PDRs4All

XII. Far-ultraviolet-driven formation of simple hydrocarbon radicals and their relation with polycyclic aromatic hydrocarbons

¹ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
² Institut de Radioastronomie Millimétrique, 38406 Saint Martin d’Hères, France
³ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 75014 Paris, France
⁴ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES, Toulouse, France
⁵ Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, Orsay, France
⁶ Joint Quantum Institute, Department of Physics, University of Maryland, College Park, MD 20742, USA
⁷ Department of Physics, Temple University, Philadelphia, PA 19122, USA
⁸ Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes), UMR6251, 35000 Rennes, France
⁹ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
¹⁰ Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
¹¹ Institute for Earth and Space Exploration, University of Western Ontario, London, Ontario, Canada
¹² Carl Sagan Center, SETI Institute, Mountain View, CA, USA
¹³ Leiden Observatory, Leiden University, Leiden, The Netherlands
¹⁴ Astronomy Department, University of Maryland, College Park, MD, USA
¹⁵ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
¹⁶ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
¹⁷ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, 33615 Pessac, France
¹⁸ Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
¹⁹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain
²⁰ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, Orsay, France
²¹ Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611, USA

^★ Corresponding author; javier.r.goicoechea@csic.es

Received: 9 December 2024
Accepted: 4 March 2025

Abstract

The infrared emission from polycyclic aromatic hydrocarbons (PAHs), along with emission from atomic carbon and simple hydrocarbons, is a robust tracer of the interaction between stellar far-UV (FUV) radiation and molecular clouds. We present subarcsecond-resolution ALMA mosaics of the Orion Bar photodissociation region (PDR) in [C I] 609 μm (³P₁−³P₀), C₂H (N = 4−3), and C¹⁸O (J = 3−2) emission lines complemented by JWST images of H₂ and aromatic infrared band (AIB) emission. We interpreted the data using up-to-date PDR and radiative transfer models, including high-temperature C₂H (X² Σ⁺)-o/p-H₂ and C (³P)-o/p-H₂ inelastic collision rate coefficients (we computed the latter up to 3000 K). The rim of the Bar shows very corrugated and filamentary structures made of small-scale H₂ dissociation fronts (DFs). The [C I] 609 μm emission peaks very close (≲ 0.002 pc) to the main H₂-emitting DFs, suggesting the presence of gas density gradients. These DFs are also bright and remarkably similar in C₂H emission, which traces “hydrocarbon radical peaks” characterized by very high C₂H abundances, reaching up to several ×10⁻⁷. The high abundance of C₂H and of related hydrocarbon radicals, such as CH₃, CH₂, and CH, can be attributed to gas-phase reactions driven by elevated temperatures, the presence of C⁺ and C, and the reactivity of FUV-pumped H₂. The hydrocarbon radical peaks roughly coincide with maxima of the 3.4/3.3 μm AIB intensity ratio, which is a proxy for the aliphatic-to-aromatic content of PAHs. This implies that the conditions triggering the formation of simple hydrocarbons also favor the formation (and survival) of PAHs with aliphatic side groups, potentially via the contribution of bottom-up processes in which abundant hydrocarbon radicals react in situ with PAHs. Ahead of the DFs, in the atomic PDR zone (where [H] ≫ [H₂]), the AIB emission is the brightest, but small PAHs and carbonaceous grains undergo photo-processing due to the stronger FUV field. Our detection of trace amounts of C₂H in this zone may result from the photoerosion of these species. This study provides a spatially resolved view of the chemical stratification of key carbon carriers in a PDR. Overall, both bottom-up and top-down processes appear to link simple hydrocarbon molecules with PAHs in molecular clouds; however, the exact chemical pathways and their relative contributions remain to be quantified.