2024A&A...683A..62M

We report the first interstellar identification of protonated acetylene, C₂H₃⁺, a fundamental hydrocarbon, in the z = 0.89 molecular absorber toward the gravitationally lensed quasar PKS 1830-211. The molecular species is identified from clear absorption features corresponding to the 2₁₂-1₀₁ (rest frequency 494.034 GHz) and 1₁₁-0₀₀ (431.316 GHz) ground-state transitions of ortho and para forms of C₂H₃⁺, respectively, in ALMA spectra toward the southwestern image of PKS 1830-211, where numerous molecules, including other hydrocarbons, have already been detected. From the simple assumption of local thermodynamic equilibrium (LTE) with cosmic microwave background photons and an ortho-to-para ratio of three, we estimate a total C₂H₃⁺ column density of 2 × 10¹² cm^–2 and an abundance of 10^–10 compared to H₂. However, formation pumping could affect the population of metastable states, yielding a C₂H₃⁺ column density higher than the LTE value by a factor of a few. We explore possible routes to the formation of C₂H₃⁺, mainly connected to acetylene and methane, and find that the methane route is more likely in PDR environment. As one of the initial hydrocarbon building blocks, C₂H₃⁺ is thought to play an important role in astrochemistry, in particular in the formation of more complex organic molecules.