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

We present F435W (B), F606W (broad V), and F814W (broad I) coronagraphic images of the debris disk around β Pictoris obtained with the Hubble Space Telescope's Advanced Camera for Surveys. These images provide the most photometrically accurate and morphologically detailed views of the disk between 30 and 300 AU from the star ever recorded in scattered light. We confirm that the previously reported warp in the inner disk is a distinct secondary disk inclined by ∼5° from the main disk. The projected spine of the secondary disk coincides with the isophotal inflections, or "butterfly asymmetry," previously seen at large distances from the star. We also confirm that the opposing extensions of the main disk have different position angles, but we find that this "wing-tilt asymmetry" is centered on the star rather than offset from it, as previously reported. The main disk's northeast extension is linear from 80 to 250 AU, but the southwest extension is distinctly bowed with an amplitude of ∼1 AU over the same region. Both extensions of the secondary disk appear linear, but not collinear, 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 the spine of the main disk are fitted with four distinct radial power laws between 40 and 250 AU, while those of the secondary disk between 80 and 150 AU are fitted with single power laws. These discrepancies suggest that the two disks have different grain compositions or size distributions. The F606W/F435W and F814W/F435W flux ratios of the composite disk are nonuniform and asymmetric about both projected axes of the disk. The disk's northwest region appears 20%-30% redder than its southeast region, which is inconsistent with the notion that forward scattering from the nearer northwest side of the disk should diminish with increasing wavelength. Within ∼120 AU, the mF435W - mF606W and mF435W - mF814W colors along the spine of the main disk are ∼10% and ∼20% redder, respectively, than those of β Pic. These colors increasingly redden beyond ∼120 AU, becoming 25% and 40% redder, respectively, than the star at 250 AU. These measurements overrule previous determinations that the disk is composed of neutrally scattering grains. The change in color gradient at ∼120 AU nearly coincides with the prominent inflection in the surface brightness profile at ∼115 AU and the expected water-ice sublimation boundary. We compare the observed red colors within ∼120 AU with the simulated colors of nonicy grains having a radial number density ∝r-3 and different compositions, porosities, and minimum grain sizes. The observed colors are consistent with those of compact or moderately porous grains of astronomical silicate and/or graphite with sizes ≳0.15-0.20 μm, but the colors are inconsistent with the blue colors expected from grains with porosities ≳90%. The increasingly red colors beyond the ice sublimation zone may indicate the condensation of icy mantles on the refractory grains, or they may reflect an increasing minimum grain size caused by the cessation of cometary activity.