A New Window into Prebiotic Nitrogen Chemistry in Protoplanetary Disks

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Title: A Survey of CH₃CN and HC₃N in Protoplanetary Disks
Author: Jennifer B. Bergner, Viviana G. Guzman, Karin I. Öberg, Ryan A. Loomis, Jamila Pegues
First Author’s Institution: Smithsonian Astrophysical Observatory and Sternberg Astronomical Institute
Status: Published in ApJ

The Prebiotic Importance of Nitrogen-Bearing Molecules

Did you know that you are made out of primordial dust and gas? It’s true! Our solar system formed from a gravitationally collapsing cloud of interstellar dust and hydrogen gas, birthing a proto-Sun in the center of this hot dense material. The various planets are then thought to have formed out of the material in the solar nebula, the disk-shaped cloud of gas and dust left over from the Sun’s formation, which ultimately takes the shape of a rotating protoplanetary disk. As discussed in a previous astrobite, these disks exhibit an intriguing variety of chemistry that is only beginning to be probed with the higher sensitivity of telescopes such as ALMA. Understanding how the early inventory of organic molecules present in this stage of planet formation developed into the vast complexity of biochemistry we see today is key to the study of the origins of life.

Chemists and astronomers alike have been especially interested in nitrile-bearing molecules, which contain a carbon-nitrogen triple bond. These molecules likely play a crucial role in prebiotic chemistry, as recent studies have shown that this particular bond is involved in prebiotic synthesis of RNA and protein precursors. In today’s astrobite, we take a look at these important types of molecules in a recent survey of protoplanetary disks, and we explore the implications for nitrogen-based chemistry in our early solar system.

Surveying CH₃CN and HC₃N in Planet-Forming Disks

Up to this point, few disks had well-characterized nitrile abundances. More importantly, it was unknown if other disks contain similar nitrile abundances as the solar nebula. Also, it was unclear how robust the nitrile chemistry is in different circumstellar environments. For instance, do differences in the density and temperature around young stars lead to significantly different amounts of nitrogen-bearing molecules, or can they survive in a wide range of physical conditions? The authors of today’s bite sought to obtain a larger sample of observations to answer these questions.

The authors used ALMA to survey six nearby protoplanetary disks at distances ranging from 235 to 466 light-years from Earth. Cyanoacetylene (HC₃N) is detected in all but one of the disks, while methyl cyanide (CH₃CN) is firmly detected in three of the six disks with additional tentative detections in another two disks. A face-on view of the HC₃N and CH₃CN gas, along with the observed spectra in half of the observed disks, is shown in Figure 1.

Discovery of a Robust Nitrile Chemistry

In protoplanetary disks, the underlying chemistry is determined by both the presence of gas-phase molecules and dust grains. In the case of these two molecules, astrochemists have discerned that HC₃N is only able to form efficiently in the gas phase, while CH₃CN forms either in the gas phase or on the surfaces of dust grains. To investigate not only these formation mechanisms, but also to test the chemical dependence on various physical conditions, the disks selected for this study were carefully chosen to span a range of physical conditions, including their age, mass, and luminosity. However, after careful modeling, the authors find no strong trends in nitrile emission strength or abundances with these environmental differences. This result implies that nitrile chemistry is chemically robust and is likely to be found with similar outcomes even in substantially different physical conditions.

Protostellar and Cometary Comparisons

To further investigate this unexpected chemical resilience, the authors compare their results in protoplanetary disks with previous observations of protostellar envelopes, as well as with solar-system comets, which are thought to be relatively pristine records of our own solar nebula. To do so, the authors compare abundance ratios among HC₃N, CH₃CN, and HCN with those measured in protostars and comets in Figure 2. For each of these disks, the molecule HCN, which is a precursor molecule to HC₃N, had previously been observed and thus is also included in their comparisons.

Nitrile abundance ratios are surprisingly consistent across the surveyed disks as well as with comets and protostellar envelopes, despite the inherent evolutionary differences of these objects. This consistency again demonstrates that complex nitrile species should be reliably produced in a variety of different star- and planet-forming environments. These results also suggest that the solar system is not unique in its nitrile chemistry, which means that the raw materials needed for the biochemical beginnings of life may be common around newly forming planets in other solar systems!

About the author, Charles Law:

Hi! I’m a first-year graduate student at Harvard/CfA. I’m interested in observationally studying the chemical complexity found in space, including throughout high-mass star-forming regions and in protoplanetary disks. In my free time, I enjoy hiking, bicycling, and traveling.