In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar CH ice
arXiv:2609.17867 · doi:10.1051/0004-6361/202556672
Abstract
Acetylene (CH) is commonly observed in star-forming regions, young stellar objects, and the Solar System. Laboratory and theoretical studies have linked this simplest alkyne to volatile hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) through UV- or cosmic-ray-driven chemistry, but it remains unclear whether refractory material can efficiently form through solid-state reactions of CH on dust grains. We experimentally investigate the chemical complexity induced by UV irradiation of pure CH ice and characterize both volatile and nonvolatile photoproducts. Experiments were performed with MATRICES under ultra-high-vacuum conditions at 15 K using 7.2--10.2 eV photons. UV-processed ices were monitored in situ by laser desorption post-ionization reflection time-of-flight mass spectrometry (LDPI ReTOF-MS) combined with pulsed ion deflection (PID). Volatile and refractory products were measured in situ at 15 and 300 K, respectively. After a fluence of photons cm, corresponding to about years in dense clouds, large saturated and unsaturated hydrocarbons containing up to 13 carbon atoms are formed. After sublimation of the volatile products, the 300 K residue shows a rich and distinct mass spectrum consistent with refractory material containing conjugated triple (-CC-) and double (-C=C-) bonds. These results demonstrate that UV processing of pure CH ice can produce substantial molecular complexity and refractory hydrocarbons under astronomically relevant conditions, with possible implications for unidentified infrared emission bands.
25 pages, 9 figures
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