  We propose that the two armed spiral features seen in visible Hubble Space
Telescope images of scattered light in HD100546's circumstellar disk are caused
by the illumination of a warped outer disk. A tilt of 6-15 degrees from the
symmetry plane can cause the observed surface brightness variations providing
the disk is very twisted (highly warped) at radii greater than 200 AU where the
spiral features are seen. Dust lanes are due in part to shadowing in the
equatorial plane from the inner disk within a radius of 100 AU. HD100546's
outer disk, if viewed edge-on, would appear similar to that of Beta Pictorus. A
disk initially misaligned with a planetary system, becomes warped due to
precession induced by planetesimal bodies and planets. However, the twistedness
of HD100546's disk cannot be explained by precession during the lifetime of the
system induced by a single Jovian mass planet within the clearing at ~13 AU.
One possible explanation for the corrugated disk is that precession was induced
by massive of bodies embedded in the disk at larger radius. This would require
approximately a Jupiter mass of bodies well outside the central clearing at 13
AU and within the location of the spiral features or at radii approximately
between 50-200 AU.
