Spitzer 24 um Images of Planetary Nebulae

Spitzer MIPS 24 um images were obtained for 36 Galactic planetary nebulae(PNe) whose central stars are hot white dwarfs(WDs) or pre-WDs with effective temperaturesof ~100,000 K or higher. Diffuse 24 um emissionis detected in 28 of these PNe. The eight non-detections are angularly large PNe with very low H-alpha surface brightnesses. We find three types of correspondence between the 24 um emissionand H-alphaline emissionof these PNe: six show 24 um emissionmore extended than H-alpha emission, nine have a similar extent at 24 um and H-alpha, and 13 show diffuse 24 um emissionnear the center of the H-alphashell. The sizes and surface brightnessesof these three groups of PNe and the non-detections suggest an evolutionary sequence, with the youngest ones being brightest and the most evolved ones undetected. The 24 um band emissionfrom these PNe is attributed to [O IV] 25.9 um and [Ne V] 24.3 um line emissionand 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.