Knowledge Bank

The $H3+$ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many interstellar molecules. In dense 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. On the other hand, observations of diffuse clouds have suggested that $H3+$ is considerably more abundant than expected from the chemical models. However, diffuse cloud models have been hampered by the uncertain values of three key parameters: the rate of $H3+$ destruction by electrons, the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the $H3+$ dissociative recombination rate under nearly interstellar conditions, using a supersonic expansion discharge source that has been shown (using cavity ringdown spectroscopy) to produce rotationally cold $H3+$ ions. We also report the observation of $H3+$ in a diffuse cloud (towards $\zeta$ Persei) where the electron fraction is already known from ultraviolet spectroscopy. Taken together, these results allow us to derive the value of the third uncertain model parameter: we find that the cosmic-ray ionization rate in this sightline is forty times faster than previously assumed. If such a high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of $H3+$ can be resolved.