THE CHEMISTRY OF INTERSTELLAR OH+, H2O+, AND H3O+: INFERRING THE COSMIC-RAY IONIZATION RATES FROM OBSERVATIONS OF MOLECULAR IONS

We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using a steady-state photodissociation region code that treats the freezeout of gas species, grain surface chemistry, and desorption of ices from grains. The code includes polycyclic aromatic hydrocarbons (PAHs), which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A(V) less than or similar to 1 and one at A(V) similar to 3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far-ultraviolet flux on the cloud (chi less than or similar to 1000; chi = 1 = local interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic-ray primary ionization rate zeta(crp) divided by the hydrogen nucleus density n. The H3O+ column is dominated by the second peak, and we show that if PAHs are present, N(H3O+) similar to 4 x 10(13) cm(-2) independent of zeta(crp) or n. If there are no PAHs or very small grains at the second peak, N(H3O+) can attain such columns only if low-ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used to estimate zeta(crp)/n, chi/n and A(V) in them. We compare our models to Herschel observations of these two ions, and estimate zeta(crp) similar to 4-6 x 10(-16)(n/100 cm(-3)) s(-1) and chi/n = 0.03 cm(3) for diffuse foreground clouds toward W49N.