Production of OH Molecules in Interstellar Shock Waves

Abstract

MEASUREMENTS made recently at Berkeley have shown that there is interstellar OH absorption in most low latitude radio sources¹. Most indications are that the excitation, unlike that in the intensely emitting regions², is fairly normal, and that the state temperature is T < 10° K. If it is assumed that T = 3° K (a minimum value, from the cosmic microwave background), the median value of OH/H is about 4 × 10⁻⁸ in sources in which both OH and H lines can be measured. This ratio, which corresponds to OH/O ∼ 6 × 10⁻⁵, is not far from that of CH and CH⁺, seen optically. An attempt by Bates and Spitzer to explain the abundance of CH by two body radiative association³ failed by more than three orders of magnitude. This led Stecher and Williams⁴ to propose that chemical exchange reactions on grains of the form GX + Y→G + XY (where X = H, Y = O for OH) could operate in the heated regions behind interstellar shock fronts. They calculated that OH/H = 3 × 10⁻⁹ after a shock passes. Photodissociation (τ = 10⁴ yr) between shock passages (T = 10⁷ yr) reduces OH/H to an average value of 3 × 10⁻¹², which is four orders of magnitude too low. According to Carroll and Salpeter⁵, the chemical exchange reaction O + H₂→OH + H produces comparable amounts of OH if H₂/H ∼ 10⁻⁴. According to Stecher and Williams⁶, however, H₂/H is kept to the extremely low value of 10⁻⁷ by photodissociation of H₂.