Cambridge, MA (March 18, 2021) – Scientists have discovered a vast, previously unknown reservoir of new aromatic material in a dark, cold 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?
“I’ve always believed that polycyclic aromatic hydrocarbons formed primarily in the atmospheres of dying stars,” said Brett McGuire, an assistant professor of chemistry at the Massachusetts Institute of Technology and principal investigator of the project for GOTHAM, or Green Bank Telescope Observations (GBT). of TMC-1: Hunting of aromatic molecules. “In this study, we found them in cold, dark clouds, where stars have not even begun to form yet.”
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. PAHs are also pollutants formed by burning many fossil fuels and are even among the carcinogens formed when vegetables and meat are charred at high temperatures. “Polycyclic aromatic hydrocarbons are believed to contain up to 25% 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 to their chemistry, which will allow us to study in detail how this massive carbon reservoir 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 papers published in the last seven months, provides the first definitive evidence of their existence in molecular clouds. To search for evasive molecules, the team focused 100-meter 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 one day will take place collapsed themselves to form stars – and what they found was amazing: not only were the accepted scientific models incorrect, but in TMC-1 much more was happening than the team could have imagined.
“From decades of previous modeling, I thought I understood the chemistry of molecular clouds quite well,” said Michael McCarthy, astrochemist and deputy director of CfA, whose research group conducted accurate laboratory measurements that allowed many of the 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 larger than predicted by standard models.
McGuire added that previous studies have only revealed that there are PAH molecules there, but not specific ones. “For the past 30 years, scientists have observed the massive signature of these molecules in our galaxy and other infrared galaxies, but we could not see which individual molecules made up that mass. With the addition of radio astronomy, instead of seeing this large mass that we cannot distinguish, we see individual molecules. “
To their surprise, the team did not discover a single new molecule that was hidden in TMC-1. Detailed in several papers, the team observed 1-cyanonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans- (E) -cyanovinylacetylene propene, HC. “It’s like walking into a boutique store and just browsing through the front-end inventory without ever knowing there’s a back room. We’ve been collecting small molecules for about 50 years, and that’s how I found out that “There’s a back door. When we opened that door and looked inside, we found this giant storehouse of molecules and chemistry that we didn’t expect,” McGuire said. “He was there, all the time, lurking just beyond where we had looked before.”
McGuire and other scientists from the GOTHAM project have been “hunting” TMC-1 molecules for more than two years, after 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 stumbled upon a new set of molecules, unlike anything we were able to detect before, and that will completely change our understanding of how these molecules interact with each other. It has ramifications downstream,” McGuire said. that eventually these molecules grow large enough to begin to aggregate into interstellar dust seeds. “When these molecules become large enough to become interstellar dust seeds, they have the potential to affect the composition of asteroids, comets and planets, the surfaces on which frosts form and perhaps, in turn, even the locations where planets form in within stellar systems. ”
The discovery of new molecules in TMC-1 also has implications for astrochemistry, and although the team does not yet have all the answers, the ramifications here will take several 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 the ones that a chemist knows and recognizes and can produce on Earth,” McCarthy said. . “And this is just the tip of the iceberg. Whether these organic molecules were synthesized there or transported there, they exist and only knowledge is a fundamental advance in the field.”
Prior to the launch of GOTHAM in 2018, scientists cataloged approximately 200 individual molecules in the interstellar environment of the Milky Way. These new discoveries led the team to wonder and, rightly, what is there. “The amazing thing about these observations, this discovery and these molecules, is that no one looked or looked hard enough,” McCarthy said. “It makes you wonder what else is out there that I just didn’t look for.”
This new aromatic chemistry that scientists are finding is not isolated from TMC-1. A companion survey to GOTHAM, known as ARKHAM – A Rigorous K / Ka-Band Hunting for Aromatic Molecules – recently found benzonitrile in several additional objects. “Incredibly, I found benzonitrile in each of the first four objects observed by ARKHAM,” said Andrew Burkhardt, a post-doctoral trophy Submillimeter Array at CfA and co-principal investigator for GOTHAM. “This is important, because while GOTHAM exceeds the limit of chemistry we thought possible in space, these findings imply that the things we learn in TMC-1 about aromatic molecules could be applied to dark clouds anywhere. These dark clouds are the original birthplaces of stars and planets. So, these previously invisible aromatic molecules will also need to be considered at every subsequent step along the way 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 for this project: Kin Long Kelvin Lee from MIT; Ryan Loomis, Anthony Remijan and Emmanuel Momjian from 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 Kalensky from 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 & Smithsonian
Center for Astrophysics Harvard & Smithsonian is a collaboration between Harvard and Smithsonian designed to ask – and ultimately answer – humanity’s greatest 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
MIT’s chemistry department 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 cooperation agreement by the Associated Universities, Inc. Founded in 1956, NRAO offers state-of-the-art radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers, regardless of institutional or national affiliation. Time observation on NRAO telescopes is competitively available to qualified scientists, after evaluating research proposals based on scientific merit, the ability of the instruments to work and the availability of the telescope in the required time. NRAO also offers formal and informal programs in education and public information for teachers, students, the general public and the media.
About the Green Bank Observatory
The Green Bank Observatory is home to one of the largest fully orientable radio telescopes in the world, the Green Bank Telescope (GBT) of the National Science Foundation. The Observatory houses several additional instruments and panels and is protected by two complementary radio interference protection zones, the National Radio Quiet Zone and the West Virginia Radio Astronomy Zone.
reference
“Detection of interstellar HC4NC and an investigation of isocyanopolytic chemistry under TMC-1 conditions.” C. Xue et al., 2020 Sept. 1, The Astrophysical Journal Letters [https:/
“Early Science at GOTHAM: Project Overview, Methods and Detection of Interstellar Propargyl Cyanide (HCCCH2CN) in TMC-1.” B. McGuire et al., 2020 Sept. 1, 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:/
“The chemistry of aromatic carbon ubiquitous in the early stages of star formation.” AM Burkhardt et al., 2021, January 11 Nature Astronomy [https:/
“Interstellar detection of five-polar ring 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., 2021 February 11, The Astrophysical Journal Letters [https:/
“Interstellar detection of 2-cyanocyclopentadiene, C5H5N, a second five-membered ring to 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 & McGuire 2021, Journal of Physical Chemistry [https:/