<i>Herschel</i>/HIFI discovery of interstellar chloronium (H<sub>2</sub>Cl<sup>+</sup>)
<i>Herschel</i>/HIFI discovery of interstellar chloronium (H<sub>2</sub>Cl<sup>+</sup>)
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$^+$ …