We propose that the two-armed spiral features seen in visible Hubble Space Telescope images of scattered light in HD 100546's circumstellar disk are caused by the illumination of a warped outer disk. A tilt of 6°-15° 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. HD 100546's outer disk, if viewed edge-on, would appear similar to that of Beta Pictoris. A disk initially misaligned with a planetary system becomes warped due to precession induced by planetesimal bodies and planets. However, the twistedness of HD 100546'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 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 and 200 AU.
Stars: Individual: Henry Draper Number: HD 100546