Cambridge, MA (18 March 2021) – Scientists have discovered an enormous, 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. thus begins to answer a three-decade-old scientific mystery: how and where are these molecules formed in space?
“We have always thought that polycyclic aromatic hydrocarbons are formed primarily in the atmosphere of dying stars,” said Brett McGuire, assistant professor of chemistry at the Massachusetts Institute of Technology, and project lead researcher for GOTHAM, or Green Bank Telescope (GBT) Observations, said. of TMC-1: Hunting of aromatic molecules. “In this study, we found them in cold, dark clouds where stars had not even begun to form.”
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 the combustion of many fossil fuels and are even one of the carcinogenic substances formed when vegetables and meat are charred at high temperatures. “Polycyclic aromatic hydrocarbons presumably contain as much as 25 percent of the carbon in the universe,” said McGuire, who is also a research fellow at the Center for Astrophysics. Harvard & Smithsonian (CfA). “Now, for the first time, we have a direct window into their chemistry that will enable us to study in detail how this massive carbon reservoir responds and evolves through the formation of stars and planets.”
Scientists have suspected the presence of PAHs in space since the 1980s, but the new research, published in nine articles published over the past seven months, is the first definitive proof of their existence in molecular clouds. To investigate the elusive molecules, the team focused the 100 meter radioactivity GBT on the Taurus Molecular Cloud, or TMC-1 – a large, pre-star cloud of dust and gas located about 450 light-years from Earth, which would one day collapse on its own to form stars – and what they found was astounding: not only were the accepted scientific models wrong, but there was also much more going on in TMC-1 than the team could have imagined .
“From decades of previous modeling, we believed we understood the chemistry of molecular clouds fairly well,” said Michael McCarthy, an astrochemist and acting deputy director of CfA. The research team made the exact laboratory measurements that made many of these possibilities possible. astronomical traces to be determined with confidence. “What these new astronomical observations show is that these molecules occur not only in molecular clouds, but also in sizes of greater order than standard models predict.”
McGuire added that previous studies have only revealed that there are PAH molecules, but not which specifics. “For the past thirty years or so, scientists have been observing the largest signature of these molecules in our galaxy and other galaxies in the infrared, but we could not see what individual molecules the mass consists of. With the addition of radio astronomy, instead from seeing this great mass that we cannot distinguish, we see individual molecules. ‘
To their surprise, the team does not discover just one new molecule hiding in TMC-1. The team observed in detail in 1 article 1-cyanonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanoacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans- (E) -cyanovinylacetylene, HC4Nc, and others. “It’s like going into a boutique store and just looking at the stock on the front without ever knowing there’s a back room. We’ve been collecting small molecules for 50 years and now we’ve discovered that there’s a back door “When we opened that door and looked in. we found this giant warehouse with molecules and chemistry that we did not expect,” McGuire said. “It kept peeking out 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 the initial detection of benzonitrile in McGuire in 2018. The results of the latest observations of the project could have implications for astrophysics for years to come. . “We’ve come up with a whole new set of molecules, unlike anything we could detect before, and that will change our understanding of how these molecules interact with each other. It has downstream consequences,” McGuire said. and adds that these molecules eventually become large enough to begin accumulating in the seeds of interstellar matter. “If these molecules become large enough to be the seeds of interstellar matter, it could affect the composition of asteroids, comets and planets, the surfaces on which ice forms, and perhaps also the places where planets are forming within star systems.”
The discovery of new molecules in TMC-1 also has consequences for astrochemistry, and although the team does not yet have all the answers, the consequences here will also last decades. “We went 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 molecules that a chemist knows and recognizes and can produce on earth,” he said. . McCarthy. “And that’s just the tip of the iceberg. Whether these organic molecules are synthesized there or transported there, it exists, and knowledge alone is a fundamental advance in the field.”
Prior to the launch of GOTHAM in 2018, scientists cataloged approximately 200 individual molecules in the interstellar medium of the Milky Way. These new discoveries forced the team to, and rightly so, wonder what was there. “The amazing thing about these observations, of this discovery and of these molecules, is that no one looked or looked hard enough,” McCarthy said. “It makes you wonder what else we’re not looking for.”
This new aromatic chemistry that scientists find is not isolated from TMC-1. In a companion survey to GOTHAM, known as ARKHAM – a rigorous K / Ka Band test for aromatic molecules – benzonitrile was recently found in several additional objects. “Incredibly, we found benzonitrile in every one of the first four objects observed by ARKHAM,” said Andrew Burkhardt, a sub-doctoral fellow at the CfA and a co-principal investigator for GOTHAM. “This is important because although GOTHAM shifts the limit of the chemistry we thought was possible in space, these discoveries mean that the things we learn in TMC-1 about aromatic molecules can be broadly applied to dark clouds everywhere. “These dark clouds are the initial birthplaces of stars and planets. So, these previously invisible aromatic molecules will also have to be thought of at every subsequent step towards the creation of stars, planets and solar systems such as our own.”
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In addition to McGuire, McCarthy, and Burkhardt, the following researchers contributed to and led research for 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.
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The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a partnership agreement by Associated Universities, Inc. The NRAO, founded in 1956, provides modern radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers, regardless of institutional or national commitment. The observation of time on NRAO telescopes is available on a competitive basis to qualified scientists after evaluating research proposals based on scientific merit, the ability of the instruments to do the job and the availability of the telescope during the required time. NRAO also offers formal and informal programs in education and public outreach to teachers, students, the general public and the media.
About the Green Bank Observatory
The Green Bank Observatory is home to one of the world’s largest fully steerable radio telescopes, the National Bank’s Green Bank Telescope (GBT). The observatory is home to several additional instruments and arrays and is protected by two additional radio interference protection zones, the National Radio Quiet Zone and the West Virginia Radio Astronomy Zone.
References
“Detection of interstellar HC4NC and an investigation into Isocyanopolyyne Chemistry under TMC-1 conditions.” C. Xue et al, 2020 September 1, The Astrophysical Journal Letters [https:/
“GOTHAM Early Science: Project Overview, Methods, and Detection of Interstellar Propargyl Cyanide (HCCCH2CN) in TMC-1.” B. McGuire et al., September 1, 2020, The astrophysical journal letters [https:/
“An Investigation into Techniques for Spectral Line Stacking and Application to HC11N Detection.” R. Loomis et al, 2021 January 11, Natural Astronomy [https:/
“Widespread aromatic carbon chemistry in the earliest stages of star formation.” AM Burkhardt et al, 2021 January 11, Natural Astronomy [https:/
“Interstellar detection of the highly polar ring cyanocyclopentadiene.” M. McCarthy et al, 2021 Feb. Natural Astronomy [https:/
“Discovery of interstellar trans-cyanovinylacetylene (HCCCH = CCHCN) and Vinylcyanoacetylene (H2C = CHC3N) in GOTHAM Observations of TMC-1.” K. Lee et al., February 20, 2021, 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, pre-print PDF: https: /
“Detection of two interstellar polycyclic aromatic hydrocarbons by spectral matching filtering,” McGuire et al, 2021 March 19, Science [https:/
“Aromatic and Cyclic Molecules in Molecular Clouds: A New Dimension of Interstellar Organic Chemistry,” McCarthy & McGuire 2021, The Journal of Physical Chemistry A [https:/