ISO detects benzene in space
22 January 2001The first 'ringed molecule' found around stars
Life as we know it is based on the ability of the carbon atom to form ring-shaped molecules. But rings of carbon are not exclusive to Earth, as experts in space chemistry now know. A Spanish team of astronomers that observed with ESA's Infrared Space Observatory (ISO) report this week the first detection in interstellar space of benzene, the ring molecule par excellence. They think benzene is produced by stars at a specific stage of evolution, and that it is an essential chemical step towards the synthesis of more complex organic molecules whose true nature is still unclear -although their fingerprints are very conspicuous in the Universe. In industry, benzene is obtained from petroleum and has many uses.
Benzene is made of six atoms of carbon chained together to form a ring, plus six atoms of hydrogen, one per carbon. This structure was discovered in 1865 by the German chemist August Kekule -who would later say a dream inspired him. Chemists know today that benzene-type molecules make a whole family of compounds, called 'aromatic' hydrocarbons because of their smell (they are basic constituents, for instance, of perfumes and candles).
Astronomers also expected to find these ringed molecules in space, where long strings of carbon atoms have been detected. Moreover, it had been postulated that certain compounds of yet unknown nature, that are known to be very abundant in space, are actually aromatic hydrocarbons. These compounds have left their chemical fingerprint, the so-called 'Unidentified Infrared Bands' or 'UIBs', in many places in the Universe, and although the labs have not yet been able to confirm their nature for certain, many astronomers already call them 'Polycyclic Aromatic Hydrocarbons', or PAHs.
Where to look for the 'ringed molecules'? Around carbon-rich old stars, thought the astronomers. When stars of intermediate mass -up to about three solar masses- become old, and reach the 'red giant' phase, they begin to release huge amounts of gas and dust into their environment; because carbon is produced by the nuclear reactions in the core of the star, many carbonaceous compounds are present in the dust expelled by the red giant star.
The team, led by Josi Cernicharo (Instituto de Estructura de la Materia, CSIC), chose a typical red giant star to start the search. But it did not work: the star did have carbon-based molecules, such as acetylene, but not ringed molecules. It did not have any UIBs either. So the astronomers turned to an even older star, a 'protoplanetary nebula' -a star that is about to 'die' via becoming a 'white dwarf' star surrounded by a beautiful cloud of glowing dust and gas. They focused on the protoplanetary nebula CRL618 (See image taken by the ESA/NASA Hubble Space Telescope).
"We knew that, in a protoplanetary nebula, the dust ejected in the previous red-giant stage is bathed in powerful ultraviolet radiation coming from the central star, which also emits high velocity winds. The radiation and the winds break up the carbonaceous compounds in the dust and trigger new reactions. We thought benzene could be formed in this way, a process that we could call polymerization of acetylene in evolved stars", explains Cernicharo.
This time the idea was correct. As published in the 10 January issue of 'The Astrophysical Journal', they have found benzene in the surroundings of the CRL618 protoplanetary nebula. The authors think that there could be a few molecules of benzene per cubic centimetre, a value considered to be high, although the estimated density of molecules of all kinds in the area observed is ten million per cubic centimetre.
A 'missing link'
The team thinks that this molecule is the 'missing link' between the simple carbon molecules observed in red giants, made of no more than eight carbon atoms, and the complex molecules responsible for UIBs, known to be made of hundreds of carbon atoms. This theory therefore implies that the UIBs are indeed due to aromatic hydrocarbons, a possibility that, according to the authors, becomes stronger after the first confirmed detection of an aromatic molecule in space.
The 'missing-link' idea is based on observations of objects at each stage of evolution. UIBs have been detected around stars that are already 'dead', the 'planetary nebula', but not in the previous evolutionary stage of protoplanetary nebula -such as CRL618, where benzene has been found. The transformation from protoplanetary to planetary nebula lasts for no longer than a thousand years, a quick process in astronomical terms -as an example, the central star of CRL618 was still a red giant only a few centuries ago, and will become a 'fully formed' planetary nebula in a few thousand years.
As Cernicharo says, "the molecules causing the UIBs must form in the relatively short period from protoplanetary nebula to planetary nebulae. It seems that carbon-rich protoplanetary nebula are the best organic chemistry factories in space".
The European Space Agency's infrared space telescope, ISO, operated from November 1995 till May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO successfully made nearly 30 000 scientific observations.
FOR MORE INFORMATION
Josi Cernicharo (Instituto de Estructura de la Materia, CSIC, Madrid)
Martin F. Kessler, ISO Project Scientist (ESA, Villfranca Satellite Tracking Station, Spain)
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