Cambridge, MA (March 18, 2021) – Scientists have discovered a huge deposit of hitherto unknown aromatic material in a cold, dark molecular cloud by detecting individual polycyclic aromatic hydrocarbon molecules in the interstellar medium for the first time . so they begin to answer a three-decade-old scientific mystery: how and where do these molecules form in space?
“We had always thought that polycyclic aromatic hydrocarbons formed primarily in the atmospheres of dying stars,” said Brett McGuire, adjunct professor of chemistry at the Massachusetts Institute of Technology and lead researcher on the project for GOTHAM, or Observations of the TMC-1 Green Bank Telescope (GBT): Hunting Aromatic Molecules. “In this study, we found them in cold, dark clouds, where they have not yet begun to form stars.”
Aromatic molecules and PAHs (short for polycyclic aromatic hydrocarbons) are well known to scientists. Aromatic molecules exist in the chemical composition of humans and other animals, and are found in food and medicine. In addition, PAHs are pollutants formed by the combustion of many fossil fuels and are even among the carcinogens formed when vegetables and meat are carbonized at high temperatures. “Polycyclic aromatic hydrocarbons are believed to contain up to 25% of the universe’s carbon,” said McGuire, who is also an associate researcher at the Center for Astrophysics. Harvard and Smithsonian (CfA). “Now, for the first time, we have a direct window into its chemistry that will allow us to study in detail how this huge carbon deposit reacts and evolves through the process of star and planet formation.”
Scientists have suspected the presence of PAHs in space since the 1980s, but new research, detailed in nine articles published over the past seven months, provides the first definitive proof of its existence in molecular clouds. To search for the fleeting molecules, the team centered the giant 100m GBT radio astronomy on the Taurus molecular cloud, or TMC-1, a large pre-stellar cloud of dust and gas located about 450 light-years from Earth that someday it will come crashing down on itself to form stars – and what they found was amazing: not only were the accepted scientific models wrong, but there were many more things in TMC-1 than the team could have imagined.
“From decades of previous models, we believed we had a good understanding of molecular cloud chemistry,” said Michael McCarthy, an astrochemist and acting deputy director of CfA, whose research group made accurate laboratory measurements. which allowed many of these astronomical detections to be established with confidence. “What these new astronomical observations show is that these molecules are not only present in molecular clouds, but in quantities that are orders of magnitude higher than those predicted by standard models.”
McGuire added that previous studies only revealed that there were PAH molecules, but not specific ones. “For the last 30 years or so, scientists have been observing the massive signature of these molecules in our galaxy and other infrared galaxies, but we have not been able to see which individual molecules formed this mass. With the addition of radio astronomy, instead of seeing this great mass that we cannot distinguish, we see individual molecules. “
To his surprise, the team didn’t just discover a new molecule hiding in the TMC-1. Detailed in several documents, the team observed 1-cyanoonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans- (E) -cyanovinylacetylene, HC4NC, and propargylcyan. “It’s like going into a store and just browsing the front inventory without ever knowing there was a room in the back. We’ve been collecting small molecules for about 50 years and now we’ve discovered that there’s a back door. we opened the door and looked inside, we found this giant storehouse of molecules and chemistry that we didn’t expect, ”McGuire said. “There he was, all the time, lurking just beyond where we had looked before.”
McGuire and other scientists in the GOTHAM project have been “searching” 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 during the next years. . “We stumbled upon a whole new set of molecules, unlike anything we’ve been able to detect before, and that will completely change our understanding of how these molecules interact with each other. It has branches downstream,” McGuire said, adding that eventually these molecules grow long enough to begin to aggregate into interstellar dust seeds. “When these molecules are large enough to be interstellar dust seeds, they have the potential to affect the composition of asteroids, comets and planets, the surfaces on which ice cream is formed, and perhaps in turn , even the places where the planets are located. They form within the stellar systems. ”
The discovery of new molecules in TMC-1 also has implications for astrochemistry, and while the team does not yet have all the answers, the ramifications here will also last for decades. “We have gone from one-dimensional carbon chemistry, which is very easy to detect, to real organic chemistry in space in the sense that newly discovered molecules are what a chemist knows and recognizes and can produce on Earth,” he said. McCarthy. “And that’s just the tip of the iceberg. Whether those organic molecules were synthesized there or transported there, they exist, and only that knowledge is a fundamental breakthrough in the field.”
Prior to the launch of GOTHAM in 2018, scientists had cataloged approximately 200 individual molecules in the interstellar medium of the Milky Way. These new discoveries have motivated the team to ask themselves and rightly so what is here. “The amazing thing about these observations, this discovery, and these molecules, is that no one had looked at it or looked hard enough,” McCarthy said. “It makes you wonder what’s more we just haven’t looked for.”
This new aromatic chemistry that scientists are finding is not isolated from TMC-1. A complementary survey to GOTHAM, known as ARKHAM – A rigorous investigation into the K / Ka band of aromatic molecule hunting – recently found benzonitrile in multiple additional objects. “Incredibly, we found benzonitrile in all the first four objects observed by ARKHAM,” said Andrew Burkhardt, a CfA submillimeter matrix postdoctoral fellow and GOTHAM co-principal investigator. “This is important because while GOTHAM pushes the limit of chemistry that we believe is possible in space, these findings imply that the things we learn in TMC-1 about aromatic molecules could be broadly applied to dark clouds. from anywhere.These dark clouds are the initial birthplaces of stars and planets, so these previously invisible aromatic molecules will also need to be thought of at every subsequent step on the path to the creation of stars, planets, and solar systems. like ours. “
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In addition to McGuire, McCarthy, and Burkhardt, the following researchers contributed and led the research of this project: Kin Long Kelvin Lee of MIT; Ryan Loomis, Anthony Remijan and Emmanuel Momjian of the National Radio Astronomy Observatory; Christopher N. Shingledecker of Benedictine College; Steven B. Charnley and Martin A. Cordiner of NASA Goddard; Eric Herbst, Eric R. Willis, Ci Xue, and Mark Siebert of the University of Virginia; and Sergei Kalenskii of the Lebedev Physical Institute. The project also received research support from the University of Stuttgart, the Max Planck Institute, and the Catholic University of America.
About the Center for Astrophysics | Harvard and Smithsonian
The Center for Astrophysics | Harvard & Smithsonian is a collaboration between Harvard and the Smithsonian designed to ask – and ultimately answer – humanity’s biggest unresolved questions about the nature of the universe. The Center for Astrophysics is headquartered in Cambridge, MA, with research facilities in the United States and around the world.
About the Massachusetts Institute of Technology
The MIT Department of Chemistry is an inclusive, supportive, and innovative community whose common goal is to create new chemical knowledge and guide the next generation of the best and brightest students who will define the next frontiers of chemical science.
About the National Radio Astronomy Observatory
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc. Founded in 1956, the NRAO provides state-of-the-art radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers regardless of their institutional or national affiliation. Time observation in NRAO telescopes is competitively available to qualified scientists after evaluation of research proposals based on scientific merit, the ability of the instruments to do the work, and the availability of the telescope during the requested time. NRAO also provides formal and informal programs in education and public outreach for teachers, students, the general public, and the media.
About the Green Bank Observatory
The Green Bank Observatory is home to one of the most fully orientable radio telescopes in the world, the Green Bank Telescope (GBT) of the National Science Foundation. The Observatory houses several additional instruments and matrices and is protected by two complementary radio interference protection zones, the Radio Quiet Radio Zone and the West Virginia Radio Astronomy Zone.
References
“Detection of interstellar HC4NC and investigation of isocyanopolyline chemistry under TMC-1 conditions.” C. Xue et al, September 1, 2020, The Letters from astrophysical journals [https:/
“Early Science from GOTHAM: Project Overview, Methods, and Detection of Interstellar Propargyl Cyanide (HCCCH2CN) in TMC-1.” B. McGuire et al, 1 September 2020, The Astrophysical Journal Letters [https:/
“An investigation of spectral line stacking techniques and application to HC11N detection.” R. Loomis et al, 2021, January 11, Nature Astronomy [https:/
“Aromatic carbon chemistry located in the early stages of star formation.” AM Burkhardt et al, 2021, January 11, Nature Astronomy [https:/
“Interstellar detection of the highly polar five-membered cyanocyclopentadiene ring.” M. McCarthy et al, February 2021, Nature Astronomy [https:/
“Discovery of interstellar trans-cyanovinylacetylene (HCCCH = CCHCN) and vinylcyanacetylene (H2C = CHC3N) in GOTHAM observations of TMC-1.” K. Lee et al, February 11, 2021, The Astrophysical Journal Letters [https:/
“Interstellar detection of 2-cyanocyclopentadiene, C5H5N, a second five-membered ring toward TMC-1,” K. Lee et al. 2021, accepted The Astrophysical Journal Letters, prepress PDF: https: /
“Detection of two interstellar polycyclic aromatic hydrocarbons by spectral filtering,” McGuire et al, 2021, March 19, Science [https:/
“Aromatics and Cyclic Molecules in Molecular Clouds: A New Dimension of Interstellar Organic Chemistry,” McCarthy and McGuire 2021, The Journal of Physical Chemistry A [https:/