  We present ACS/HST coronagraphic observations of HD 100546, a B9.5 star, 103
pc away from the sun, taken in the F435W, F606W, and F814W bands. Scattered
light is detected up to 14'' from the star. The observations are consistent
with the presence of an extended flattened nebula with the same inclination as
the inner disk. The well-known ``spiral arms'' are clearly observed and they
trail the rotating disk material. Weaker arms never before reported are also
seen. The inter-arm space becomes brighter, but the structures become more
neutral in color at longer wavelengths, which is not consistent with models
that assume that they are due to the effects of a warped disk. Along the major
disk axis, the colors of the scattered-light relative to the star are \Delta
(F435W-F606W) ~ 0.0--0.2 mags and \Delta (F435W-F814W)~0.5--1 mags. To explain
these colors, we explore the role of asymmetric scattering, reddening, and
large minimum sizes on ISM-like grains. We conclude each of these hypotheses by
itself cannot explain the colors. The disk colors are similar to those derived
for Kuiper Belt objects, suggesting that the same processes responsible for
their colors may be at work here. We argue that we are observing only the
geometrically thick, optically thin envelope of the disk, while the optically
thick disk responsible for the far-IR emission is undetected. The observed
spiral arms are then structures on this envelope. The colors indicate that the
extended nebulosity is not a remnant of the infalling envelope but reprocessed
disk material.

Astrophysics