H3+ and other species in the diffuse cloud towards zeta Persei: A new detailed model
Résumé
McCall et al. have recently shown that a large column density for the molecular ion H3+ of ≈8 × 1013 cm-2 exists in the classical diffuse cloud towards zeta Persei. They have used this observation to infer that the cosmic ray ionization rate zeta for this source is approximately 40 times larger than previously assumed. But, although the value of zeta they infer (≈1.2 × 10-15 s-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 zeta 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 (4 pc) diffuse region at 60 K and a tiny (≈100 AU) dense region at 20 K, both with an ionization rate zeta 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+.
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