Spitzer IRAC Observations of White Dwarfs. II. Massive Planetary and   Cold Brown Dwarf Companions to Young and Old Degenerates
  This paper presents a sensitive and comprehensive IRAC 3-8 $\mu$m photometric survey of white dwarfs for companions in the planetary mass regime with temperatures cooler than the known T dwarfs. The search focuses on descendents of intermediate mass stars with $M\ga3$ $M_{\odot}$ whose inner, few hundred AU regions cannot be probed effectively for massive planets and brown dwarfs by any alternative existing method. Furthermore, examination for mid-infrared excess explores an extensive range of orbital semimajor axes, including the intermediate 5-50 AU range poorly covered and incompletely accessible by other techniques at main sequence or evolved stars. Three samples of white dwarfs are chosen which together represent relatively young as well as older populations of stars: 9 open cluster white dwarfs, 22 high mass field white dwarfs, and 17 metal-rich field white dwarfs. In particular, these targets include: 7 Hyads and 4 field white dwarfs of similar age; 1 Pleiad and 19 field white dwarfs of similar age; van Maanen 2 and 16 similarly metal-rich white dwarfs with ages between 1 and 7 Gyr. No substellar companion candidates were identified at any star. By demanding a 15% minimum photometric excess at 4.5 $\mu$m to indicate a companion detection, upper limits in the planetary mass regime are established at 34 of the sample white dwarfs, 20 of which have limits below 10 $M_{\rm J}$ according to substellar cooling models. Specifically, limits below the minimum mass for deuterium burning are established at all Pleiades and Hyadeswhite dwarfs, as well as similarly young field white dwarfs, half a dozen of which receive limits at or below 5 $M_{\rm J}$. Two IRAC epochs at vMa 2 rule out $T\ga200$ K proper motion companions within 1200 AU. 
J. Farihi, E. E. Becklin, B. Zuckerman