A New Window into Prebiotic Nitrogen Chemistry in Protoplanetary Disks

Title: A Survey of CH3CN and HC3N in Protoplanetary Disks

Authors: Jennifer B. Bergner, Viviana G. Guzman, Karin I. Öberg, Ryan A. Loomis, Jamila Pegues

First Author’s Institution: Harvard University, Department of Chemistry and Chemical Biology

Status: Published in The Astrophysical Journal [open access on arXiv]

The Prebiotic Importance of Nitrogen-Bearing Molecules

Our solar system formed from a gravitationally-collapsing cloud of interstellar dust and hydrogen gas, forming 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 disc-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 previous astrobites, these disks exhibit an intriguing variety of chemistry that is only beginning to be probed with the higher sensitivity of telescopes such as ALMA. In particular, understanding how the early inventory of organic molecules that are 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 syntheses of RNA and protein precursors. In today’s astrobite, we take a look at a survey of these important types of molecules in a recent survey of protoplanetary disks and 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 and 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 was 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? Thus, today’s authors sought to obtain a larger sample of observations to answer these questions.

Today’s authors used ALMA to survey six nearby protoplanetary disks at distances ranging from 235 to 466 light years from Earth. Cyanoacetylene (HC3N) is detected toward all but one of the disks, while methyl cyanide (CH3CN) is firmly detected toward three of the six disks with an additional tentative detections in another two disks. A face-on view of the HC3N and CH3N 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, 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 due to 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.

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 form either in the gas-phase or on the surfaces of dust grains. To investigate not only these formation mechanisms, but also test the chemical dependence on various physical conditions, the disks selected for this study were carefully chosen to span a range of physical conditions, namely 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 environments.

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 though to be relatively pristine records of our own solar nebula. To do so, the authors compare abundance ratios between HC3N and CH3CN 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 CH3CN/HCN (a), HC3N/HCN (b), and CH3CN/HC3N (b). Due to uncertainties in the determined gas temperatures, a range of temperatures from 30-70 K is shown. The top two panels show comparisons against cometary fractions, while the bottom panel is compared with the measured fraction in protostellar envelopes from a previous study by the same authors.

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 therefore be reliably produced in a variety of different star- and planet-forming environments.

About Charles Law

Hi! I'm a third-year graduate student at Harvard/CfA. I study chemical complexity in protoplanetary disks and star-forming regions using telescopes such as the SMA, VLA, and ALMA. In my free time, I enjoy hiking, bicycling, and traveling.

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