AN EMPIRICAL-TEST OF THE THEORY OF CRYSTALLIZATION IN STELLAR INTERIORS

Our theoretical calculations indicate that the massive pulsating DA wh ite dwarf, BPM 37093, has a crystallized ion interior. Our estimates f or the crystallized mass fraction depend on our assumption for the int erior composition and range from 10% for pure carbon and low total mas s to more than 90% for pure oxygen and high total mass; the mass of th is white dwarf suggests on theoretical grounds that the core compositi on is unlikely to be fighter than oxygen. Our best estimate is that th e crystallized mass fraction exceeds 90%. BPM 37093 is the first known example of a pulsating white dwarf star with a potentially crystallin e interior We point out that future observations of the pulsation prop erties of this object are essential for carrying out asteroseismologic al measurements of the crystallized mass fraction of its interior. Unc ertainties in the nature and extent of crystallization and its associa ted effects are the largest source of uncertainty in calculating the a ges of the coolest white dwarf stars-an important chronometer for the Galactic disk. In addition, crystallization plays an important role in our understanding of neutron star crusts. Investigations of the pulsa tion properties of this star promise to provide us with the first empi rical tests of the theory of crystallization in stellar interiors, now more than 35 yr old.