Variations in H_2O^+/H_2O ratios toward massive star-forming regions

Abstract

Early results from the Herschel Space Observatory revealed the water cation H_2O^+ to be an abundant ingredient of the interstellar medium. Here we present new observations of the H_2O and 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 H_2O and H_2O^+. Nine out of ten sources show absorption from H_2O^+ 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 H_2O^+ are found in the range of 10^(12) to a few 10^(13) cm^(−2). The highest N(H_2O^+) column densities are found in the outflows of the sources. The ratios of H_2O^+/H_2O 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 H_2O in emission, H_2O^+ is still seen in absorption.

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