A Spanish team of astronomers observing 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 toward the synthesis of more complex organic molecules whose true nature is still unclear -- although their fingerprints are very conspicuous in the Universe.

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 Kekulé -- 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 José Cernicharo (Instituto de Estructura de la Materia, CSIC), chose a typical red giant star to start the search. But it didn't work: the star did have carbon-based molecules, such as acetylene, but not ringed molecules. It didn't have any UIBs, either.

So the astronomers turned to an even older star, a "protoplanetary nebula" -- a star that is about to die by becoming a white dwarf star surrounded by a beautiful cloud of glowing dust and gas. They focused on the protoplanetary nebula CRL618.

"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," Cernicharo explains.

This time the idea was correct. As published in the January 10 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 centimeter, 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.

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 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.

[Contact: José Cernicharo, Martin F. Kessler]

26-Jan-2001