CORONAGRAPHIC IMAGING OF THE BETA-PICTORIS CIRCUMSTELLAR DISK - EVIDENCE OF CHANGING DISK STRUCTURE WITHIN 100 AU

New R-band images of the beta Pictoris circumstellar disk obtained with the Adaptive Optics Coronagraph expose the disk inward to 40 AU from the star. From these images, the first reliable optical photometry of the disk within 100 AU of beta Pic is reported. Across a radius of 100 AU, the radial power-law dependence of the disk-midplane surface brightness undergoes an abrupt transition, with power-law indices changing within 100 AU from -3.5 to -2.4 in the NE extension and from -4.2 to -1.9 in the SW extension. This result confirms the previously noted asymmetry in the brightness gradient beyond 250 AU, and suggests an inverted asymmetry within 100 AU. The geometrical thickness of the disk appears nearly constant within approximately 115 AU and increases proportionally with radius beyond approximately 115 AU. These changes in brightness gradient and disk thickness are consistent with the two-component disk models of Backman et al. The observed changes in disk structure at approximately 100 AU may mark the boundary of rapid ice sublimation within which only refractory grains exist, but may also reflect a flattened grain distribution associated with planetary formation within 40 AU.