  Two types of dust disks around white dwarfs (WDs) have been reported: small
dust disks around cool metal-rich WDs consisting of tidally disrupted
asteroids, and a large dust disk around the hot central WD of the Helix
planetary nebula (PN) possibly produced by collisions among Kuiper Belt-like
objects. To search for more dust disks of the latter type, we have conducted a
Spitzer MIPS 24 um survey of 71 hot WDs or pre-WDs, among which 35 are central
stars of PNe (CSPNs). Nine of these evolved stars are detected and their 24 um
flux densities are at least two orders of magnitude higher than their expected
photospheric emission. Considering the bias against detection of distant
objects, the 24 um detection rate for the sample is >~15%. It is striking that
seven, or ~20%, of the WD and pre-WDs in known PNe exhibit 24 um excesses,
while two, or 5-6%, of the WDs not in PNe show 24 um excesses and they have the
lowest 24 um flux densities. We have obtained follow-up Spitzer IRS spectra for
five objects. Four show clear continuum emission at 24 um, and one is
overwhelmed by a bright neighboring star but still show a hint of continuum
emission. In the cases of WD 0950+139 and CSPN K1-22, a late-type companion is
present, making it difficult to determine whether the excess 24 um emission is
associated with the WD or its red companion. High-resolution images in the
mid-IR are needed to establish unambiguously the stars responsible for the 24
um excesses.
