OH-IR stars. I. Physical properties of circumstellar envelopes

Abstract

A theoretical model of the circumstellar envelope which surrounds a OH-IR star is developed. The circumstellar gas is ejected by radiation pressure which acts on dust grains that condense in the atmosphere of the central star. The dust grains transfer momentum to the gas by collisions with the gas molecules. These collisions are the dominant source of heat input to the circumstellar gas. The major sources of cooling are the emission of radiation by H_2O molecules and adiabatic expansion. The gas temperature decreases from T ≈ 2 x 10^3 K near the stellar surface at r ≈ 6 x 10^(13) cm, to T ≈ 8 x 10^2 K at r = 10^(15) cm and to T ≈ 10^2 K at r = 10^(16) cm. The OH molecule abundance in the circumstellar envelope is controlled by chemical exchange reactions and by the dissociation of H^2O molecules. The reaction OH + H_2 ↔ H_2O + H + 0.69 eV, which has an activation energy of 0.3 eV, rapidly converts OH molecules into H_2O molecules in the warm (T ≳ 5 x 10^2 K) inner (r ≾ 2 x 10^(15) cm) region of the circumstellar envelope. Beyond r ≈ 2 x 10^(15) cm, T is so low that the exchange reaction is very slow and the mean lifetime of an OH molecule is greater than the expansion time scale for the circumstellar envelope. In the outer region of the circumstellar envelope, OH molecules are produced from the photodissociation of H_2O molecules by the interstellar ultraviolet radiation and from the dissociation of H_2O molecules by collisions with dust grains. These processes are capable of producing OH number densities greater than 1 cm^(-3) at r ≈ 10^(16) cm. The predicted values of the gas temperature, T, and the OH abundance, n_(OH), depend upon the rate of mass loss from the central star, Ф. The results quoted above are based on a calculation with Ф = 3 x 10^(-5) M_☉ yr^(-1). In general, T varies inversely and n_(OH) varies directly with Ф.