  Only a small number of exoplanets has been identified in stellar cluster
environments. We initiated a high angular resolution direct imaging search
using the Hubble Space Telescope (HST) and its NICMOS instrument for
self-luminous giant planets in orbit around seven white dwarfs in the 625 Myr
old nearby (45 pc) Hyades cluster. The observations were obtained with NIC1 in
the F110W and F160W filters, and encompass two HST roll angles to facilitate
angular differential imaging. The difference images were searched for companion
candidates, and radially averaged contrast curves were computed. Though we
achieve the lowest mass detection limits yet for angular separations >0.5
arcsec, no planetary mass companion to any of the seven white dwarfs, whose
initial main sequence masses were >2.8 Msun, was found. Comparison with
evolutionary models yields detection limits of 5 to 7 Jupiter masses according
to one model, and between 9 and 12 Mjup according to another model, at physical
separations corresponding to initial semimajor axis of >5 to 8 A.U. (i.e.,
before the mass loss events associated with the red and asymptotic giant branch
phase of the host star). The study provides further evidence that initially
dense cluster environments, which included O- and B-type stars, might not be
highly conducive to the formation of massive circumstellar disks, and their
transformation into giant planets (with m>6 Mjup and a>6 A.U.). This is in
agreement with radial velocity surveys for exoplanets around G- and K-type
giants, which did not find any planets around stars more massive than about 3
Msun.
