Dissociative Recombination of Rotationally Cold OH$^+$ and Its Implications for the Cosmic Ray Ionization Rate in Diffuse Clouds

TL;DR

This study experimentally determined a higher dissociative recombination rate for OH$^+$, leading to a revised, higher estimate of the cosmic ray ionization rate in diffuse clouds, impacting our understanding of cosmic ray propagation and shielding.

Contribution

The paper provides the first experimental measurement of the dissociative recombination rate of OH$^+$ at diffuse cloud temperatures, significantly updating previous theoretical and experimental values.

Findings

The new rate coefficient is about 5 times larger than previous experimental estimates.

The inferred cosmic ray ionization rate is more than twice previous estimates.

Results suggest greater cosmic ray shielding in interstellar clouds.

Abstract

Observations of OH$^+$ are used to infer the interstellar cosmic ray ionization rate in diffuse atomic clouds, thereby constraining the propagation of cosmic rays through and the shielding by interstellar clouds, as well as the low energy cosmic ray spectrum. In regions where the H$_2$ to H number density ratio is low, dissociative recombination (DR) is the dominant destruction process for OH$^+$ and the DR rate coefficient is important for predicting the OH$^+$ abundance and inferring the cosmic ray ionization rate. We have experimentally studied DR of electronically and vibrationally relaxed OH$^+$ in its lowest rotational levels, using an electron--ion merged-beams setup at the Cryogenic Storage Ring. From these measurements, we have derived a kinetic temperature rate coefficient applicable to diffuse cloud chemical models, i.e., for OH$^+$ in its electronic, vibrational, and rotational ground level. At typical diffuse cloud temperatures, our kinetic temperature rate coefficient is a factor of $\sim 5$ times larger than the previous experimentally derived value and a factor of $\sim 33$ times larger than the value calculated by theory. Our combined experimental and modelling results point to a significant increase for the cosmic ray ionization rate inferred from observations of OH$^+$ and H$_2$O$^+$, corresponding to a geometric mean of $(6.6 \pm 1.0) \times 10^{-16}\,\mathrm{s}^{-1}$, which is more than a factor of two larger than the previously inferred values of the cosmic ray ionization rate in diffuse atomic clouds. Combined with observations of diffuse and dense molecular clouds, these findings indicate a greater degree of cosmic ray shielding in interstellar clouds than has been previously inferred.