Cyclopropenone (c-C₃H₂O) as a Tracer of the Nonequilibrium Chemistry Mediated by Galactic Cosmic Rays in Interstellar Ices

While gas-phase astrochemical reaction networks nicely replicate the abundance of hydrogen-deficient organics like linear cyanopolyynes, pathways to complex organic molecules (COMs)-organic molecules with six or more atoms-have not been completely understood, with gas-phase models often significantly underestimating fractional abundances of the astronomically observed organics by orders of magnitude. Here, by exploiting cyclopropenone (c-C3H2O) as a tracer, laboratory experiments on the processing of an ice mixture of acetylene(C2H2) and carbon monoxide (CO) by energetic electrons coupled with astrochemical model simulations expose a previously poorly explored reaction class leading to COMs via galactic cosmic-ray-mediated nonequilibrium chemistry. These processes occur within interstellar ices at ultralow temperatures, but not through traditional radical-radical pathways on grain surfaces in the warm-up phase of the ices as hypothesized for the last decades, but more likely through barrierless excited state reactions during the irradiation.