H+3 and other species in the diffuse cloud towards ζ Persei:: A new detailed model

McCall et al. have recently shown that a large column density for the molecular ion H-3(+) of approximate to8x10(13) 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 (approximate to1.2x10(-15) s(-1)) can explain the abundance of H-3(+), it is not at all clear that such a hi h 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 H-3(+). We consider two phases-a long (4 pc) diffuse region at 60 K and a tiny (approximate to100 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 H-3(+), to within a factor of three or better. To reproduce the CH+ abundance and those of the excited rotational populations of H-2, we consider the addition of shocks. This phase has little effect on our calculated abundance for H-3(+).