In a series of nine papers, scientists from the GOTHAM – Green Bank Telescope Observations of TMC-1: Hunting Aromatic Molecules – project described the detection of more than a dozen polycyclic aromatic hydrocarbons in the Taurus Molecular Cloud, or TMC-1. These complex molecules, never previously detected in the interstellar medium, allow scientists to better understand the formation of stars, planets and other bodies in space. In this artist’s view, some of the molecules detected include, from left to right: 1-cyano-naphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyan-naphthalene, vinyl cyano-acetylene, 2-cyano-cyclopentadiene, benzonitrile, trans- (E) – cyanovinylacetylene, HC4NC and propargyl cyanide, among others. Credit: M. Weiss / Center for Astrophysics | Harvard and Smithsonian
Radio observations of a cold, dense cloud of molecular gas reveal more than a dozen unexpected molecules.
Scientists have discovered a vast, previously unknown reservoir of new aromatic material in a cold, dark molecular cloud by detecting for the first time individual polycyclic aromatic hydrocarbon molecules in the interstellar medium, beginning to answer a three-decade-old scientific mystery: how and where are these molecules formed in space?
“We always thought that polycyclic aromatic hydrocarbons were formed mainly in the atmosphere 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. TMC-1: Hunting Aromatic Molecules. “In this study, we found them in cold, dark clouds where stars are not yet forming.”
Aromatic molecules and PAHs – short for polycyclic aromatic hydrocarbons – are well known to scientists. Aromatic molecules are found in the chemical makeup of humans and other animals, and are found in food and medicine. PAHs are also pollutants formed from the combustion of many fossil fuels and are even among 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 universe’s carbon,” says McGuire, who is also a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA“Now, for the first time, we have a direct look at their chemistry, which allows us to study in detail how this huge carbon store reacts and evolves through the process of star and planet formation.”
Green Bank Telescope in West Virginia, USA Credit: GBO / AUI / NSF
Scientists have suspected PAHs in space since the 1980s, but the new research, detailed in nine papers published over the past seven months, provides the first definitive evidence of their existence in molecular clouds. To find the elusive molecules, the team focused the 100-meter-long, gigantic radio astronomy GBT on the Taurus Molecular Cloud, or TMC-1 – a large, pre-stellar cloud of dust and gas about 450 light years from Earth that will one day. to collapse. on its own to form stars – and what they found was astonishing: Not only were the accepted scientific models incorrect, but there was much more going on in TMC-1 than the team could have imagined.
“Based on decades of previous models, we thought we had a fairly good understanding of molecular cloud chemistry,” said Michael McCarthy, an astrochemist and acting deputy director of CfA, whose research group performed the precise laboratory measurements that many of these astronomical detections must be established with confidence. “What these new astronomical observations show is that these molecules are not only present in molecular clouds, but also in quantities orders of magnitude higher than standard models predict.”
McGuire added that previous studies have only shown that there are PAH molecules, but not which specific ones. “For the past 30 years, scientists have observed the mass signature of these molecules in our galaxy and other galaxies in the infrared, but we couldn’t tell which individual molecules made up that mass. With the addition of radio astronomy, instead of this great mass that we cannot distinguish, we see individual molecules. “
To their surprise, the team did not discover that only one new molecule was hidden in TMC-1. In detail in multiple papers, the team observed 1-cyano-naphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyan-naphthalene, vinyl cyano-acetylene, 2-cyano-cyclopentadiene, benzonitrile, trans- (E) -cyan-vinyl acetylene, HC4NC, and propargyl cyanide, among others. It’s like entering a boutique store and just flipping through the inventory out front without ever knowing there was a back room. We’ve been collecting small molecules for about 50 years, and now we’ve discovered there is a back door. When we opened that door and looked in, we found this gigantic warehouse of molecules and chemistry that we were not expecting, ”said McGuire. “There it was, all the time, lurking just past where we had looked before.”
McGuire and other scientists from the GOTHAM project have been ‘hunting’ for molecules in TMC-1 for more than two years, following McGuire’s first detection of benzonitrile in 2018. The results of the project’s latest observations could have implications in the coming years for astrophysics. . “We’ve come across a whole new set of molecules that we haven’t been able to detect before, and that will completely change our understanding of how these molecules interact. It has downstream branches, ”said McGuire, adding that these molecules eventually grow so large that they begin to aggregate in the seeds of interstellar dust. “When these molecules grow so large that they are the seeds of interstellar dust, they have the potential to influence the composition of asteroids, comets and planets, the surfaces on which ice forms, and perhaps even the locations where planets form. forms within galaxies. “
The discovery of new molecules in TMC-1 has implications for astrochemistry as well, and while the team doesn’t have all the answers yet, the consequences will last for decades here too. “We’ve moved 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 them 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 that knowledge alone is a fundamental advance in the field. “
Before the launch of GOTHAM in 2018, scientists had about 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 at them hard enough,” McCarthy said. “You wonder what else is there that we just haven’t been looking for.”
This new aromatic chemistry that scientists are discovering has not been isolated for TMC-1. An accompanying study of 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 each of the first four objects observed by ARKHAM,” said Andrew Burkhardt, a Submillimeter Array Postdoctoral Fellow at CfA and a co-principal investigator for GOTHAM. “This is important because while GOTHAM pushes the limits of the chemistry we thought was possible in space, these discoveries imply that the things we learn in TMC-1 about aromatic molecules can be broadly applied to dark clouds, where also. These dark clouds are the first birthplaces of stars and planets. Thus, these previously invisible aromatic molecules must also be considered in every later step towards the creation of stars, planets and solar systems like ours. “
Reference: March 18, 2021, Science
In addition to McGuire, McCarthy and Burkhardt, the following researchers contributed and led research for this project: Kin Long Kelvin Lee from WITHRyan 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 from NASA Goddard; Eric Herbst, Eric R. Willis, Ci Xue, and Mark Siebert of the University of Virginia; and Sergei Kalenskii from Lebedev Physical Institute. The project also received research support from the University of Stuttgart, the Max Planck Institute and the Catholic University of America.
Funding: Center for Astrophysics | Harvard & Smithsonian, Massachusetts Institute of Technology, National Radio Astronomy Observatory, Benedictine College, National Aeronautics and Space Administration Goddard Flight Center, University of Virginia, Lebedev Physica