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

TL;DR

This study investigates whether the white dwarf WD0433+270 is a member of the Hyades cluster and explores its core composition, which affects its cooling time and implications for stellar evolution and Galactic dynamics.

Contribution

It presents a detailed analysis of WD0433+270's membership in the Hyades and proposes that an Fe core could reconcile its cooling age with the cluster's age.

Findings

White dwarf's core could be C/O or Fe, affecting cooling time.

If Fe core, white dwarf's age aligns with Hyades cluster.

System's membership in Hyades stream suggests older age than cluster.

Abstract

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 ~4 Gyr. Although it has not been pointed out before explicitly, this result is 6 times larger than the age of the Hyades cluster, giving rise to an apparent conflict between the physics of white dwarfs and cluster main-sequence fitting. We investigate whether this system belongs to the Hyades cluster and, accordingly, give a plausible explanation to the nature of the white dwarf member. 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 ~4 Gyr and ~1 Gyr, respectively. We distinguish two posssible scenarios. If the pair does not belong to the Hyades cluster but only to the Hyades stream, this would indicate that such stream contains rather old objects and definitely not coeval with the cluster. This has interesting consequences for Galactic dynamics. On the contrary, if the white dwarf has an Fe core, it will have 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.