SPITZER 24 μm IMAGES OF PLANETARY NEBULAE

Spitzer MIPS 24 mu m images were obtained for 36 Galactic planetary nebulae (PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective temperatures of similar to 100,000 K or higher. Diffuse 24 mu m emission is detected in 28 of these PNe. The eight nondetections are angularly large PNe with very low H alpha surface brightnesses. We find three types of correspondence between the 24 mu m emission and H alpha line emission of these PNe: six show 24 mu m emission more extended than H alpha emission, nine have a similar extent at 24 mu m and H alpha, and 13 show diffuse 24 mu m emission near the center of the H alpha 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 24 mu m band emission from these PNe is attributed to [O IV] 25.9 mu m and [Ne V] 24.3 mu m 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.