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The comet-like composition of a protoplanetary disk as revealed by complex cyanides

Nature volume 520, pages 198–201 (2015)Cite this article

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Abstract

Observations of comets and asteroids show that the solar nebula that spawned our planetary system was rich in water and organic molecules. Bombardment brought these organics to the young Earth’s surface1. Unlike asteroids, comets preserve a nearly pristine record of the solar nebula composition. The presence of cyanides in comets, including 0.01 per cent of methyl cyanide (CH3CN) with respect to water, is of special interest because of the importance of C–N bonds for abiotic amino acid synthesis2. Comet-like compositions of simple and complex volatiles are found in protostars, and can readily be explained by a combination of gas-phase chemistry (to form, for example, HCN) and an active ice-phase chemistry on grain surfaces that advances complexity3. Simple volatiles, including water and HCN, have been detected previously in solar nebula analogues, indicating that they survive disk formation or are re-formed in situ4,5,6,7. It has hitherto been unclear whether the same holds for more complex organic molecules outside the solar nebula, given that recent observations show a marked change in the chemistry at the boundary between nascent envelopes and young disks due to accretion shocks8. Here we report the detection of the complex cyanides CH3CN and HC3N (and HCN) in the protoplanetary disk around the young star MWC 480. We find that the abundance ratios of these nitrogen-bearing organics in the gas phase are similar to those in comets, which suggests an even higher relative abundance of complex cyanides in the disk ice. This implies that complex organics accompany simpler volatiles in protoplanetary disks, and that the rich organic chemistry of our solar nebula was not unique.

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Figure 1: ALMA detections of simple and complex cyanides in the MWC 480 protoplanetary disk.
Figure 2: Spectra of detected cyanides in the MWC 480 protoplanetary disk.
Figure 3: Models of cyanide emission and radial distributions in the MWC 480 disk.

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Acknowledgements

We acknowledge comments from E. van Dishoeck. This Letter makes use of ALMA data. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. K.I.Ö. acknowledges A. Leroy and the NAASC for assistance with calibration and imaging, and also acknowledges funding from the Simons Collaboration on the Origins of Life (SCOL), the Alfred P. Sloan Foundation, and the David and Lucile Packard Foundation. D.J.W. acknowledges funding from NASA Origins of Solar Systems (grant no. NNX11AK63).

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Authors and Affiliations

  1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA,

    Karin I. Öberg, Viviana V. Guzmán, Chunhua Qi, Sean M. Andrews, Ryan Loomis & David J. Wilner

  2. Leiden Observatory, Leiden University, PO Box 9513, 2300 CA Leiden, The Netherlands,

    Kenji Furuya

  3. Kobe University, 1-1 Rokkodaicho, Nada Ward, Kobe, Hyogo Prefecture 657-0013, Japan,

    Yuri Aikawa

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  1. Karin I. Öberg
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Contributions

K.I.Ö. led the overall project, reduced the data, assisted by V.V.G. and R.L., and wrote the manuscript with revisions from S.M.A. and D.J.W. V.V.G., assisted by C.Q., performed the parametric modelling and abundance extraction. K.F. performed the astrochemical modelling, and interpreted the results with Y.A. All authors contributed to discussions of the results and commented on the manuscript.

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Correspondence to Karin I. Öberg.

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The authors declare no competing financial interests.

Additional information

The ALMA program number for the presented data is 2013.1.00226.

Extended data figures and tables

Extended Data Figure 1 Model of the physical structure of the MWC 480 protoplanetary disk.

a, Radial (distance R) and vertical (distance Z) disk temperature profile (colour: see colour scale on right, contours: the gas temperature Tkin = 20, 30, 50, 100 and 1,000 K). b, Radial (R) and vertical (Z) density profile (colour: see colour scale on the right, contours: hydrogen density nH 1010, 108, 106 and 104 cm−3). Z/R = 0.2 is marked with a dashed line.

Extended Data Figure 2 Synthetic observations of H13CN, HC3N and CH3CN for different density

slopes α. The models are based on best fit to data for different choices of α, with the ranges chosen based on the emission pattern for each molecule. Left column, H13CN; middle column, HC3N; right column, CH3CN. Top row, α = 0; middle row, α = 1; bottom row, α = 2. a–g, Integrated emission maps (colour: see colour scale on the right). Black contours are the observed [3, 4, 5, 7, 10]σ in Fig. 1. The synthesized beam is shown in the bottom left corner of each panel. Note the change in emission profile between α = 1 and 2 for HC3N.

Extended Data Figure 3 Models of gaseous CH3CN/HCN abundance ratios under different physical conditions.

a–l, The CH3CN/HCN abundance ratio on a logarithmic scale (colour: see colour scale on the bottom and numbers on contours). The ultraviolet radiation flux increases from left to right from G0 = 1 (a, d, g, j) to G0 = 10 (b, e, h, k) to G0 = 100 (c, f, i, l), where G0 is the scaling factor in multiples of the local interstellar radiation field. The ionization rate of H2 increases from top to bottom from 10−17 s−1 (a–c) to 10−16 s−1 (d–f) to 10−15 s−1 (g–i) to 10−14 s−1 (j–l).

Extended Data Figure 4 Models of gaseous CH3CN in disks with and without turbulent diffusion.

a, The abundance of CH3CN with respect to the hydrogen density nH (colour: see colour scale on the right) as a function of disk radius (R) and height scaled by the radius (Z/R) in a model without turbulence. The dashed lines indicate gas temperatures of [30, 50, 100] K. b, c, As a but in disk models that include turbulence parameterized by αz = 10−3 (b) and αz = 10−2 (c). d, The vertically integrated column density of CH3CN from a–c (solid line: αz = 0, dashed line: αz = 10−3, dotted line: αz = 10−2).

Extended Data Figure 5 Models of gaseous CH3CN/HCN ratios in disks with and without turbulent diffusion.

a–d, As in Extended Data Fig. 4 but for CH3CN/HCN ratio.

Extended Data Figure 6 Models of gas-to-ice ratios of HCN in disks with and without turbulent diffusion.

a–d, As in Extended Data Fig. 4 but for ice-to-gas ratios of HCN.

Extended Data Figure 7 Models of gas-to-ice ratios of CH3CN in disks with and without turbulent diffusion.

a–d, As in Extended Data Fig. 4 but for ice-to-gas ratios of CH3CN.

Extended Data Table 1 Physical model for the disk of MWC 480
Full size table

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Öberg, K., Guzmán, V., Furuya, K. et al. The comet-like composition of a protoplanetary disk as revealed by complex cyanides. Nature 520, 198–201 (2015). https://doi.org/10.1038/nature14276

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