JWST observations of photodissociation regions

I. Aliphatic and aromatic carbonaceous dust, ices, and gas-phase spectral line inventory

¹ Steward Observatory, University of Arizona, Tucson, AZ 85721-0065, USA
² Ritter Astrophysical Research Center, University of Toledo, Toledo, OH 43606, USA
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁵ Department of Physics & Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
⁶ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, 91405 Orsay, France
⁷ Sorbonne Université, CNRS, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁸ United Kingdom Astronomy Technology Centre, Edinburgh, GB, UK
⁹ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse, France

^★ Corresponding author.

Received: 28 March 2025
Accepted: 20 June 2025

Abstract

Aims. This paper provides an overview of the spectroscopic data obtained by the JWST Guaranteed Time Observations (GTO) program 1192, titled The Physics and Chemistry of PDR Fronts, including an inventory of the spatially resolved dust, gas, and molecular content in the Horsehead Nebula and the NW filament of NGC 7023. We demonstrate the unique capability of this high spatial resolution data set to elucidate the evolution of gas and dust at the interface between stars and their natal clouds at the scale at which the physics and chemistry are at work.

Methods. The disassociation regions (PDRs) in the Horsehead Nebula and the northwest (NW) filament of NGC 7023 were mapped at a spectral resolving power between 1000–3000 and a spatial resolution of 2–7 × 10⁻⁴ pc between 0.97–28 μm. The data were obtained as long, narrow strips with the long axis aligned with the line of sight between the exciting start and perpendicular to the PDR front.

Results. The spectra extracted from the template regions yield a large number of atomic, ionized, and molecular lines. Full line lists and extracted spectra for all ten regions are provided through CDS. Absorption from H₂O, CO₂, and CO ices are identified in three regions in NGC 7023. In this overview, we have focused on the spectral region between three and five μm, which is dominated by emission from aromatic and aliphatic carbon bonds to illustrate the power of the dataset. We confirm the entrainment of aromatic carbonaceous species in the photoevaporative flow from the PDR surface into the H II region in the Horsehead. No aliphatic emission is present in the outflow, indicating the complete removal of aliphatic bonds when exposed to strong UV fields. There is a clear detection of deuterium substitution in the carbon bonds. Aliphatic D-substitution is more efficient relative to aromatic D-substitution, ranging from N_D/N_H ~ 0.1–0.3 for aliphatics, compared to ~0.03 for the aromatics.