WD0433+270: an old Hyades stream member or an Fe-core white dwarf?

Abstract

Context. G39-27/289 is a common proper-motion pair formed by a white dwarf (WD0433+270) and a main-sequence star (BD+26 730) that apparently has been classified as a member of the Hyades open cluster. Previous studies of the white dwarf component yielded a cooling time of similar to 4 Gyr. Although it has not been pointed out before explicitly, this result is 6 times greater than the age of the Hyades cluster, giving rise to an apparent conflict between the physics of white dwarfs and cluster main-sequence fitting. Aims. We investigate whether this system belongs to the Hyades cluster and, accordingly, give a plausible explanation for the nature of the white dwarf member. Methods. We have performed and analyzed spectroscopic observations to better characterize these objects, and used their kinematic properties to evaluate their membership in the Hyades. Then, different mass-radius relations and cooling sequences for different core compositions (He, C/O, O/Ne and Fe) were employed to infer the mass and cooling time of the white dwarf. Results. From kinematic and chemical composition considerations, we believe that the system was a former member of the Hyades cluster and therefore has an evolutionary link with it. However, the evidence is not conclusive. With regards to the nature of the white dwarf component, we find that two core compositions -C/O and Fe - are compatible with the observed effective temperature and radius. These compositions yield very different cooling times of similar to 4 Gyr and similar to 1 Gyr, respectively. Conclusions. We distinguish two possible scenarios. If the pair does not belong to the Hyades cluster but only to the Hyades stream, this would indicate that such a stream contains rather old objects and is definitely not coeval with the cluster. This has interesting consequences for Galactic dynamics. However, our favored scenario is that of a white dwarf with a rather exotic Fe core, having a cooling time compatible with the Hyades age. This is a tantalizing result that would have implications for the thermonuclear explosion of white dwarfs and explosion theories of degenerate nuclei.