An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H+3-e- recombination rate

The H-3(+) molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules(1,2). In dense interstellar clouds, the H-3(+) abundance is understood using a simple chemical model, from which observations of H-3(+) yield valuable estimates of cloud path length, density and temperature(3,4). But observations of diffuse clouds have suggested that H-3(+) is considerably more abundant than expected from the chemical models(5-7). Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H-3(+) destruction by electrons (e(-)), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H-3(+) destruction rate under nearly interstellar conditions. We also report the observation of H-3(+) in a diffuse cloud ( towards zeta 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(5-7) of H-3(+) can be resolved.