The element carbon is a building block for life, both on Earth and potentially elsewhere in the vast reaches of space. There should be a lot of carbon in space, but surprisingly, it’s not always easy to find. While it can be observed in many places, it doesn’t add up to the volume astronomers would expect to see. The discovery of a new, complex molecule (1-cyanopyrene), challenges these expectations, about where the building blocks for carbon are found, and how they evolve. This research was published today in the journal Science.
1-cyanopyrene is an organic molecule made up of multiple fused benzene rings and belongs to a class of compounds known as Polycyclic Aromatic Hydrocarbons (PAHs), which were previously believed to form only at high temperatures, in areas with lots of energy, like the environments surrounding aging stars. On Earth, PAHs are found in burning fossil fuels, and as char marks on grilled food. However, these newly observed 1-cyanopyrene molecules were found in Taurus Molecular Cloud-1 (TMC-1), a cold interstellar cloud. Located in the Taurus constellation, TMC-1 has not yet begun forming stars, and the temperature is only about 10 degrees above absolute zero.
Brett McGuire, an Assistant Professor of Chemistry at MIT and an adjunct astronomer at the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO), has been studying PAHs, and those found in TMC-1, for most of his career. “TMC-1 is a natural laboratory for studying these molecules that go on to form the building blocks of stars and planets,” shares McGuire, “These are the largest molecules we’ve found in TMC-1 to date. This discovery pushes the boundaries of our understanding of the complexity of molecules that can exist in interstellar space.”
Astronomers used the NSF NRAO Green Bank Telescope, the largest fully steerable radio telescope in the world, to discover 1-cyanopyrene. Every molecule has a unique rotational spectrum, like a fingerprint, which allows for its identification. However, their large size and lack of a permanent dipole moment, can make some PAHs difficult – or even impossible – to detect. The observations of this new cyanopyrene isomer can provide indirect evidence for the presence of even larger and more complex molecules in future observations.
“Identifying the unique rotational spectrum of 1-cyanopyrene required the work of an interdisciplinary scientific team,” explains Harshal Gupta, NSF Program Director for the Green Bank Observatory and a co-author on this paper. “This research combined the expertise of astronomy and chemistry, with measurements and analysis conducted in the molecular spectroscopy laboratory of Dr. Michael McCarthy at the Center for Astrophysics | Harvard & Smithsonian. The team involved researchers at all levels, with an MIT postdoc, Gabi Wenzel, leading the lab work and the paper.”
Astronomers study PAHs not just to learn about their particular lifecycle, but to learn more about how they interact with and reveal more about the interstellar medium (ISM) and celestial bodies around them. PAHs are believed to be responsible for the unidentified infrared bands observed in many astronomical objects. These bands arise from the infrared fluorescence of PAHs after they absorb ultraviolet (UV) photons from stars. The intensity of these bands reveal PAHs could account for a significant fraction of carbon in the ISM.
PAHs are also found much closer to home. Last December, a team of researchers examining samples returned from an asteroid in our own solar system found it not only contained PAHs, but specifically large quantities of pyrene, as well as a PAH known as naphthalene that McGuire and his colleagues also previously found evidence for in TMC-1. What’s more, by analyzing the signatures of a rare isotope of carbon – carbon-13 – in the sample, they were able to determine that both of these PAHs had to have formed at very low temperatures. As low, in fact, as the 10 K of TMC-1!
“This is hinting to us that these PAHs we’re finding in our own Solar System may have formed long before our star, in the cold dark cloud of gas and dust like TMC-1 that gave birth to the Sun,” McGuire says. “It’s wild to think we may be looking at the chemical archaeological record of our molecular origins in these asteroids… and at the very beginnings of that record for whatever solar systems and planets eventually form in TMC-1.”
McGuire and his colleagues will continue to search for other PAHs in TMC-1, to create a more complete picture of the molecular population in this region. The detection of PAHs in this cold dense cloud challenges the traditional view of their formation, and has implications for their role in interstellar chemistry and astrobiology. PAHs are believed to be the precursors to molecules essential for the origin of life. Future research will provide insights into the formation of stars and planets, and ultimately contribute to our understanding of the conditions that lead to the emergence of life in our Universe.
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The National Radio Astronomy Observatory and the Green Bank Observatory are major facilities of the U.S. National Science Foundation and are operated by Associated Universities, Inc.
