OH-ir stars. II. A model for the 1612 MHz masers.

A model is presented for the 1612 MHz OH masers associated with infrared stars. Its principal conclusions are as follows. The central stars are losing 3 x l0- M0 yr -1, and the masers operate in the outer regions (r > 1016 cm) of the circumstellar envelopes. The maser radiation is narrowly beamed in the radial direction, both inward and outward. Thus the two maser emission features originate in the near and the far sides of the expanding circumstellar gas. The 1612 MHz maser is powered by the absorption of 35 photons which excite the OH molecules from the 211312, J = 312 ground state to the 211112, J = 512 state. The excited OH molecules return to the ground state by a series of radiative decays. In most cases, the radiative cascade proceeds directly down the 211112 ladder. The final and the most important step in the pump cycle is the radiative decay from the 211112, J = 112 state to the 211312, J = 3/2 state. If the transitions which link these two states are optically thick, a strong inversion of the F = 1 F = 2 1612 MHz transition is produced. Subject headings: infrared: sources - masers - stars: circumstellar shells