Search for giant extrasolar planets around white dwarfs: direct imaging with NICMOS/HST and NACO/VLT

Abstract

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 orbital separations of about 25 AU. At the distance of the Hyades (45 pc) this corresponds to angular separations of about 0.5 arcsec which can be resolved with NICMOS/HST; the expected contrast in the J and H bands amount to 7.5 $\pm$ 1.5 mag. Evaluation of our NICMOS data set did not reveal any evidence for 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 arcsec. However, we detected a low-mass, probably stellar, companion to a field white dwarf (WD1847-223J, distance $\sim$ 50 pc, age $\sim$ 1 Gyr; separation $\sim$ 0.5 arcsec, $\Delta$ H $\sim$ 2.5 mag), using the NACO adaptive optics system at the VLT.