OH as a probe of the warm-water cycle in planet-forming disks

• Published in: Nature astronomy Vol. 8; no. 5; pp. 577 - 586
• Main Authors: Zannese, Marion, Tabone, Benoît, Habart, Emilie, Goicoechea, Javier R., Zanchet, Alexandre, van Dishoeck, Ewine F., van Hemert, Marc C., Black, John H., Tielens, Alexander G. G. M., Veselinova, A., Jambrina, P. G., Menendez, M., Verdasco, E., Aoiz, F. J., Gonzalez-Sanchez, L., Trahin, Boris, Dartois, Emmanuel, Berné, Olivier, Peeters, Els, He, Jinhua, Sidhu, Ameek, Chown, Ryan, Schroetter, Ilane, Van De Putte, Dries, Canin, Amélie, Alarcón, Felipe, Abergel, Alain, Bergin, Edwin A., Bernard-Salas, Jeronimo, Boersma, Christiaan, Bron, Emeric, Cami, Jan, Dicken, Daniel, Elyajouri, Meriem, Fuente, Asunción, Gordon, Karl D., Issa, Lina, Joblin, Christine, Kannavou, Olga, Khan, Baria, Languignon, David, Le Gal, Romane, Maragkoudakis, Alexandros, Meshaka, Raphael, Okada, Yoko, Onaka, Takashi, Pasquini, Sofia, Pound, Marc W., Robberto, Massimo, Röllig, Markus, Schefter, Bethany, Schirmer, Thiébaut, Vicente, Sílvia, Wolfire, Mark G.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.05.2024; Nature Publishing Group
• Subjects: 639/33/34/4122; 639/33/34/865; 639/766/36; Astronomy; Astrophysics and Cosmology; Deuterium; Hydrologic cycle; Hydroxyl radicals; Oceans; Physics; Physics and Astronomy; Planets; Radiation; Space telescopes; Ultraviolet radiation
• ISSN: 2397-3366; 2397-3366
• DOI: 10.1038/s41550-024-02203-0

Water is a key ingredient for the emergence of life as we know it. Yet, its destruction and reformation in space remain unprobed in warm gas ( T  > 300 K). Here we detect with the James Webb Space Telescope the emission of the hydroxyl radical (OH) from d203-506, a planet-forming disk exposed to external far-ultraviolet (FUV) radiation. These observations were made as part of the Early Release Science programme PDRs4All, which is focused on the Orion bar. The observed OH spectrum is compared with the results of quantum dynamical calculations to reveal two essential molecular processes. The highly excited rotational lines of OH in the mid-infrared are telltale signs of H 2 O destruction by FUV radiation. The OH rovibrational lines in the near-infrared are attributed to chemical excitation by the key reaction O + H 2  → OH + H, which seeds the formation of water in the gas phase. These results show that under warm and irradiated conditions, water is destroyed and efficiently reformed through gas-phase reactions. We infer that, in this source, the equivalent of Earth oceans’ worth of water is destroyed per month and replenished. This warm-water cycle could reprocess some water inherited from cold interstellar clouds and explain the lower deuterium fraction of water in Earth’s oceans compared with that found around protostars. The hydroxyl radical OH has been detected in a planet-forming disk exposed to ultraviolet radiation and in a rovibrationally excited state. These JWST observations, when coupled with quantum calculations, reveal the ongoing photodissociation of water and its reformation in the gas phase.