Herschel/HIFI discovery of interstellar chloronium (H$_2$Cl$^+$)
  We report the first detection of chloronium, H$_2$Cl$^+$, in the interstellar
medium, using the HIFI instrument aboard the \emph{Herschel} Space Observatory.
The $2_{12}-1_{01}$ lines of ortho-H$_2^{35}$Cl$^+$ and ortho-H$_2^{37}$Cl$^+$
are detected in absorption towards NGC~6334I, and the $1_{11}-0_{00}$
transition of para-H$_2^{35}$Cl$^+$ is detected in absorption towards NGC~6334I
and Sgr~B2(S). The H$_2$Cl$^+$ column densities are compared to those of the
chemically-related species HCl. The derived HCl/H$_2$Cl$^+$ column density
ratios, $\sim$1--10, are within the range predicted by models of diffuse and
dense Photon Dominated Regions (PDRs). However, the observed H$_2$Cl$^+$ column
densities, in excess of $10^{13}$~cm$^{-2}$, are significantly higher than the
model predictions. Our observations demonstrate the outstanding spectroscopic
capabilities of HIFI for detecting new interstellar molecules and providing key
constraints for astrochemical models.
