Discovery of H2CCCH+ 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 spectroscopic constants this asymmetric rotor determined with accuracy. Using Yebes 40 m IRAM 30 radio telescopes, we detect four towards cold dense core TMC-1. With dipole moment about 0.55 Debye obtained from high-level ab initio calculations, derive column density 5.4$\pm$1$\times$10$^{11}$ cm$^{-2}$ 1.6$\pm$0.5$\times$10$^{11}$ for ortho para species, respectively, an abundance ratio N(H$_2$CCC)/N(H$_2$CCCH$^+$)= 2.8$\pm$0.7. The chemistry is modelled using most recent chemical network reactions involving formation H$_2$CCCH$^+$. We find reasonable agreement between model predictions observations, new insights into C$_3$ bearing species TMC-1 are obtained.