their lives there. It is not the only stellar nursery in the galaxy but, being the closest forming massive stars (more than 8 times the mass of the Sun), it offers us the opportunity to study in detail how the stars are born; how, once formed, they interact with the interstellar environment that surrounds them; and, in particular, how intense stellar ultraviolet radiation fields end up “destroying” (photodissociating) the molecular cloud where they were born.
Ultraviolet radiation that ionizes atoms and dissociates molecules in the Orion Bar comes from the famous set of massive stars in the Trapezium cluster, which takes its name from the asterism that make up its four brightest stars.
Having a chemistry controlled by ultraviolet radiation makes that, in these regions, very peculiar species are produced, such as radicals (C2H, OH, HCO…) and ions (SO+, CO+, CH+, HOC+, etc.). These species do not exist naturally on our planet, as they are extremely reactive and unstable and quickly react with other molecules to form new, more stable compounds. They can only be formed in the laboratory under very specific controlled conditions.
The study of the Orion Bar
In order to establish the limits of the chemical complexity of the interstellar environment, and using data obtained with a spectral mapping  carried out with the IRAM-30m radio telescope (located in Sierra Nevada, Granada, Spain), an ASTROMOL team has managed to expand our knowledge of which molecules exist in environments radiated by strong fields of ultraviolet radiation and how they form.
Although the Orion Bar is a hostile environment where you would expect only very simple molecules, observations show a spectrum with more than 500 lines coming from the emission of more than 60 different molecules containing 2 to 6 atoms. What is surprising is that approximately 40% of the detected lines belong to hydrocarbons ! So, it’s all about making a word game and claiming that we have an area with a hydrocarbon open bar in Orion.
But how do these hydrocarbons form in the interstellar environment and why are they so abundant? Until now, in studies in other regions radiated by less intense ultraviolet radiation fields, such as the famous Horsehead Nebula, or in interstellar diffuse clouds, the results obtained through the analysis of observations did not match the theoretical results obtained from gas phase models . The abundances of hydrocarbon measured in these regions were much greater than those predicted by these models. That is, gas chemistry was not enough to explain these high abundances.
Researchers looked for alternative sources of carbon that might be contributing to the amount of hydrocarbons formed through gas reactions, and thought about polycyclic aromatic hydrocarbons (PAHs). PAHs are powerful environmental pollutants, but they are also present ubiquitously in the universe (see image 2). In these regions, the incidence of radiation on PAHs would completely break down the cyclical structure of these compounds, forming small hydrogen and carbon molecules, and contributing to the amount of hydrocarbons formed by gas phase reactions.
However, the ASTROMOL team has discovered that, to explain the high abundances of hydrocarbons in the Orion Bar, there is no need to resort to the destruction of PAHs (or their contribution is not the dominant one) as Trapezium stars illuminate the region with ultraviolet radiation fields so intense that molecular gas reaches very high temperatures, bringing into action new gas chemical reactions that need very high energies to occur .
This takes a step further in understanding the results and details of photodissociation in gas clouds, helping us improve our knowledge of interstellar carbon chemistry and learn more about how chemical complexity in space increases.
 Generally, massive OB-type stars, at least 8 times more massive than the Sun and main source of ultraviolet radiation in galaxies like ours.
 Spectral maps are one of the most important tools in the field of astrochemistry to study the interstellar medium, as they allow to carry out a complete chemical characterization of the region under study. In this case, spectral lines have been obtained in the millimeter range, one of the lowest energy in the electromagnetic spectrum and whose emission is dominated by low-energy transitions produced by molecules.
 C2H, C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H, l-C3H+ and l-H2C in decreasing order of abundance.
 These models attempt to computationally simulate the physical and chemical conditions of interstellar clouds, simulating hundreds of chemical reactions and processes that occur in different regions.
 Endothermal reactions (or with barriers, i.e. those that only occur from certain temperatures) in gas phase between C+, radicals and H2, can dominate chemistry and promote the formation of hydrocarbons. However, photodissociation of PAHs, hydrogenated amorphous carbons (HACs) and very small grains (VSGs) may be required, as well as a greater knowledge of surface chemistry in carbonous grains to explain the abundances of the most complex hydrocarbons.
This work has been published in the scientific paper “The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR”, and the authors are S. Cuadrado (Molecular Astrophysics Group of the Institute of Materials Science of Madrid (ICMM, CSIC); Astrobiology Center (CAB/CSIC-INTA), Spain; J. R. Goicoechea (Molecular Astrophysics Group of the ICMM-CSIC; CAB/CSIC-INTA, Spain); P. Pilleri (Université Toulouse III – Paul Sabatier, UPS- Observatoire Midi-Pyrénées, OMP – Institut de Recherche en Astrophysique et Planétologie, IRAP); Centre national de la recherche scientifique, CNRS – IRAP, France); J. Cernicharo (Molecular Astrophysics Group of the ICMM-CSIC; CAB/CSIC-INTA, Spain); A. Fuente (National Astronomical Observatory, OAN-IGN, Spain); and C. Joblin (Université de Toulouse UPS-OMP, IRAP; CNRS, IRAP, France).
- Paper: “The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR” (DOI: http://dx.doi.org/10.1051/0004-6361/201424568)
- IRAM-30m Radio Telescope.
1. The Trapezium in the Orion Nebula.
At the center of this image, surrounded by dust and gas, we see the intense brightness of the stars that make up the Trapezium, the four most massive stars of the Orion Nebula. The ultraviolet radiation they emit alters the chemistry of their entire environment. http://hubblesite.org/newscenter/archive/releases/2006/01/image/e/
2. Center of the Orion Nebula
Image of the center of Orion Nebula in the infrared (at 8 microns) taken by the IRAC camera aboard the Spitzer Space Telescope (data from NASA/Spitzer’s public file: http://archive.spitzer.caltech.edu). At these wavelengths the emission is dominated by polycyclic aromatic hydrocarbons (PAHs). It also shows the position of the Trapezium Cluster (marked with stars) and the region studied in this work (the green box). Credits: NASA/Spitzer; Javier R. Goicoechea
3. Hydrocarbon Spectra in the Orion Bar
Spectra of the Orion Bar at 85 GHz. Three lines (rotational transitions) of two different hydrocarbons (C4H and C3H2) and a hydrogen recombination line from the atomic and ionized gas region (HII region) can be observed.
Originally published in Spanish on the Naukas website: Barra libre de hidrocarburos en Orión (2015/03/11).