The Crystal Method: Asteroseismology of BPM 37093 (original) (raw)
2004
More than 40 years have passed since Ed Salpeter and others predicted that the carbon/oxygen cores of the coolest white dwarf stars in our Galaxy will theoretically crystallize. This effect has a dramatic impact on the calculated ages of cool white dwarfs, but until recently we have had no way of testing the theory. In 1992, pulsations were discovered in the massive potentially crystallized white dwarf BPM 37093, and in 1999 the theoretical effects of crystallization on the pulsation modes were determined. Observations from two Whole Earth Telescope campaigns in 1998 and 1999, combined with a new model-fitting method using a genetic algorithm, are now giving us the first glimpse inside of a crystallized star.
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The effects of crystallization on the pulsations of white dwarf stars
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The theory of white dwarf crystallization lias been around for over forty years, yet there exists few direct observational tests to verify it. A littie over a decade has past since a possible testing-ground for the theory lias been discovered, the star BPM 37093, yet our ability to proceed with such tests using current methods is stiil under debate. This DAV star lias the cliaracteristic pulsations of tlie otliers of its type, yet it also lias a mass high enougli so that, by theoretical calculations, we expect tlie center to already be in tlie process of crystallizing, leading some to propose that the pulsations could serve as indicators of the crystallized fraction of tlie star. The validitidy of this daim, tlie use of current tecliniques and stellar knowledge. to determine tlie exact fraction of the star whicli is crystallized is tlie basis of this research. Using extensive, and careful modeling techniques, the relative importance of the crystallization as a parameter of pulsation...
1. Star Formation History and Physics from the White Dwarf Stars
We explore the physics of crystallization in the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of white dwarf stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn (1968). We outline how this data can be used to observationally constrain the value of Γ ≡ ECoulomb/Ethermal near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation. Subject headings: white dwarfs — dense matter — equation of state
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