Discovery of H₂CCCH⁺ in TMC-1

Based on a novel laboratory method, 14 millimeter-wave lines of the molecular ion H 2 CCCH + have been measured in high resolution, and the spectroscopic constants of this asymmetric rotor determined with high accuracy. Using the Yebes 40 m and IRAM 30 m radio telescopes, we detected four lines of H 2 CCCH + toward the cold dense core TMC-1. With a dipole moment of about 0.55 D obtained from high-level ab initio calculations, we derive a column density of 5.4±1×10 11 cm −2 and 1.6±0.5×10 11 cm −2 for the ortho and para species, respectively, and an abundance ratio N(H 2 CCC)/N(H 2 CCCH + ) = 2.8±0.7. The chemistry of H 2 CCCH + is modeled using the most recent chemical network for the reactions involving the formation of H 2 CCCH + . We find a reasonable agreement between model predictions and observations, and new insights into the chemistry of C 3 -bearing species in TMC-1 were obtained.