2004A&A...417..993L

McCall et al. have recently shown that a large column density for the molecular ion H₃⁺ of ≃8x10¹³cm^–2 exists in the classical diffuse cloud towards ζ Persei. They have used this observation to infer that the cosmic ray ionization rate ζ for this source is approximately 40 times larger than previously assumed. But, although the value of ζ they infer (≃1.2x10^–15s^–1) can explain the abundance of H₃⁺, it is not at all clear that such a high ionization rate is consistent with the many other detailed atomic and molecular observations made along the same line of sight. In particular, the abundances of the species OH and HD were previously used to determine a much lower ionization rate. In this paper, we report a detailed chemical model of the diffuse cloud towards ζ Persei which appears to fit to a reasonable extent both the older atomic and molecular observations and the new detection of H₃⁺. We consider two phases - a long (4pc) diffuse region at 60K and a tiny (≃100AU) dense region at 20K, both with an ionization rate ζ in between the standard value and that advocated by McCall et al. The model reproduces almost all abundances, including that of H₃⁺, to within a factor of three or better. To reproduce the CH⁺ abundance and those of the excited rotational populations of H₂, we consider the addition of shocks. This phase has little effect on our calculated abundance for H₃⁺.