Variations in H2O+/H2O ratios toward massive star-forming regions
  Early results from the Herschel Space Observatory revealed the water cation
H2O+ to be an abundant ingredient of the interstellar medium. Here we present
new observations of the H2O and H2O+ lines at 1113.3 and 1115.2 GHz using the
Herschel Space Observatory toward a sample of high-mass star-forming regions to
observationally study the relation between H2O and H2O+ . Nine out of ten
sources show absorption from H2O+ in a range of environments: the molecular
clumps surrounding the forming and newly formed massive stars, bright
high-velocity outflows associated with the massive protostars, and unrelated
low-density clouds along the line of sight. Column densities per velocity
component of H2 O+ are found in the range of 10^12 to a few 10^13 cm-2 . The
highest N(H2O+) column densities are found in the outflows of the sources. The
ratios of H2O+/H2O are determined in a range from 0.01 to a few and are found
to differ strongly between the observed environments with much lower ratios in
the massive (proto)cluster envelopes (0.01-0.1) than in outflows and diffuse
clouds. Remarkably, even for source components detected in H2O in emission,
H2O+ is still seen in absorption.
