A team of scientists using the National Science Foundation’s (NSF)
Robert C. Byrd Green Bank Telescope (GBT) has discovered two new
molecules in an interstellar cloud near the center of the Milky Way
Galaxy. This discovery is the GBT’s first detection of new molecules,
and is already helping astronomers better understand the complex
processes by which large molecules form in space.

The 8-atom molecule propenal and the 10-atom molecule propanal were
detected in a large cloud of gas and dust some 26,000 light-years away
in an area known as Sagittarius B2. Such clouds, often many
light-years across, are the raw material from which new stars are
formed.

“Though very rarefied by Earth standards, these interstellar clouds are
the sites of complex chemical reactions that occur over
hundreds-of-thousands or millions of years,” said Jan M. Hollis of the
NASA Goddard Space Flight Center in Greenbelt, Md. “Over time, more
and more complex molecules can be formed in these clouds. At present,
however, there is no accepted theory addressing how interstellar
molecules containing more than 5 atoms are formed.”

So far, about 130 different molecules have been discovered in
interstellar clouds. Most of these molecules contain a small number of
atoms, and only a few molecules with eight or more atoms have been
found
in interstellar clouds. Each time a new molecule is discovered, it
helps to constrain the formation chemistry and the nature of
interstellar dust grains, which are believed to be the formation sites
of most complex interstellar molecules.

Hollis collaborated with Anthony Remijan, also of NASA Goddard; Frank
J. Lovas of the National Institute of Standards and Technology in
Gaithersburg, Md.; Harald Mollendal of the University of Oslo, Norway;
and Philip R. Jewell of the National Radio Astronomy Observatory (NRAO)
in Green Bank, W.Va. Their results were accepted for publication in
the Astrophysical Journal Letters.

In the GBT experiment, three aldehyde molecules were observed and
appear to be related by simple hydrogen addition reactions, which
probablyoccur on the surface of interstellar grains. An aldehyde is
a moleculethat contains the aldehyde group (CHO): a carbon atom singly
bonded to a hydrogen atom and double-bonded to an oxygen atom; the
remaining bond on that same carbon atom bonds to the rest of the
molecule.

Starting with previously reported propynal (HC2CHO), propenal
(CH2CHCHO) is formed by adding two hydrogen atoms. By the same
process propanal (CH3CH2CHO) is formed from propenal.

After these molecules are formed on interstellar dust grains, they may
be ejected as a diffuse gas. If enough molecules accumulate in the
gas, they can be detected with a radio telescope. As the molecules
rotate end-for-end, they change from one rotational energy state to
another, emitting radio waves at precise frequencies. The “family” of
radio frequencies emitted by a particular molecule forms a unique
“fingerprint” that scientists can use to identify that molecule. The
scientists identified the two new aldehydes by detecting a number of
frequencies of radio emission in what is termed the K-band region (18
to 26 GHz) of the electromagnetic spectrum.

“Interstellar molecules are identified by means of the frequencies that
are unique to the rotational spectrum of each molecule,” said Lovas.
“These are either directly measured in the laboratory or calculated
from the measured data. In this case we used the calculated spectral
frequencies based on an analysis of the literature data.”

Complex molecules in space are of interest for many reasons, including
their possible connection to the formation of biologically significant
molecules on the early Earth. Complex molecules might have formed on
the early Earth, or they might have first formed in interstellar clouds
and been transported to the surface of the Earth.

Molecules with the aldehyde group are particularly interesting since
several biologically significant molecules, including a family of sugar
molecules, are aldehydes.

“The GBT can be used to fully explore the possibility that a significant
amount of prebiotic chemistry may occur in space long before it occurs
on a newly formed planet,” said Remijan. “Comets form from interstellar
clouds and incessantly bombard a newly formed planet early in its
history. Craters on our Moon attest to this. Thus, comets may be the
delivery vehicles for organic molecules necessary for life to begin on
a new planet.”

Laboratory experiments also demonstrate that atomic addition reactions
— similar to those assumed to occur in interstellar clouds — play a
role in synthesizing complex molecules by subjecting ices containing
simpler molecules such as water, carbon dioxide, and methanol to
ionizing radiation dosages. Thus, laboratory experiments can now be
devised with various ice components to attempt production of the
aldehydes observed with the GBT.

“The detection of the two new aldehydes, which are related by a common
chemical pathway called hydrogen addition, demonstrates that evolution
to more complex species occurs routinely in interstellar clouds and
that a relatively simple mechanism may build large molecules out of
smaller ones. The GBT is now a key instrument in exploring chemical
evolution in space,” said Hollis.

The GBT is the world’s largest fully steerable radio telescope; it is
operated by the NRAO.

“The large diameter and high precision of the GBT allowed us to study
small interstellar clouds that can absorb the radiation from a bright
background source. The sensitivity and flexibility of the telescope
gave us an important new tool for the study of complex interstellar
molecules,” said Jewell.

The NRAO is a facility of the National Science Foundation, operated
under a cooperative agreement with Associated Universities, Inc.

Editors:

A graphic of the molecules is located at:
http://www.nrao.edu/pr/2004/GBTMolecules/molecules.JPG

Images of the Green Bank Telescope are here:
http://www.nrao.edu/imagegallery/php/level2a.php?class=3DTelescopes&subclass=3DGBT