ESA Science & Technology - ISO finds the precursors of the complex organic molecules in space

The study of the complex organic molecules in space, the so-called 'PAH' (Polycyclic Aromatic Hydrocarbons), is rapidly advancing with ISO. Researchers from Canada and the US reported last week at the American Astronomical Society meeting in Atlanta (US) that the making of PAH in stellar envelopes can take as little as a few thousand years (see previous story "Complex organic molecules form quickly in old stars"). Now, the finding by the Spanish team adds information about the intermediate chemical steps that lead from one of the simplest organic molecules, such as the acetylene, to the complex PAH.

"ISO has provided an important database for the study of these large and complex organic molecules", says Josi Cernicharo, from the Centro Superior de Investigaciones Cientmficas (CSIC) in Madrid. "This will allow us to investigate their role in the chemistry of interstellar space, and to answer important questions that remain open. For instance, how are these large species formed?"

Using ISO, Cernicharo and co-workers observed two stars in the process of dying, CRL618 and CRL2688, which have been blasting out huge amounts of material over the last thousand years and thus have become the central stars illuminating a large shell of gas and dust - a structure called 'a protoplanetary nebula'. The astronomers studied the chemical composition of the gas around the stars and realised that many new molecules had been synthesised. Many of these molecules are unknown, but the researchers were able to identify at least two of them: C₄H₂ and C₆H₂, di- and tri- acetylene.

"The large number of unknown molecular bands revealed by ISO left us astonished. Among them we quickly identified two new molecules, di- and tri-acetylene, which are present in the planets of the Solar System but had not been found before in the interstellar space. The unknown molecular species and the di- and tri-acetylene might very well be the 'small bricks' that will combine to make the complex molecules like PAHs", Cernicharo explains.

In the proto-planetary nebula CRL618 Cernicharo and Fabrice Herpin (CSIC) have found also water (H₂O) and OH, an unexpected result because CRL618 is a carbon-rich object and those are oxygen-bearing molecules. To the researchers, these are examples of how powerful the stars are when it comes to the production of new molecules... molecules that are likely to end up in planets like the Earth.

As Cernicharo explains, "when an old star is evolving towards the proto-planetary phase, it produces violent phenomena such as high velocity winds and high flux of high energy photons; these phenomena modify completely the chemistry of the gas around the star, and allow the formation of new molecules. With time, they will escape from the gravity of the central star and will reach the interstellar medium, where they will joint the molecular clouds out of which new stars will form. When a new star with its planetary system is formed, highly complex molecular species, many of them containing a large number of carbon atoms, are already present to form part of comets and planets".

A paper about this findings will appear in the February issue of The Astrophysical Journal Letters.

Footnote about ISO

The European Space Agency's infrared space observatory, ISO, operated from November 1995 to May 1998, almost a year longer than expected. An unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO made nearly 30 000 scientific observations.

Contacts:

Martin Kessler, ISO Project Scientist
mkessleriso.vilspa.esa.es
+34 91 8131254

Josi Cernicharo, Consejo Superior de Investigaciones Cientmficas
cerniastro.iem.csic.es
+34 91 5901611