Hubble Space TelescopeACS Multiband Coronagraphic Imaging of the Debris Disk around β Pictoris

(Abridged.) We present F435W (B), F606W (Broad V), and F814W (Broad I)coronagraphic images of the debris disk around Beta Pictoris obtained withHST's Advanced Camera for Surveys. We confirm that the previously reported warpin the inner disk is a distinct secondary disk inclined by ~5 deg from the maindisk. The main disk's northeast extension is linear from 80 to 250 AU, but thesouthwest extension is distinctly bowed with an amplitude of ~1 AU over thesame region. Both extensions of the secondary disk appear linear, but notcollinear, from 80 to 150 AU. Within ~120 AU of the star, the main disk is ~50%thinner than previously reported. The surface-brightness profiles along thespine of the main disk are fitted with four distinct radial power laws between40 and 250 AU, while those of the secondary disk between 80 and 150 AU arefitted with single power laws. These discrepancies suggest that the two diskshave different grain compositions or size distributions. The F606W/F435W andF814W/F435W flux ratios of the composite disk are nonuniform and asymmetricabout both projected axes of the disk. Within ~120 AU, the m_F435W-m_F606W andm_F435W-m_F814W colors along the spine of the main disk are ~10% and ~20%redder, respectively, than those of Beta Pic. These colors increasingly reddenbeyond ~120 AU, becoming 25% and 40% redder, respectively, than the star at 250AU. We compare the observed red colors within ~120 AU with the simulated colorsof non-icy grains having a radial number density ~r^-3 and differentcompositions, porosities, and minimum grain sizes. The observed colors areconsistent with those of compact or moderately porous grains of astronomicalsilicate and/or graphite with sizes >0.15-0.20 um, but the colors areinconsistent with the blue colors expected from grains with porosities >90%.