Expansion of a Photoionized Stellar Wind

Abstract

In the late stages of evolution, a star may make a transition from an extended, cool state where it has a slow, dense wind to a compact, hot state where it has a fast wind. The compact star can be a source of photoionizing radiation and may completely photoionize the dense wind. If the photoionized gas is isothermal, with sound speed a, and the fast wind power is steady, the subsequent expansion of the slow wind gas becomes self-similar. Starting from the center, the structure is characterized by a hot bubble of shocked fast wind, a region of shocked, photoionized slow wind, and a region of slow wind that is accelerated by the photoionization heating. In the limit of pure photoionization (previously studied by Meyer), a shock wave is generated with a minimum velocity of 2.56a. Even a weak fast wind tends to fill the central region of the flow. For conditions typical of planetary nebulae and a weak fast wind, the preshock flow is accelerated by roughly a above the initial slow wind velocity and the shock compression is roughly a factor of 2. As the inner wind power rises, the shock compression rises and the shocked slow wind region becomes thinner. Although the evolution in the actual stellar properties is likely to cause deviations from these solutions, they do give insight into the combined effects of stellar winds and photoionization.