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
  We report additional detections of the chloronium molecular ion, H$_2$Cl$^+$,
toward four bright submillimeter continuum sources: G29.96, W49N, W51, and
W3(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 781.627
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 $N({\rm
H_2Cl^+})/N({\rm H})$ ratio lie in the range $(0.9 - 4.8) \times 10^{-9}$.
These chloronium abundances relative to atomic hydrogen can exceed 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 $\rm
H_2^{35}Cl^+/H_2^{37}Cl^+$ 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.
