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Keywords:

  • stars: abundances;
  • circumstellar matter;
  • stars: evolution;
  • stars: magnetic field;
  • planetary systems;
  • white dwarfs

ABSTRACT

An accumulation of multi-epoch, high-resolution optical spectra reveal that the nearby star G77-50 is a very cool DAZ white dwarf externally polluted by Mg, Fe, Al, Ca and possibly Na, Cr, Mn. The metallic and hydrogen absorption features all exhibit multiple components consistent with Zeeman splitting in a B≈ 120 kG magnetic field. Ultraviolet through infrared photometry combined with trigonometric parallaxes yield Teff= 5310 K, M= 0.60 M and a cooling age of 5.2 Gyr. The space velocity of the white dwarf suggests possible membership in the Galactic thick disc, consistent with an estimated total age of 8.6 Gyr. G77-50 is spectrally similar to G165-7 and LHS 2534; these three cool white dwarfs comprise a small group exhibiting both metals and magnetism.

The photospheric metals indicate accretion of rocky debris similar to that contained in asteroids, but the cooling age implies that a remnant planetary system should be stable. A possibility for G77-50 and similarly old, polluted white dwarfs is a recent stellar encounter that dynamically rejuvenated the system from the outside-in. Metal abundance measurements for these cooler white dwarfs have the potential to distinguish material originating in outer region planetesimals injected via fly-by. If common envelope evolution can generate magnetic fields in white dwarfs, then G77-50 and its classmates may have cannibalized an inner giant planet during prior evolution, with their metals originating in terrestrial bodies formed further out. Although speculative, this scenario can be ruled out if terrestrial planet formation is prohibited in systems where a giant planet has migrated to the inner region nominally engulfed during the post-main sequence.