Protonated acetylene in the z = 0.89 molecular absorber toward PKS 1830-211

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: mullers@chalmers.se
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Institut de Radioastronomie Μillimétrique, 300 rue de la piscine, 38406 St-Martin-d’Hères, France
⁴ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
⁵ CNRS and Sorbonne Université, UMR 7095, Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
⁶ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 75014 Paris, France
⁷ Observatoire de Paris, LERMA, College de France, CNRS, PSL Univ., Sorbonne Univ., 75014 Paris, France

Received: 18 December 2023
Accepted: 17 January 2024

Abstract

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.