Observations of H3+ in the Diffuse Interstellar Medium

TL;DR

This study reports widespread detection of H3+ in diffuse interstellar clouds, revealing discrepancies with existing chemical models and suggesting the need for revised parameters or processes.

Contribution

It provides observational evidence of H3+ ubiquity in diffuse clouds and challenges current chemical models by indicating they underestimate H3+ density.

Findings

H3+ detected in seven diffuse cloud sightlines

Standard models imply unrealistically long path lengths and low densities

Chemical models likely underestimate H3+ density, requiring revisions

Abstract

Surprisingly large column densities of H3+ have been detected using infrared absorption spectroscopy in seven diffuse cloud sightlines (Cygnus OB2 12, Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard model of diffuse cloud chemistry, our H3+ column densities imply unreasonably long path lengths (~1 kpc) and low densities (~3 cm^-3). Complimentary millimeter-wave, infrared, and visible observations of related species suggest that the chemical model is incorrect and that the number density of H3+ must be increased by one to two orders of magnitude. Possible solutions include a reduced electron fraction, an enhanced rate of H2 ionization, and/or a smaller value of the H3+ dissociative recombination rate constant than implied by laboratory experiments.