Letter to the Editor

PDRs4All

XV. CH radical and H${}_{\mathsf{3}}^{+}$ molecular ion in the irradiated protoplanetary disk d203-506

¹ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
² Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
³ Department of Space, Earth, and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str.77, D-50937 Köln, Germany
⁶ NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, USA
⁷ HFML-FELIX, Toernooiveld 7, 6525ED Nijmegen, The Netherlands
⁸ Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525AJ Nijmegen, The Netherlands
⁹ Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
¹⁰ Institute for Earth and Space Exploration, The University of Western Ontario, London, ON N6A 3K7, Canada
¹¹ Carl Sagan Center, Search for ExtraTerrestrial Intelligence Institute, Mountain View, CA 94043, USA
¹² Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, 91405 Orsay, France
¹³ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹⁴ Laboratoire de Physique de l’École normale supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 75005 Paris, France
¹⁵ Observatoire de Paris, Université PSL, Sorbonne Université, LERMA, CNRS UMR 8112, 75014 Paris, France
¹⁶ Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
¹⁷ LUX, UMR 8262, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
¹⁸ Astronomy Department, University of Maryland, College Park, MD 20742, USA
¹⁹ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
²⁰ Institut d’Astrophysique Spatiale, Université Paris-Saclay, Centre National de la Recherche Scientifique, 91405 Orsay, France

^⋆ Corresponding author: ilane.schroetter@gmail.com

Received: 6 June 2025
Accepted: 3 July 2025

Abstract

Most protoplanetary disks experience a phase in which they are subjected to strong ultraviolet radiation from nearby massive stars. This UV radiation can substantially alter their chemistry by producing numerous radicals and molecular ions. In this Letter we present a detailed analysis of the JWST-NIRSpec spectrum of the d203-506 obtained as part of the PDRs4All Early Release Science program. Using state-of-the-art spectroscopic data, we searched for species using a multi-molecule fitting tool, PAHTATMOL, which we developed for this purpose. Based on this analysis, we report the clear detection of ro-vibrational emission of the CH radical and the likely detection of the H₃⁺ molecular ion, with estimated abundances of a few times 10⁻⁷ and approximately 10⁻⁸, respectively. The presence of CH is predicted by gas-phase models and is well explained by hydrocarbon photochemistry. Interstellar H₃⁺ is usually formed through reactions of H₂ with H₃⁺ originating from cosmic ray ionization of H₂. However, recent theoretical studies suggest that H₃⁺ also forms through far-UV (FUV)-driven chemistry in strongly irradiated (G₀ > 10³), dense (n_H > 10⁶ cm⁻³) gas. The latter is favored as an explanation for the presence of hot H₃⁺ (T_ex ≳ 1000 K) in the outer disk layers of d203-506, coinciding with the emission of FUV-pumped H₂ and other photodissociation region (PDR) species, such as CH⁺, CH₃⁺, and OH. Our detection of infrared emission from vibrationally excited H₃⁺ and CH raises questions about their excitation mechanisms and underscores that FUV radiation can have a profound impact on the chemistry of planet-forming disks. They also demonstrate the power of JWST to push the limit of the detection of elusive species in protoplanetary disks.