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UV-driven chemistry as a signpost of late-stage planet formation

Abstract

The chemical reservoir within protoplanetary disks has a direct impact on planetary compositions and the potential for life. A long-lived carbon- and nitrogen-rich chemistry at cold temperatures (≤ 50 K) is observed within cold and evolved planet-forming disks. This is evidenced by bright emission from small organic radicals in 1–10 Myr aged systems that would otherwise have frozen out onto grains within 1 Myr. We explain how the chemistry of a planet-forming disk evolves from a cosmic-ray/X-ray-dominated regime to a ultraviolet-dominated chemical equilibrium. This, in turn, will bring about a temporal transition in the chemical reservoir from which planets will accrete. This photochemical dominated gas phase chemistry develops as dust evolves via growth, settling and drift, and the small grain population is depleted from the disk atmosphere. A higher gas-to-dust mass ratio allows for deeper penetration of ultraviolet photons is coupled with a carbon-rich gas (C/O > 1) to form carbon-bearing radicals and ions. This further results in gas phase formation of organic molecules, which then would be accreted by any actively forming planets present in the evolved disk.

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Fig. 1: A schematic highlighting the physical evolution of a disk and how that physical environment can affect the chemistry.
Fig. 2: Radial intensity profiles of the observed complex organic molecules and HCN towards the disk around HD 163296.
Fig. 3: The radial and vertical number density distributions of CH3CN and HC3N.
Fig. 4: A comparison of the MRI active region within a non-elevated UV environment and an elevated UV environment.

Data availability

The data that support the findings of this study can be obtained as part of the MAPS programme and are publicly available via alma-maps.info. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.01055.L. and 2016.1.01046.S. Data regarding TW Hya can be obtained via data-rich figures in the online version of ref. 58. ALMA is a partnership of European Southern Observatory (ESO) (representing its member states), NSF (USA) and National Institutes of Natural Sciences (Japan), together with National Research Council (Canada), Ministry of Science and Technology and Academia Sinica Insitute of Astronomy and Astrophysics (Taiwan), and Korea Astronomy and Space Science Insitute (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities Inc/National Radio Astronomy Observatory and National Astronomical Observatory of Japan. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Code availability

This study relied on the following publicly available coding packages: rac2d https://github.com/fjdu/rac-2d, RADMC-3D https://www.ita.uni-heidelberg.de/~dullemond/software/radmc-3d/ and GoFish https://github.com/richteague/gofish. TORUS is a private code developed by T. Harries and collaborators.

References

  1. Miotello, A., Kamp, I., Birnstiel, T., Cleeves, L. I. & Kataoka, A. Setting the stage for planet formation: measurements and implications of the fundamental disk properties. Preprint at arXiv:2203.09818 (2022). https://arxiv.org/abs/2203.09818

  2. Ansdell, M. et al. ALMA survey of Lupus protoplanetary disks. II. Gas disk radii. Astrophys. J. 859, 21 (2018).

    Article  ADS  Google Scholar 

  3. Haisch Jr, K. E., Lada, E. A. & Lada, C. J. Disk frequencies and lifetimes in young clusters. Astrophys. J. Lett. 553, L153–L156 (2001).

    Article  ADS  Google Scholar 

  4. Andrews, S. M. & Birnstiel, T. in Handbook of Exoplanets (eds. Deeg, H. J. & Belmonte, J. A.) 136 (Springer International Publishing AG, 2018).

  5. Öberg, K. I., Murray-Clay, R. & Bergin, E. A. The effects of snowlines on C/O in planetary atmospheres. Astrophys. J. Lett. 743, L16 (2011).

    Article  ADS  Google Scholar 

  6. Johansen, A. & Lambrechts, M. Forming planets via pebble accretion. Annu. Rev. Earth Planet. Sci. 45, 359–387 (2017).

    Article  ADS  Google Scholar 

  7. Lambrechts, M., Johansen, A. & Morbidelli, A. Separating gas-giant and ice-giant planets by halting pebble accretion. Astron. Astrophys. 572, A35 (2014).

    Article  ADS  Google Scholar 

  8. Powner, M. W., Gerland, B. & Sutherland, J. D. Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions. Nature 459, 239–242 (2009).

    Article  ADS  Google Scholar 

  9. Ritson, D. & Sutherland, J. D. Prebiotic synthesis of simple sugars by photoredox systems chemistry. Nat. Chem. 4, 895–899 (2012).

    Article  Google Scholar 

  10. Sutherland, J. D. The origin of life - out of the blue. Angew. Chem. 55, 104–121 (2015).

    Article  Google Scholar 

  11. Bergner, J. B., Guzmán, V. G., Öberg, K. I., Loomis, R. A. & Pegues, J. A survey of CH3CN and HC3N in protoplanetary disks. Astrophys. J. 857, 69 (2018).

    Article  ADS  Google Scholar 

  12. Ilee, J. D. et al. Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and properties of the large organic molecules HC3N, CH3CN, and c-C3H2. Astrophys. J. Suppl. Ser. 257, 9 (2021).

    Article  ADS  Google Scholar 

  13. Kastner, J. H. et al. A subarcsecond ALMA Molecular Line Imaging Survey of the circumbinary, protoplanetary disk orbiting V4046 Sgr. Astrophys. J. 863, 106 (2018).

    Article  ADS  Google Scholar 

  14. Öberg, K. I. et al. The comet-like composition of a protoplanetary disk as revealed by complex cyanides. Nature 520, 198–201 (2015).

    Article  ADS  Google Scholar 

  15. Loomis, R. A. et al. The distribution and excitation of CH3CN in a solar nebula analog. Astrophys. J. 859, 131 (2018).

    Article  ADS  Google Scholar 

  16. Chapillon, E. et al. Chemistry in disks. VII. First detection of HC3N in protoplanetary disks. Astrophys. J. 756, 58 (2012).

    Article  ADS  Google Scholar 

  17. Goldsmith, P. F. & Langer, W. D. Population diagram analysis of molecular line emission. Astrophys. J. 517, 209–225 (1999).

    Article  ADS  Google Scholar 

  18. Corazzi, M. A. et al. Thermal desorption of astrophysically relevant ice mixtures of acetaldehyde and acetonitrile from olivine dust. Astrophys. J. 913, 128 (2021).

    Article  ADS  Google Scholar 

  19. Canta, A., Teague, R., Le Gal, R. & Öberg, K. I. The first detection of CH2CN in a protoplanetary disk. Astrophys. J. 922, 62 (2021).

    Article  ADS  Google Scholar 

  20. Öberg, K. I. et al. The Spitzer ice legacy: ice evolution from cores to protostars. Astrophys. J. 740, 109 (2011).

    Article  ADS  Google Scholar 

  21. Bergner, J. B. & Ciesla, F. Ice inheritance in dynamical disk models. Astrophys. J. 919, 45 (2021).

    Article  ADS  Google Scholar 

  22. Walsh, C. et al. Complex organic molecules in protoplanetary disks. Astron. Astrophys. 563, A33 (2014).

    Article  Google Scholar 

  23. Vasyunin, A. I. & Herbst, E. Reactive desorption and radiative association as possible drivers of complex molecule formation in the cold interstellar medium. Astrophys. J. 769, 34 (2013).

    Article  ADS  Google Scholar 

  24. Basalgète, R. et al. Photodesorption of acetonitrile CH3CN in UV-irradiated regions of the interstellar medium: experimental evidence. Astrophys. J. 922, 213 (2021).

    Article  ADS  Google Scholar 

  25. Miotello, A. et al. Bright C2H emission in protoplanetary discs in Lupus: high volatile C/O > 1 ratios. Astron. Astrophys. 631, A69 (2019).

    Article  Google Scholar 

  26. Bosman, A. D. et al. Molecules with ALMA at Planet-forming Scales (MAPS). VII. Substellar O/H and C/H and superstellar C/O in planet-feeding gas. Astrophys. J. Suppl. Ser. 257, 7 (2021).

    Article  ADS  Google Scholar 

  27. Cleeves, L. I. et al. The TW Hya Rosetta Stone Project IV: a hydrocarbon-rich disk atmosphere. Astrophys. J. 911, 29 (2021).

    Article  ADS  Google Scholar 

  28. Le Gal, R., Brady, M. T., Öberg, K. I., Roueff, E. & Le Petit, F. The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks. Astrophys. J. 886, 86 (2019).

    Article  ADS  Google Scholar 

  29. Asplund, M., Amarsi, A. M. & Grevesse, N. The chemical make-up of the Sun: a 2020 vision. Astron. Astrophys. 653, A141 (2021).

    Article  ADS  Google Scholar 

  30. Law, C. J. et al. Molecules with ALMA at Planet-forming Scales (MAPS). IV. Emission surfaces and vertical distribution of molecules. Astrophys. J. Suppl. Ser. 257, 4 (2021).

    Article  ADS  Google Scholar 

  31. Birnstiel, T., Andrews, S. M. & Ercolano, B. Can grain growth explain transition disks? Astron. Astrophys. 544, A79 (2012).

    Article  ADS  Google Scholar 

  32. Öberg, K. I. et al. Molecules with ALMA at Planet-forming Scales (MAPS). I. Program overview and highlights. Astrophys. J. Suppl. Ser. 257, 1 (2021).

    Article  ADS  Google Scholar 

  33. Bergner, J. B. et al. A survey of C2H, HCN, and C18O in protoplanetary disks. Astrophys. J. 876, 25 (2019).

    Article  ADS  Google Scholar 

  34. Gaia Collaboration. Gaia Data Release 2. Summary of the contents and survey properties. Astron. Astrophys. 616, A1 (2018).

    Article  Google Scholar 

  35. Booth, A. S. et al. Molecules with ALMA at Planet-forming Scales (MAPS). XVI. Characterizing the impact of the molecular wind on the evolution of the HD 163296 system. Astrophys. J. Suppl. Ser. 257, 16 (2021).

    Article  ADS  Google Scholar 

  36. Xie, C. et al. A MUSE view of the asymmetric jet from HD 163296. Astron. Astrophys. 650, L6 (2021).

    Article  ADS  Google Scholar 

  37. Andrews, S. M. et al. The Disk Substructures at High Angular Resolution Project (DSHARP). I. Motivation, sample, calibration, and overview. Astrophys. J. Lett. 869, L41 (2018).

    Article  ADS  Google Scholar 

  38. Bergner, J. B. et al. Molecules with ALMA at Planet-forming Scales (MAPS). XI. CN and HCN as tracers of photochemistry in disks. Astrophys. J. Suppl. Ser. 257, 11 (2021).

    Article  ADS  Google Scholar 

  39. Zhang, K. et al. Molecules with ALMA at Planet-forming Scales (MAPS). V. CO gas distributions. Astrophys. J. Suppl. Ser. 257, 5 (2021).

    Article  ADS  Google Scholar 

  40. Calahan, J. K. et al. Molecules with ALMA at Planet-forming Scales (MAPS). XVII. Determining the 2D thermal structure of the HD 163296 disk. Astrophys. J. Suppl. Ser. 257, 17 (2021).

    Article  ADS  Google Scholar 

  41. Bosman, A. D., Alarcón, F., Zhang, K. & Bergin, E. A. Destruction of refractory carbon grains drives the final stage of protoplanetary disk chemistry. Astrophys. J. 910, 3 (2021).

    Article  ADS  Google Scholar 

  42. Schwarz, K. R. et al. Unlocking CO depletion in protoplanetary disks. I. The warm molecular layer. Astrophys. J. 856, 85 (2018).

    Article  ADS  Google Scholar 

  43. Krijt, S., Schwarz, K. R., Bergin, E. A. & Ciesla, F. J. Transport of CO in protoplanetary disks: consequences of pebble formation, settling, and radial drift. Astrophys. J. 864, 78 (2018).

    Article  ADS  Google Scholar 

  44. Van Clepper, E., Bergner, J. B., Bosman, A. D., Bergin, E. & Ciesla, F. J. Chemical feedback of pebble growth: impacts on CO depletion and C/O ratios. Astrophys. J. 927, 206 (2022).

    Article  ADS  Google Scholar 

  45. Bosman, A. D., Walsh, C. & van Dishoeck, E. F. CO destruction in protoplanetary disk midplanes: Inside versus outside the CO snow surface. Astron. Astrophys. 618, A182 (2018).

    Article  Google Scholar 

  46. McElroy, D. et al. The UMIST database for astrochemistry 2012. Astron. Astrophys. 550, A36 (2013).

    Article  Google Scholar 

  47. Najita, J. R., Ádámkovics, M. & Glassgold, A. E. Formation of organic molecules and water in warm disk atmospheres. Astrophys. J. 743, 147 (2011).

    Article  ADS  Google Scholar 

  48. Anderson, D. E. et al. Observing carbon and oxygen carriers in protoplanetary disks at mid-infrared wavelengths. Astrophys. J. 909, 55 (2021).

    Article  ADS  Google Scholar 

  49. Woitke, P., Kamp, I. & Thi, W. F. Radiation thermo-chemical models of protoplanetary disks. I. Hydrostatic disk structure and inner rim. Astron. Astrophys. 501, 383–406 (2009).

    Article  ADS  Google Scholar 

  50. Brewer, J. M., Fischer, D. A. & Madhusudhan, N. C/O and O/H ratios suggest some hot Jupiters originate beyond the snow line. Astron. J. 153, 83 (2017).

    Article  ADS  Google Scholar 

  51. Gammie, C. F. & Ostriker, E. C. Can nonlinear hydromagnetic waves support a self-gravitating cloud? Astrophys. J. 466, 814 (1996).

    Article  ADS  Google Scholar 

  52. Czekala, I. et al. Molecules with ALMA at Planet-forming Scales (MAPS). II. CLEAN strategies for synthesizing images of molecular line emission in protoplanetary disks. Astrophys. J. Suppl. Ser. 257, 2 (2021).

    Article  ADS  Google Scholar 

  53. Law, C. J. et al. Molecules with ALMA at Planet-forming Scales (MAPS). III. Characteristics of radial chemical substructures. Astrophys. J. Suppl. Ser. 257, 3 (2021).

    Article  ADS  Google Scholar 

  54. McMullin, J. P., Waters, B., Schiebel, D., Young, W. & Golap, K. in Astronomical Data Analysis Software and Systems XVI, Vol. 376 of Astronomical Society of the Pacific Conference Series (eds. Shaw, R. A. et al.) 127 (Astronomical Society of the Pacific, 2007).

  55. Jorsater, S. & van Moorsel, G.A. High resolution neutral hydrogen observations of the barred spiral galaxy NGC 1365. Astron. J. 110, 2037 (1995).

  56. Du, F. & Bergin, E. A. Water vapor distribution in protoplanetary disks. Astrophys. J. 792, 2 (2014).

    Article  ADS  Google Scholar 

  57. Dullemond, C. P. et al. RADMC-3D: A multi-purpose radiative transfer tool (Astrophysics Source Code Library, 2012).

  58. Calahan, J. K. et al. The TW Hya Rosetta Stone Project. III. Resolving the gaseous thermal profile of the disk. Astrophys. J. 908, 8 (2021).

    Article  ADS  Google Scholar 

  59. Lynden-Bell, D. & Pringle, J. E. The evolution of viscous discs and the origin of the nebular variables. Mon. Not. R. Astron. Soc. 168, 603–637 (1974).

    Article  ADS  Google Scholar 

  60. Mathis, J. S., Rumpl, W. & Nordsieck, K. H. The size distribution of interstellar grains. Astrophys. J. 217, 425–433 (1977).

    Article  ADS  Google Scholar 

  61. Isella, A. et al. The Disk Substructures at High Angular Resolution Project (DSHARP). IX. A High-definition study of the HD 163296 planet-forming disk. Astrophys. J. Lett. 869, L49 (2018).

    Article  ADS  Google Scholar 

  62. Birnstiel, T. et al. The Disk Substructures at High Angular Resolution Project (DSHARP). V. Interpreting ALMA maps of protoplanetary disks in terms of a dust model. Astrophys. J. Lett. 869, L45 (2018).

    Article  ADS  Google Scholar 

  63. Warren, S. G. & Brandt, R. E. Optical constants of ice from the ultraviolet to the microwave: a revised compilation. J. Geophys. Res. (Atmos.) 113, D14220 (2008).

    Article  ADS  Google Scholar 

  64. Draine, B. T. Interstellar dust grains. Annu. Rev. Astron. Astrophys. 41, 241–289 (2003).

    Article  ADS  Google Scholar 

  65. Henning, T. & Stognienko, R. Dust opacities for protoplanetary accretion disks: influence of dust aggregates. Astron. Astrophys. 311, 291–303 (1996).

    ADS  Google Scholar 

  66. Schöier, F. L., van der Tak, F. F. S., van Dishoeck, E. F. & Black, J. H. An atomic and molecular database for analysis of submillimetre line observations. Astron. Astrophys. 432, 369–379 (2005).

    Article  ADS  Google Scholar 

  67. Müller, H. S. P., Thorwirth, S., Roth, D. A. & Winnewisser, G. The Cologne Database for Molecular Spectroscopy, CDMS. Astron. Astrophys. 370, L49–L52 (2001).

    Article  ADS  Google Scholar 

  68. Müller, H. S. P., Schlöder, F., Stutzki, J. & Winnewisser, G. The Cologne Database for Molecular Spectroscopy, CDMS: a useful tool for astronomers and spectroscopists. J. Mol. Struct. 742, 215–227 (2005).

    Article  ADS  Google Scholar 

  69. Teague, R. Gofish: Fishing for line observations in protoplanetary disks. J. Open Source Softw. 4, 1632 (2019).

    Article  ADS  Google Scholar 

  70. Allen, M. & Robinson, G. W. The molecular composition of dense interstellar clouds. Astrophys. J. 212, 396–415 (1977).

    Article  ADS  Google Scholar 

  71. Hasegawa, T. I., Herbst, E. & Leung, C. M. Models of gas-grain chemistry in dense interstellar clouds with complex organic molecules. Astrophys. J. Suppl. Ser. 82, 167 (1992).

    Article  ADS  Google Scholar 

  72. Goumans, T. P. M., Uppal, M. A. & Brown, W. A. Formation of CO2 on a carbonaceous surface: a quantum chemical study. Mon. Not. R. Astron. Soc. 384, 1158–1164 (2008).

    Article  ADS  Google Scholar 

  73. Ioppolo, S., Cuppen, H. M., Romanzin, C., van Dishoeck, E. F. & Linnartz, H. Laboratory evidence for efficient water formation in interstellar ices. Astrophys. J. 686, 1474–1479 (2008).

    Article  ADS  Google Scholar 

  74. Ioppolo, S., Cuppen, H. M., Romanzin, C., van Dishoeck, E. F. & Linnartz, H. Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration. Phys. Chem. Chem. Phys.12, 12065 (2010).

    Article  Google Scholar 

  75. Fuchs, G. W. et al. Hydrogenation reactions in interstellar CO ice analogues. A combined experimental/theoretical approach. Astron. Astrophys. 505, 629–639 (2009).

    Article  ADS  Google Scholar 

  76. Oba, Y. et al. Water formation through a quantum tunneling surface reaction, OH + H2, at 10 K. Astrophys. J. 749, 67 (2012).

    Article  ADS  Google Scholar 

  77. Ruffle, D. P. & Herbst, E. New models of interstellar gas-grain chemistry – III. Solid CO2. Mon. Not. R. Astron. Soc. 324, 1054–1062 (2001).

    Article  ADS  Google Scholar 

  78. Bergner, J. B., Rajappan, M. & Öberg, K. I. HCN snow lines in protoplanetary disks: constraints from ice desorption experiments. Astrophys. J. 933, 206 (2022).

    Article  ADS  Google Scholar 

  79. Guzmán, V. V. et al. Molecules with ALMA at Planet-forming Scales (MAPS). VI. Distribution of the small organics HCN, C2H, and H2CO. Astrophys. J. Suppl. Ser. 257, 6 (2021).

    Article  ADS  Google Scholar 

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Acknowledgements

J.K.C. acknowledges support from the National Science Foundation Graduate Research Fellowship under grant no. DGE 1256260 and the National Aeronautics and Space Administration FINESST grant, under grant no. 80NSSC19K1534. E.A.B. acknowledges support from National Science Foundation (NSF) AAG grant no. 1907653. A.D.B. acknowledges support from NSF AAG grant no. 1907653. E.A.R. acknowledges support from NSF AST grant no. 1830728. J.B.B. acknowledges support from NASA through the NASA Hubble Fellowship grant no. HST-HF2-51429.001-A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. Support for J.H. was provided by NASA through the NASA Hubble Fellowship grant no. HST-HF2-51460.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract no. NAS5-26555. This work was supported by a grant from the Simons Foundation 686302 and by an award from the Simons Foundation 321183FY19 (to K.I.Ö.). This material is based upon work supported by the National Science Foundation under grant no. AST-1907832. J.D.I. acknowledges support from an STFC Ernest Rutherford Fellowship (no. ST/W004119/1) and a University Academic Fellowship from the University of Leeds. C.W. acknowledges financial support from the University of Leeds, the Science and Technology Facilities Council and UK Research and Innovation (grant nos. ST/T000287/1 and MR/T040726/1). V.V.G. gratefully acknowledges support from FONDECYT Regular 1221352, ANID BASAL projects nos. ACE210002 and FB210003, and ANID – Millennium Science Initiative Program – NCN19_171. We thank T. Harries for help with and providing access to the TORUS modelling program.

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Authors

Contributions

J.K.C. is the lead author, produced thermochemical models and led the group effort to put forward this new chemical theory. E.A.B. contributed to the text, provided advisement throughout the modelling process, contributed heavily to the scientific theory, and is a principle investigator of the large observing programme these models and theory rely on. A.D.B contributed to the text, provided advisement throughout the modelling process, and contributed heavily to the scientific theory and the SED modelling. E.A.R. contributed to the text and provided and contributed to the SED modelling. K.I.Ö. and V.V.G. contributed to the text and discussion on the scientific theory, and are principle investigators of the large observing programme these models and theory rely on. C.W. contributed to the text and discussion on the scientific theory, compiled the gas-grain chemical network and is a PI of the large observing programme these models and theory rely on. S.M.A., J.B.B., L.I.C., J.H., J.D.I., C.J.L, R.LG., R.T., D.J.W. and K.Z. contributed significantly to discussion of the theory and writing and editing of the text.

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Correspondence to Jenny K. Calahan.

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Calahan, J.K., Bergin, E.A., Bosman, A.D. et al. UV-driven chemistry as a signpost of late-stage planet formation. Nat Astron 7, 49–56 (2023). https://doi.org/10.1038/s41550-022-01831-8

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