ABSTRACT. Spitzer MIPS 24 μm 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 24 μm 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 24 μm emission and Hα line emission of these PNe: six show 24 μm emission more extended than Hα emission, nine have a similar extent at 24 μm and Hα, and 13 show diffuse 24 μm 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 24 μm band emission from these PNe is attributed to [O  IV] 25.9 μm and [Ne V] 24.3 μm 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.

Key words: circumstellar matter; infrared: stars; planetary nebulae: general; white dwarfs

Print publication: Issue 2 (2009 August)
Received 2009 April 10, accepted for publication 2009 June 18
Published 2009 July 15