Discovery of H${}_2$CCCH${}^{+}$ in TMC-1

Based on a novel laboratory method, 14 mm-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 detect four lines of H${}_2$CCCH${}^{+}$ towards the cold dense core TMC-1. With a dipole moment of about 0.55 Debye obtained from high-level ab initio calculations, we derive a column density of 5.4$\pm$1$\times$10${}^{11}$ cm${}^{-2}$ and 1.6$\pm$0.5$\times$10${}^{11}$ cm${}^{-2}$ for the ortho and para species, respectively, and an abundance ratio N(H${}_2$CCC)/N(H${}_2$CCCH${}^{+}$)= 2.8$\pm$0.7. The chemistry of H${}_2$CCCH${}^{+}$ is modelled 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 are obtained.