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

Abstract

We report the first interstellar identification of protonated acetylene a fundamental hydrocarbon, in the $z=0.89$ molecular absorber toward the gravitationally lensed quasar The molecular species is identified from clear absorption features corresponding to the $ (rest frequency 494.034 GHz) and $1_ $ (431.316 GHz) ground-state transitions of ortho and para forms of respectively, in ALMA spectra toward the southwestern image of 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 column density of $2 $ cm$^ $ and an abundance of 10$^ $ compared to H$_2$. However, formation pumping could affect the population of metastable states, yielding a column density higher than the LTE value by a factor of a few. We explore possible routes to the formation of 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 is thought to play an important role in astrochemistry, in particular in the formation of more complex organic molecules.