Dust and molecules in the central region of our Galaxy: The background image shows the dust emission in a combination of ... [more]
© MPIfR/A. Weiß (background image), University of Cologne/M. Koerber (molecular models), MPIfR/A. Belloche (montage).
Detection of iso-propyl cyanide with ALMA, the Atacama Large Millimeter/submillimeter Array
Scientists from the Max Planck Institute for Radio Astronomy
(Bonn, Germany), Cornell University (USA), and the University of Cologne
(Germany) have for the first time detected a carbon-bearing molecule
with a "branched" structure in interstellar space. The molecule, iso-propyl cyanide (i-C3H7CN),
was discovered in a giant gas cloud called Sagittarius B2, a region of
ongoing star formation close to the center of our galaxy that is a
hot-spot for molecule-hunting astronomers. The branched structure of the
carbon atoms within the iso-propyl cyanide molecule is unlike
the straight-chain carbon backbone of other molecules that have been
detected so far, including its sister molecule normal-propyl cyanide. The discovery of iso-propyl
cyanide opens a new frontier in the complexity of molecules found in
regions of star formation, and bodes well for the presence of amino
acids, for which this branched structure is a key characteristic.
The results are published in this week’s issue of “Science”.
The results are published in this week’s issue of “Science”.
While various types of molecules have been detected in space, the
kind of hydrogen-rich, carbon-bearing (organic) molecules that are most
closely related to the ones necessary for life on Earth appear to be
most plentiful in the gas clouds from which new stars are being formed.
"Understanding the production of organic material at the early stages of
star formation is critical to piecing together the gradual progression
from simple molecules to potentially life-bearing chemistry," says
Arnaud Belloche from the Max Planck Institute for Radio Astronomy, the
lead author of the paper.
The search for molecules in interstellar space began in the 1960's,
and around 180 different molecular species have been discovered so far.
Each type of molecule emits light at particular wavelengths, in its own
characteristic pattern, or spectrum, acting like a fingerprint that
allows it to be detected in space using radio telescopes.
Until now, the organic molecules discovered in star-forming regions
have shared one major structural characteristic: they each consist of a
"backbone" of carbon atoms that are arranged in a single and more or
less straight chain. The new molecule discovered by the team, iso-propyl
cyanide, is unique in that its underlying carbon structure branches off
in a separate strand. "This is the first ever interstellar detection of
a molecule with a branched carbon backbone," says Holger Müller, a
spectroscopist at the University of Cologne and co-author on the paper,
who measured the spectral fingerprint of the molecule in the laboratory,
allowing it to be detected in space.
But it is not just the structure of the molecule that surprised the
team - it is also plentiful, at almost half the abundance of its
straight-chain sister molecule, normal-propyl cyanide (n-C3H7CN),
which the team had already detected using the single-dish radio
telescope of the Institut de Radioastronomie Millimétrique (IRAM) a few
years ago. "The enormous abundance of iso-propyl cyanide suggests
that branched molecules may in fact be the rule, rather than the
exception, in the interstellar medium," says Robin Garrod, an
astrochemist at Cornell University and a co-author of the paper.
The team used the Atacama Large Millimeter/submillimeter Array
(ALMA), in Chile, to probe the molecular content of the star-forming
region Sagittarius B2 (Sgr B2). This region is located close to the
Galactic Center, at a distance of about 27,000 light years from the Sun,
and is uniquely rich in emission from complex interstellar organic
molecules. "Thanks to the new capabilities offered by ALMA, we were able
to perform a full spectral survey toward Sgr B2 at wavelengths between
2.7 and 3.6 mm, with sensitivity and spatial resolution ten times
greater than our previous survey," explains Belloche. "But this took
only a tenth of the time." The team used this spectral survey to search
systematically for the fingerprints of new interstellar molecules. "By
employing predictions from the Cologne Database for Molecular
Spectroscopy, we could identify emission features from both varieties of
propyl cyanide," says Müller. As many as 50 individual features for i-propyl cyanide and even 120 for n-propyl
cyanide were unambiguously identified in the ALMA spectrum of Sgr B2.
The two molecules, each consisting of 12 atoms, are also the
joint-largest molecules yet detected in any star-forming region.
The team constructed computational models that simulate the chemistry
of formation of the molecules detected in Sgr B2. In common with many
other complex organics, both forms of propyl cyanide were found to be
efficiently formed on the surfaces of interstellar dust grains. "But,"
says Garrod, "the models indicate that for molecules large enough to
produce branched side-chain structure, these may be the prevalent forms.
The detection of the next member of the alkyl cyanide series, n-butyl cyanide (n-C4H9CN), and its three branched isomers would allow us to test this idea".
"Amino acids identified in meteorites have a composition that
suggests they originate in the interstellar medium," adds Belloche.
“Although no interstellar amino acids have yet been found, interstellar
chemistry may be responsible for the production of a wide range of
important complex molecules that eventually find their way to planetary
surfaces."
"The detection of iso-propyl cyanide tells us that amino acids
could indeed be present in the interstellar medium because the
side-chain structure is a key characteristic of these molecules", says
Karl Menten, director at MPIfR and head of its Millimeter and
Submillimeter Astronomy research department. "Amino acids have already
been identified in meteorites and we hope to detect them in the
interstellar medium in the future", he concludes.
Contact
Dr. Arnaud Belloche
Phone:+49 228 525-376
Max-Planck-Institut für Radioastronomie, Bonn
Email: belloche@mpifr-bonn.mpg.de
Dr. Robin T. Garrod
Phone:+1 607-255-8967
Center for Radiophysics and Space Research, Cornell University, U.S.A.
Email: rgarrod@astro.cornell.edu
Dr. Holger Müller
Phone:+49 221 470-4528
I. Physikalisches Institut, Universität zu Köln
Email: hspm@ph1.uni-koeln.de
Dr. Norbert Junkes
Presse- und Öffentlichkeitsarbeit
Phone:+49 228 525-399
Max-Planck-Institut für Radioastronomie, Bonn
Email: njunkes@mpifr-bonn.mpg.de
