What Excites FLIERS?

Abstract

The nature of the microstructures in planetary nebulae called fast low-ionization emission regions (FLIERs) remains somewhat of a mystery. These appear as ansae, jets, or pairs of symmetric knots on either side of the central star, are very small, and display highly supersonic motions. In this Letter we show that their properties can be understood as being the result of shocks in a strongly photoionized medium. Typically, the energy flux in the radiation field into the shock is 2 orders of magnitude higher than the flux of mechanical energy across the shock. These shocks appear to be associated with ``exit nozzles'' formed in the polar regions of the planetary nebulae, where the high-pressure photoionized gas escapes into the surrounding low-density interstellar medium.