A Spitzer Search for Substellar Companions to Low Mass White Dwarfs
  The formation scenarios for single low-mass (M < 0.45 Msol) white dwarfs
include enhanced mass loss from a metal-rich progenitor star or a common
envelope phase of a solar-like star with a close-in massive planet or a brown
dwarf. Both scenarios suggest that low-mass white dwarfs may have planets.
Here, we present a Spitzer IRAC search for substellar and planetary mass
companions to 14 low-mass white dwarfs. One of our targets, HS 1653+7753,
displays near- and mid-infrared flux excess. However, follow-up MMT
observations show that this excess is due to a nearby resolved source, which is
mostly likely a background object. Another target, PG 2257+162, shows flux
excess compatible with a late-type stellar companion. We do not detect
substellar companions to any of the remaining targets. In addition, eight of
these stars do not show any radial velocity variations, ruling out stellar mass
companions including other white dwarfs. We conclude that a significant
fraction of the low-mass white dwarfs in our sample do not have stellar or
massive brown dwarf companions.
