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.

Figure 1: Face-on view of total radio emission from cold dust (left column) and CH3CN and HC3N (middle columns) around the IM Lup, V4046 Sgr, and MWC 480 disks. Images are centered on the peak dust emission (“+”). Darker colors and additional contours represent increasing strength of the signal. The x- and y-axes are in equatorial coordinates in arcseconds and, depending on source distance, they show a total range of 360–805 AU, or nearly 7–16 times the size of our solar system. The light gray circle in the lower left indicates the resolution of the radio observations. The disk-integrated spectra from rotational transitions of each molecule (right column) are shown in terms of velocity and radio intensity. The double-peaked structures indicate that the disks are undergoing Keplerian rotation. [Bergner et al. 2018]

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.

Figure 2: Abundance comparisons between (a) CH3CN/HCN, (b) HC3N/HCN, and (c) CH3CN/HC3N. Due to uncertainties in the determined gas temperatures, the ratios are shown for a range of temperatures from 30–70 K. The top two panels show comparisons against typical cometary fractions, while the bottom panel is compared with the measured fraction in protostellar envelopes from a previous study by the same authors. The authors find that the nitrile chemistry exhibits similar abundance ratios in protoplanetary disks, comets, and protostellar envelopes. [Bergner et al. 2018]

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.

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