An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H₃⁺-e^- recombination rate

The H₃⁺ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H₃⁺ abundance is understood using a simple chemical model, from which observations of H₃⁺ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H₃⁺ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H₃⁺ destruction by electrons (e^-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H₃⁺ destruction rate under nearly interstellar conditions. We also report the observation of H₃⁺ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H₃⁺ can be resolved.