Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
² Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Vitacura, Santiago, Chile
⁴ IRAM, 300 rue de la Piscine, 38400 Saint Martin d’Hères, France
⁵ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
⁶ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁷ Instituto de Física Fundamental (CSIC), Calle Serrano 121, 28006, Madrid, Spain
⁸ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
⁹ Joint ALMA Observatory, Avenida Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹⁰ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 September 2025
Accepted: 27 January 2026

Abstract

Complex organic molecules (COMs) are considered essential precursors to prebiotic species in the interstellar and circumstellar medium. Despite their astrobiological relevance, many aspects of the formation of COMs remain unclear, particularly the role of ultraviolet (UV) radiation. While COMs were once expected to be efficiently destroyed under UV-irradiated conditions, detections in photodissociation regions (PDRs) have challenged this view. However, the mechanisms by which UV radiation contributes to their formation are still uncertain. Here we present moderately resolved maps of simple and complex organic molecules at the UV-illuminated edge of the Horsehead nebula, obtained by combining Atacama Large Millimeter/submillimeter Array (ALMA) and IRAM 30 m single-dish observations at ~15″ resolution. For the first time in this PDR environment, we analyzed the spatial distribution of species such as C¹⁷O, CH₂CO, CH₃CHO, HNCO, CH₃CN, and HC₃N. By incorporating previous C¹⁷O and C¹⁸O single-dish data as well as Plateau de Bure Interferometer (PdBI) maps of H₂CO and CH₃OH, we derived profiles of gas density, temperature, thermal pressure, and column densities of the organic species as a function of distance from the UV source. Our results show that most organic species – particularly H₂CO, CH₂CO, CH₃CHO, HNCO, and CH₃CN – exhibit enhanced column densities at the UV-illuminated edge compared to cloud interiors, possibly indicating efficient dust-grain surface chemistry driven by the diffusion of atomic C and radicals produced via photodissociation of CO and CH₃OH, as supported by recent laboratory experiments. The exceptions, HC₃N and CH₃OH, can be attributed to inefficient formation on dust grains and ineffective nonthermal desorption into the gas phase, respectively. Additionally, contributions from gas-phase hydrocarbon photochemistry, possibly seeded by grain-surface products, cannot be ruled out. Further chemical modeling is needed to confirm the efficiency of these pathways for the studied species, which could have important implications for other cold UV-irradiated environments such as protoplanetary disks.