OBSERVATIONAL PROJECTS - BERKELEY

H₃⁺ towards Zeta Persei and a High Cosmic-Ray Flux

       Our discovery of H₃⁺ in diffuse clouds (more details), where CO is not the dominant reservoir of carbon, has suggested that the models of H₃⁺ chemistry are using an incorrect value for one of three key parameters: the dissociative recombination rate constant, the electron fraction, or the cosmic-ray ionization rate. To make sense of the observed H₃⁺ column densities, one or more of these values must be changed by almost two orders of magnitude! Our recent storage-ring measurements of H₃⁺ dissociative recombination (more details) suggest that this value is not the source of the problem. This leaves the next most likely suspect as the electron fraction: until recently, we had not been able to detect H₃⁺ in a sightline with sufficient UV flux to enable spectroscopy of H₂ and C⁺ (the source of the electrons).

       In 2002, using the CGS4 spectrometer at UKIRT, we managed to detect weak absorption lines of H₃⁺ along the line of sight to zeta Persei, a classical and well-studied diffuse cloud source. This star is bright enough that the H₂ column density had long ago been measured by Copernicus, and the C⁺ column density had also been measured by Hubble Space Telescope. Significantly, the electron fraction in this sightline has exactly the same value as had been used in the models of H₃⁺ chemistry...thus ruling out the electron fraction as the cause of the overabundant H₃⁺.

       The combination of these results appears to leave the cosmic-ray ionization rate as the culprit: we suggest that, at least in the cloud towards zeta Persei, the cosmic-ray ionization rate must be a factor of 40 times higher than the canonical value in dense clouds. The fact that we have observed H₃⁺ in many diffuse cloud sightlines suggests that zeta Persei is unlikely to be an unusual case, and that there may be a significant flux of low-energy cosmic rays that have sufficient energy to permeate diffuse, but not dense, clouds. Searches for H₃⁺ in more UV-accessible sightlines continue using CGS4 at UKIRT and NIRSPEC at Keck.

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