The Formation of Condensations in Planetary Nebulae. I

The condensations in planetary nebulae are considered in terms of the "mock gravitational" instability that may arise from shadowing effects on radiation. The nebulae are assumed to contain dust particles which provide an energy sink for La radiation. Apparently, ionized regions are stable against La pressure-induced condensation whereas neutral regions are not, in general, stable against such condensations. However, conditions in the neutral parts of young planetary nebulae are probably unfavorable to La pressure-induced condensation. The condensations are also considered in terms of gravitational stability. Surprisingly, the gas in a very young planetary nebula may be gravitationally unstable. The Jeans , however, is never significantly smaller than the dimensions of the nebula, and it is doubtful that gravitational instability could be responsible for the small-scale structure of planetary nebulae.