Scientists uncover warehouse-full of complex molecules never before seen
in space
Radio observations of a cold, dense cloud of molecular gas reveal more
than a dozen unexpected molecules

Date:
March 18, 2021
Source:
Harvard-Smithsonian Center for Astrophysics
Summary:
Scientists have discovered a vast, previously unknown reservoir
of new aromatic material in a cold, dark molecular cloud by
detecting individual polycyclic aromatic hydrocarbon molecules
in the interstellar medium for the first time, and in doing so
are beginning to answer a three-decades- old scientific mystery:
how and where are these molecules formed in space? The more than a
dozen PAHs may hold clues as to the formation of comets, asteroids,
stars, and even planets.



FULL STORY ========================================================================== Scientists have discovered a vast, previously unknown reservoir of
new aromatic material in a cold, dark molecular cloud by detecting
individual polycyclic aromatic hydrocarbon molecules in the interstellar
medium for the first time, and in doing so are beginning to answer a three-decades-old scientific mystery: how and where are these molecules
formed in space?

==========================================================================
"We had always thought polycyclic aromatic hydrocarbons were primarily
formed in the atmospheres of dying stars," said Brett McGuire, Assistant Professor of Chemistry at the Massachusetts Institute of Technology, and
the Project Principal Investigator for GOTHAM, or Green Bank Telescope
(GBT) Observations of TMC-1: Hunting Aromatic Molecules. "In this study,
we found them in cold, dark clouds where stars haven't even started
forming yet." Aromatic molecules, and PAHs -- shorthand for polycyclic aromatic hydrocarbons -- are well known to scientists. Aromatic molecules
exist in the chemical makeup of human beings and other animals, and are
found in food and medicines.

As well, PAHs are pollutants formed from the burning of many fossil fuels
and are even amongst the carcinogens formed when vegetables and meat
are charred at high temperatures. "Polycyclic aromatic hydrocarbons
are thought to contain as much as 25-percent of the carbon in the
universe," said McGuire, who is also a research associate at the Center
for Astrophysics | Harvard & Smithsonian (CfA). "Now, for the first time,
we have a direct window into their chemistry that will let us study in
detail how this massive reservoir of carbon reacts and evolves through
the process of forming stars and planets." Scientists have suspected the presence of PAHs in space since the 1980s but the new research, detailed
in nine papers published over the past seven months, provides the first definitive proof of their existence in molecular clouds. To search out
the elusive molecules, the team focused the 100m behemoth radio astronomy
GBT on the Taurus Molecular Cloud, or TMC-1 -- a large, pre-stellar cloud
of dust and gas located roughly 450 light-years from Earth that will
someday collapse in on itself to form stars -- and what they found was astonishing: not only were the accepted scientific models incorrect, but
there was a lot more going on in TMC-1 than the team could have imagined.

"From decades of previous modeling, we believed that we had a fairly
good understanding of the chemistry of molecular clouds," said Michael McCarthy, an astrochemist and Acting Deputy Director of CfA, whose
research group made the precise laboratory measurements that enabled many
of these astronomical detections to be established with confidence. "What
these new astronomical observations show is these molecules are not
only present in molecular clouds, but at quantities which are orders
of magnitude higher than standard models predict." McGuire added that
previous studies revealed only that there were PAH molecules out there,
but not which specific ones. "For the last 30 years or so, scientists
have been observing the bulk signature of these molecules in our galaxy
and other galaxies in the infrared, but we couldn't see which individual molecules made up that mass. With the addition of radio astronomy,
instead of seeing this large mass that we can't distinguish, we're
seeing individual molecules." Much to their surprise, the team didn't
discover just one new molecule hiding out in TMC-1. Detailed in multiple papers, the team observed 1- cyanonaphthalene, 1-cyano-cyclopentadiene,
HC11N, 2-cyanonaphthalene, vinylcyanoacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans-(E)- cyanovinylacetylene, HC4NC, and propargylcyanide, among others. "It's like going into a boutique shop and just browsing
the inventory on the front-end without ever knowing there was a back
room. We've been collecting little molecules for 50 years or so and now
we have discovered there's a back door.

When we opened that door and looked in, we found this giant warehouse of molecules and chemistry that we did not expect," said McGuire. "There
it was, all the time, lurking just beyond where we had looked before."
McGuire and other scientists at the GOTHAM project have been "hunting"
for molecules in TMC-1 for more than two years, following McGuire's
initial detection of benzonitrile in 2018. The results of the project's
latest observations may have ramifications in astrophysics for years
to come. "We've stumbled onto a whole new set of molecules unlike
anything we've previously been able to detect, and that is going to
completely change our understanding of how these molecules interact
with each other. It has downstream ramifications," said McGuire, adding
that eventually these molecules grow large enough that they begin to
aggregate into the seeds of interstellar dust. "When these molecules
get big enough that they're the seeds of interstellar dust, these have
the possibility then to affect the composition of asteroids, comets, and planets, the surfaces on which ices form, and perhaps in turn even the locations where planets form within star systems." The discovery of new molecules in TMC-1 also has implications for astrochemistry, and while
the team doesn't yet have all of the answers, the ramifications here,
too, will last for decades. "We've gone from one- dimensional carbon
chemistry, which is very easy to detect, to real organic chemistry in
space in the sense that the newly discovered molecules are ones that a
chemist knows and recognizes, and can produce on Earth," said McCarthy.

"And this is just the tip of the iceberg. Whether these organic
molecules were synthesized there or transported there, they exist, and
that knowledge alone is a fundamental advance in the field." Before the
launch of GOTHAM in 2018, scientists had cataloged roughly 200 individual molecules in the Milky Way's interstellar medium. These new discoveries
have prompted the team to wonder, and rightly so, what's out there.

"The amazing thing about these observations, about this discovery, and
about these molecules, is that no one had looked, or looked hard enough,"
said McCarthy. "It makes you wonder what else is out there that we just
haven't looked for." This new aromatic chemistry that scientists are
finding isn't isolated to TMC- 1. A companion survey to GOTHAM, known as
ARKHAM -- A Rigorous K/Ka-Band Survey Hunting for Aromatic Molecules -- recently found benzonitrile in multiple additional objects. "Incredibly,
we found benzonitrile in every single one of the first four objects
observed by ARKHAM," said Andrew Burkhardt, a Submillimeter Array
Postdoctoral Fellow at the CfA and a co-principal investigator for
GOTHAM. "This is important because while GOTHAM is pushing the limit
of what chemistry we thought is possible in space, these discoveries
imply that the things we learn in TMC-1 about aromatic molecules could be applied broadly to dark clouds anywhere. These dark clouds are the initial birthplaces of stars and planets. So, these previously invisible aromatic molecules will also need to be thought about at each later step along the
way to the creation of stars, planets, and solar systems like our own." ========================================================================== Story Source: Materials provided by Harvard-Smithsonian_Center_for_Astrophysics. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Brett A. Mcguire, Ryan A. Loomis, Andrew M. Burkhardt, Kin Long
Kelvin
Lee, Christopher N. Shingledecker, Steven B. Charnley, Ilsa
R. Cooke, Martin A. Cordiner, Eric Herbst, Sergei Kalenskii, Mark
A. Siebert, Eric R. Willis, Ci Xue, Anthony J. Remijan, Michael
C. Mccarthy. Detection of two interstellar polycyclic aromatic
hydrocarbons via spectral matched filtering. Science, 19 Mar 2021:
Vol. 371, Issue 6535, pp. 1265-1269 DOI: 10.1126/science.abb7535 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210318142508.htm

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