SIMBAD references

Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, require external accretion of material to explain the presence of weak metal-line absorption in their photospheres. The source of this material is currently unknown, but could come from the interstellar medium, unseen companions, or relic planetesimals from asteroid belt or Kuiper Belt analogs. Accurate mid-infrared photometry of these white dwarfs provides additional information to solve the mystery of this accretion and to look for evidence of planetary systems that have survived post-main-sequence evolution. We present Spitzer Space Telescope IRAC photometry accurate to ∼3% for four DAZs and one DA with circumstellar absorption lines in the UV. We search for excesses due to unseen companions or circumstellar dust disks. We use Hubble Space Telescope NICMOS imaging of these white dwarfs to gauge the level of background contamination to our targets, as well as to rule out common proper motion companions to WD 1620-391. All of our targets show no excesses due to companions >20 M_J, ruling out all but very low mass companions to these white dwarfs at all separations. No excesses due to circumstellar disks are observed, and we place limits on what types of disks may still be present.