Asteroseismology of the Crystallized ZZ Ceti Star BPM 37093: A Different View

BPM 37093 is a pulsating white dwarf of the ZZ Ceti type massive enough to have undergone partial crystallization. Metcalfe et al. recently claimed to have measured the fraction of crystallized matter in that star on the basis of asteroseismological techniques and determined a value upward of 90%. If true, this is a most significant achievement, well worthy of further scrutiny. In this spirit, we have reexamined the data available—eight periods—with our own independent model-building code and period-matching code in parameter space. In contrast to the above authors, we find that the likely value of the fraction of solidified matter in BPM 37093 is substantially less than 90%, but also that we cannot pin it down with any reasonable accuracy. Our results instead suggest that the value probably lies between 32% and 82%, depending on the unknown chemical composition of the core. We stress that, in principle, asteroseismology can be used to derive the fundamental parameters of BPM 37093, possibly including its core composition, but that, in this specific case, the information contained in the current period data appears insufficient. Indeed, we find full families of different models in parameter space that provide equally valid matches to the available period data. We suggest that the "lack of information" that appears to characterize the set of eight observed periods in BPM 37093 is related to the fact that these periods all correspond to high-order modes reaching into the asymptotic regime (k 1). We also point out that asteroseismology cannot provide a direct test of crystallization theory; the crystallized mass fraction is merely a secondary quantity derived by fixing the interior equation of state of the models and using the inferred fundamental parameters (including the core composition).