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

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 (zeta = chi(CR)zeta(standard)) in diffuse clouds compared to the standard value, zeta(standard) (= 2: 5 x 10(-17) s(-1)), measured toward dense molecular clouds. In an earlier work we reported an enhancement chi(CR) = 20 toward HD 185418. McCall et al. have reported chi(CR) = 48 toward zeta Persei based on the observed abundance of H-3(+), while Le Petit et al. found chi(CR) approximate to 10 to be consistent with their models for this same sight line. Here we revisit zeta 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 chi(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 chi(CR) approximate to 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.