The clouds of gas and dust grains in interstellar space
contain complex organic molecules made of hundreds of chained
carbon atoms. The European Space Agency’s infrared space
telescope, ISO, has detected these molecules in many different
environments and is now unveiling the chemical paths leading to
their formation in space. A group of Spanish astronomers have
detected for the first time outside the Solar System two molecules
that could be the precursors for the formation of the more complex
organic compounds. The newly found molecules, detected in two
very old stars, are diacetylene and triacetylene (C4H2 and C6H2).

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 José Cernicharo, from
the Centro Superior de Investigaciones Científicas (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.


Martin Kessler, ISO Project Scientist

+34 91 8131254

José Cernicharo, Consejo Superior de Investigaciones Científicas

+34 91 5901611