PDRs4All: XV. CH radical and H+3molecular ion in the irradiated protoplanetary disk d203-506
Journal article, 2025
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 (nH > 106 cm-3) gas. The latter is favored as an explanation for the presence of hot H3+ (Tex ≥ 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.
ISM: molecules
Protoplanetary disks
ISM: individual objects: d203-506
Author
Ilane Schroetter
Institut de Recherche en Astrophysique et Planétologie (IRAP)
O. Berné
Institut de Recherche en Astrophysique et Planétologie (IRAP)
J.R. Goicoechea
Spanish National Research Council (CSIC)
John H Black
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
O. Roncero
Spanish National Research Council (CSIC)
Felipe Alarcon
University of Milan
P. Amiot
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Oskar Asvany
University of Cologne
Christiaan Boersma
National Aeronautics and Space Administration (NASA)
S. Brünken
Radboud University
Jan Cami
Western University
Search for Extraterrestrial Intelligence Institute
Laurent H. Coudert
University Paris-Saclay
E. Dartois
University Paris-Saclay
A. Fuente
Centro de Astrobiologia (CAB)
Bérenger Gans
University Paris-Saclay
Antoine Gusdorf
Laboratoire de Physique de l’Ecole Normale Supérieure
Paris Observatory
Ugo Jacovella
University Paris-Saclay
Marie Aline Martin-Drumel
University Paris-Saclay
T. Onaka
University of Tokyo
Els Peeters
Western University
Search for Extraterrestrial Intelligence Institute
Evelyne Roueff
Paris Observatory
A.G.G.M. Tielens
University of Maryland
Leiden University
Marion Zannese
Institut d'Astrophysique Spatiale
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 699 L13Subject Categories (SSIF 2025)
Theoretical Chemistry
Atom and Molecular Physics and Optics
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202555841