  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 PKS1830-211. The molecular species is identified
from clear absorption features corresponding to the 2_12-1_01 (rest frequency
494.034 GHz) and 1_11-0_00 (431.316 GHz) ground-state transitions of ortho and
para forms of C2H3+, respectively, in ALMA spectra toward the southwestern
image of PKS1830-211, 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 x 10^12 cm^-2 and
an abundance of 10^-10 compared to H_2. 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.
