Letter to the Editor

PDRs4All

X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon

¹ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
² Université Paris Cité, Paris, France
³ Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden
⁴ Dept. of Astronomy, University of Michigan, Ann Arbor, MI, USA
⁵ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III - Paul Sabatier, CNRS, CNES, Toulouse, France
⁶ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, Paris-Meudon, France
⁷ IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
⁸ Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
⁹ Institute for Earth and Space Exploration, University of Western Ontario, London, Ontario, Canada
¹⁰ Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
¹¹ Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, Orsay, France
¹² Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, Orsay, France
¹³ School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
¹⁴ Department of Astronomy, Graduate School of Science, University of Tokyo, Tokyo, Japan
¹⁵ Carl Sagan Center, SETI Institute, Mountain View, CA, USA
¹⁶ Leiden Observatory, Leiden University, Leiden, The Netherlands
¹⁷ Astronomy Department, Univ. of Maryland, College Park, MD, USA
¹⁸ Space Telescope Science Institute, Baltimore, MD, USA
¹⁹ Instituto de Astrofísica e Ciências do Espaçsbon, Lisbon, Portugal

Received: 4 June 2024
Accepted: 7 August 2024

Abstract

The gas-phase abundance of carbon, x_C = [C/H]_gas = x_C⁺ + x_C⁰ + x_CO + … , and its depletion factors are essential parameters for understanding the gas and solid compositions that are ultimately incorporated into (exo)planets. The majority of protoplanetary disks are born in clusters and, as a result, are exposed to external far-ultraviolet (FUV) radiation. These FUV photons potentially affect the disk’s evolution, chemical composition, and line excitation. We present the first detection of the [C I] 609 μm fine-structure (³P₁–³P₀) line of neutral carbon (C⁰), achieved with ALMA, toward one of these disks, d203-506, in the Orion Nebula Cluster. We also report the detection of [C I] forbidden and C I permitted lines (from electronically excited states up to ∼10 eV) observed with JWST in the near-infrared (NIR). These lines trace the irradiated outer disk and photo-evaporative wind. Contrary to the common belief that these NIR lines are C⁺ recombination lines, we find that they are dominated by FUV-pumping of C⁰ followed by fluorescence cascades. They trace the transition from atomic to molecular gas, and their intensities scale with G₀. The lack of outstanding NIR O I fluorescent emission, however, implies a sharper attenuation of external FUV radiation with E ≳ 12 eV (λ ≲ Lyman-β). This is related to a lower effective FUV dust absorption cross section compared to that of interstellar grains, implying a more prominent role for FUV shielding by the C⁰ photoionization continuum. The [C I] 609 μm line intensity is proportional to N(C⁰) and can be used to infer x_C. We derive x_C ≃ 1.4 × 10⁻⁴. This implies that there is no major depletion of volatile carbon compared to x_C measured in the natal cloud, hinting at a young disk. We also show that external FUV radiation impacts the outer disk and wind by vertically shifting the water freeze-out depth, which likely results in less efficient grain growth and settling. This shift leads to nearly solar gas-phase C/O abundance ratios in these irradiated layers.