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

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 C 17 O, CH 2 CO, CH 3 CHO, HNCO, CH 3 CN, and HC 3 N. By incorporating previous C 17 O and C 18 O single-dish data as well as Plateau de Bure Interferometer (PdBI) maps of H 2 CO and CH 3 OH, 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 H 2 CO, CH 2 CO, CH 3 CHO, HNCO, and CH 3 CN – 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 CH 3 OH, as supported by recent laboratory experiments. The exceptions, HC 3 N and CH 3 OH, 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.

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