PDRs4All

Springer Link (Chiba Institute of Technology)

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 H$ _{3}^{+} $ 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 H$ _{3}^{+} $ is usually formed through reactions of H2 with H$ _{2}^{+} $ originating from cosmic ray ionization of H2. However, recent theoretical studies suggest that H$ _{3}^{+} $ 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 H$ _{3}^{+} $ (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+, CH$ _{3}^{+} $, and OH. Our detection of infrared emission from vibrationally excited H$ _{3}^{+} $ 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.

Authors 3

1. L. H. Coudert

   Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut des Sciences Moléculaires d'Orsay

   Affiliation as printed

   Institut des Sciences Moléculaires d'Orsay , CNRS , Université Paris- Saclay , 91405 Orsay , France

2. Antoine Gusdorf

   Centre National de la Recherche Scientifique · Université Paris Sciences et Lettres · École Normale Supérieure - PSL · Sorbonne Université · Observatoire de Paris · Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères · Université Paris Cité · Laboratoire de Physique de l'Ecole Normale Supérieure

   Affiliation as printed

   Laboratoire de Physique de l'École normale supérieure , ENS , Uni- versité PSL , CNRS , Sorbonne Université , Université Paris Cité , 75005 Paris , France

   Observatoire de Paris , Université PSL , Sorbonne Université , LERMA , CNRS UMR 8112 , 75014 Paris , France

3. E. Roueff

   Université Paris Sciences et Lettres · Observatoire de Paris · Laboratoire d'Étude de l'Univers et des Phénomènes Extrêmes

   Affiliation as printed

   LUX , UMR 8262 , Observatoire de Paris , PSL Research University ,