Spitzer 24 μm images of planetary nebulae

Spitzer MIPS 24μm images were obtained for 36 Galactic planetary nebulae (PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective temperatures of ∼100,000 K or higher. Diffuse 24μm emission is detected in 28 of these PNe. The eight nondetections are angularly large PNe with very low Hα surface brightnesses. We find three types of correspondence between the 24μm emission and Hα line emission of these PNe: six show 24μm emission more extended than Hα emission, nine have a similar extent at 24μm and Hα, and 13 show diffuse 24μm emission near the center of the Hα shell. The sizes and surface brightnesses of these three groups of PNe and the nondetections suggest an evolutionary sequence, with the youngest ones being brightest and the most evolved ones undetected. The 24μm band emission from these PNe is attributed to [O IV] 25.9μm and [NeV] 24.3μm line emission and dust continuum emission, but the relative contributions of these three components depend on the temperature of the central star and the distribution of gas and dust in the nebula.