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We report additional detections of the chloronium molecular ion, H$_2$Cl$^+$,toward four bright submillimeter continuum sources: G29.96, W49N, W51, andW3(OH). With the use of the HIFI instrument on the Herschel Space Observatory,we observed the $2_{12}-1_{01}$ transition of ortho-H$_2^{35}$Cl$^+$ at 781.627GHz in absorption toward all four sources. Much of the detected absorptionarises in diffuse foreground clouds that are unassociated with the backgroundcontinuum sources and in which our best estimates of the $N({\rmH_2Cl^+})/N({\rm H})$ ratio lie in the range $(0.9 - 4.8) \times 10^{-9}$.These chloronium abundances relative to atomic hydrogen can exceed thepredictions of current astrochemical models by up to a factor of 5. TowardW49N, we have also detected the $2_{12}-1_{01}$ transition ofortho-H$_2^{37}$Cl$^+$ at 780.053 GHz and the $1_{11}-0_{00}$ transition ofpara-H$_2^{35}$Cl$^+$ at 485.418 GHz. These observations imply $\rmH_2^{35}Cl^+/H_2^{37}Cl^+$ column density ratios that are consistent with thesolar system $^{35}$Cl/$^{37}$Cl isotopic ratio of 3.1, and chloroniumortho-to-para ratios consistent with 3, the ratio of spin statistical weights.

Herschel observations of interstellar chloronium. II - Detections toward  G29.96-0.02, W49N, W51, and W3(OH), and determinations of the ortho-to-para  and $^{35}$Cl/$^{37}$Cl isotopic ratios