Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction

TL;DR

This study uses Herschel/HIFI observations to detect interstellar OH+ and H2O+ absorption towards W49N, revealing insights into diffuse clouds with low molecular fractions and estimating cosmic ray ionization rates.

Contribution

First detection of interstellar OH+ and H2O+ absorption towards W49N, linking their abundance ratios to small molecular fraction clouds and cosmic ray ionization rates.

Findings

OH+/H2O+ abundance ratio ranges from ~3 to ~15.

OH+ arises in clouds with 2-8% molecular fraction.

Cosmic ray ionization rate estimated at (0.6-2.4) x 10^-16 s^-1.

Abstract

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1 - 0 transition of OH+ and the 1115 GHz 1(11) - 0(00) transition of ortho-H2O+. The resultant spectra show absorption by ortho-H2O+, and strong absorption by OH+, in foreground material at velocities in the range 0 to 70 km/s with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from ~ 3 to ~ 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the 2 - 8% range. This conclusion is confirmed by the distribution of OH+ and H2O+ in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH+/H abundance ratio of a few E-8 suggests a cosmic ray ionization rate for atomic hydrogen of (0.6 - 2.4) E-16 s-1, in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H3+ and other species.