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 H2 Oa nd H 2O + 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 H2 Oa nd H 2O + . 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 H2O + 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.