On the enhanced cosmic-ray ionization rate in the diffuse cloud toward ζ persei

Abstract

The spatial distribution of the cosmic-ray flux is important in understanding the interstellar medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ = χ_CRζ_standard) in diffuse clouds compared to the standard value, ζ_standard (=2.5 × 10^-17 s^-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χ_CR = 20 toward HD 185418. McCall et al. have reported χ_CR = 48 toward ζ Persei based on the observed abundance of H⁺₃, while Le Petit et al. found χ_CR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χ_CR deduced from the H⁺₃ column density, N(H⁺₃) , in the diffuse region of the sight line depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χ_CR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.