WIDE-FIELD PLANETARY CAMERA-2 PHOTOMETRY OF THE BRIGHT, MYSTERIOUS WHITE-DWARF PROCYON-B

Procyon B is one of a select group of white dwarf stars which are memb ers of visual binary systems and represents a critically important obj ect for testing the fundamental physics of stellar degeneracy. We have accurate measurements of the binary period and the white dwarf mass. However, the extreme brightness ratio and the small separation between primary and secondary makes accurate ground-based determinations of b asic characteristics, such as effective temperature and chemical compo sition, which we require to derive stellar radius, impossible to obtai n. Furthermore, since the age of this system is known, a temperature d etermination for Procyon B constrains its cooling age, providing an es timate of the progenitor mass, a stringent test of theories of stellar mass loss. We present the results of photometry from a series of WFPC 2 images, from which we determine an apparent magnitude of m(V) = 10.9 2 +/- 0.05, and an effective temperature of 8688 +/- 200 K, based on H ubble Space Telescope equivalent UBVRI filter measurements. Our more u ncertain, problematic, narrow filter results argue a helium compositio n is more plausible for Procyon B. The stellar radius we derive from t hese parameters, assuming a mass of 0.622 +/- 0.023 M., is 0.0096 +/- 0.0005 R., inconsistent with the carbon-core, zero-temperature mass-ra dius relation of Hamada & Salpeter. This implies that Procyon B has a heavier core than carbon, calling into question the assumption of carb on-core composition commonly used for white dwarf stars.