H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate

TL;DR

This study uses high-resolution infrared spectroscopy to detect H3+ in diffuse interstellar clouds, providing new estimates of cosmic-ray ionization rates that are higher than previous measurements, confirming H3+ as a key tracer.

Contribution

The paper presents the first extensive survey of H3+ in diffuse clouds and demonstrates its effectiveness as a direct tracer for cosmic-ray ionization rates.

Findings

H3+ detected in 8 new diffuse cloud sightlines

Cosmic-ray ionization rates range from 0.5x10^-16 to 3x10^-16 s^-1

Average ionization rate is about ten times higher than previous estimates

Abstract

Using high resolution infrared spectroscopy we have surveyed twenty sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14 cm^-2. This brings to fourteen the total number of diffuse cloud sightlines where H3+ has been detected. These detections are mostly along sightlines concentrated in the Galactic plane, but well dispersed in Galactic longitude. The results imply that abundant H3+ is common in the diffuse interstellar medium. Because of the simple chemistry associated with H3+ production and destruction, these column density measurements can be used in concert with various other data to infer the primary cosmic-ray ionization rate, zeta_p. Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16 s^-1. Where H3+ is not detected the upper limits on the ionization rate are consistent with this range. The average value of zeta_p is about an order of magnitude larger than both the canonical rate and rates previously reported by other groups using measurements of OH and HD. The discrepancy is most likely due to inaccurate measurements of rate constants and the omission of effects which were unknown when those studies were performed. We believe that the observed column density of H3+ is the most direct tracer for the cosmic-ray ionization rate due to its simple chemistry. Recent models of diffuse cloud chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to reproduce observed abundances of various atomic and molecular species, in rough accord with our observational findings.