A NICMOS Direct Imaging Search for Giant Planets around the Single White Dwarfs in the Hyades

We report preliminary results from our search for massive giant planets (6-12 Jupiter masses) around the known seven single white dwarfs in the Hyades cluster at sub-arcsec separations. At an age of 625 Myr, the white dwarfs had progenitor masses of about 3 solar masses, and massive gaseous giant planets should have formed in the massive circumstellar disks around these ex-Herbig A0 stars, probably at orbital separations similar or slightly larger than that of Jupiter. Such planets would have survived the post-Main-Sequence mass loss of the parent star and would have migrated outward adiabatically to a distance of about 25 AU. At the distance of the Hyades (45 pc) this corresponds to an angular separation of 0. 5. J and H magnitudes of these giants are in the range of 20.5-23.3 mag, which can be resolved with NICMOS. The achieved sensitivities and contrast ratios agree well with simulations. Preliminary evaluation of the NICMOS data set did not reveal any evidence for neither planetary mass companions with masses down to about 10 Jupiter masses nor brown dwarfs around any of the seven white dwarfs for separations larger than 0. 5. 1. Planets around White Dwarfs Since the discovery of the first extrasolar planet by Mayor & Queloz (1995) the number of known extrasolar planets has grown to more than 100. Most discoveries are based on measurements of radial velocities of stellar lines. Due to the great contrast in optical brightness between the main sequence star and its planet(s) no planet has so far been seen directly. A much better spectral contrast between an extrasolar giant planet and a star can be achieved in the IR band, where the planet’s thermal emission peaks. The contrast can be increased further if one searches for planets around white dwarfs, which are about a factor of ten smaller than a giant planet (F ∼ R) and typically 10 to 10 times less luminous than main sequence stars of spectral type G or K. For our search we have chosen the only young cluster in the near vicinity with a large enough sample of white dwarfs: the Hyades. Compared to isolated white dwarfs this has the advantage that the age of the white dwarfs is known