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

Abstract

We report additional detections of the chloronium molecular ion, H_2C^l+, toward four bright submillimeter continuum sources: G29.96-0.02, W49N, W51, and W3(OH). With the use of the HIFI instrument on board the Herschel Space Observatory, we observed the 2_(12)-1_(01) transition of ortho-H_2^{35}Cl^+ at 781.627 GHz in absorption toward all four sources. Much of the detected absorption arises in diffuse foreground clouds that are unassociated with the background continuum sources and in which our best estimates of the N(H_2Cl^+)/N(H) ratio lie in the range (0.9–4.8) x 10^(-9). These chloronium abundances relative to atomic hydrogen can exceed the predictions of current astrochemical models by up to a factor of 5. Toward W49N, we have also detected the 2_(12)-1_(01) transition of ortho-H_2^{37}Cl^+ at 780.053 GHz and the 1_(11)-0_(00) transition of para-H_2^{35}Cl^+ at 485.418 GHz. These observations imply (H_2^{35}Cl^+)/(H_2^{37}Cl^+) column density ratios that are consistent with the solar system ^(35)Cl/^(37)Cl isotopic ratio of 3.1, and chloronium ortho-to-para ratios consistent with 3, the ratio of spin statistical weights.