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BPM 37093 is the only hydrogen-atmosphere white dwarf currently known whichhas sufficient mass (~ 1.1 M_sun) to theoretically crystallize while stillinside the ZZ Ceti instability strip (T_eff ~ 12,000 K). As a consequence, thisstar represents our first opportunity to test crystallization theory directly.If the core is substantially crystallized, then the inner boundary for eachpulsation mode will be located at the top of the solid core rather than at thecenter of the star, affecting mainly the average period spacing. This isdistinct from the "mode trapping" caused by the stratified surface layers,which modifies the pulsation periods more selectively. In this paper we reporton Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999.Based on a simple analysis of the average period spacing we conclude that alarge fraction of the total stellar mass is likely to be crystallized.Comment: 7 pages, 3 figures, 3 tables, accepted for Astronomy & Astrophysic

Whole Earth Telescope observations of BPM 37093: A seismological test of crystallization theory in white dwarfs