SPITZER 24 mu m SURVEY FOR DUST DISKS AROUND HOT WHITE DWARFS

Abstract:
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 mu m 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 mu m flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against the detection of distant objects, the 24 mu m detection rate for the sample is greater than or similar to 15%. It is striking that seven, or similar to 20%, of the WD and pre-WDs in known PNe exhibit 24 mu m excesses, while two, or 5%-6%, of the WDs not in PNe show 24 mu m excesses and they have the lowest 24 mu m flux de! nsities. We have obtained follow-up Spitzer Infrared Spectrograph spectra for five objects. Four show clear continuum emission at 24 mu m, and one is overwhelmed by a bright neighboring star but still shows a hint of continuum emission. In the cases of WD 0950+139 and CSPN K 1-22, a late-type companion is present, making it difficult to determine whether the excess 24 mu m emission is associated with the WD or its red companion. High-resolution images in the mid-infrared are needed to establish unambiguously the stars responsible for the 24 mu m excesses.