A m

Fig. 4.6. Images of the Fomalhaut disk at different wavelengths. Left, 450 ¡m, beam 7.5", from Holland et al. (2003). Center left, 70¡m, beam 18"; center right 24¡m, beam 6", from Staplefeldt et al. (2004). Right, 0.8¡m, beam 0.07", from Kalas et al. (2005). The figure is approximately 70" high.

Fig. 4.7. Images of the Vega disk at different wavelengths. Left, 450 ¡m image of Fomalhaut to the same physical scale. Center left, 350¡m, beam 11", from Marsh et al. (2006). Center right 70¡m, beam 18"; right, 24¡m, beam 6", from Su et al. (2005). The figure is approximately 3' in height.

taken place on the order of a million years ago, setting up the collisional cascade that is responsible for the small grains.

Mature debris disks are difficult to image in scattered light because their surface brightness is generally too low. Fomalhaut is a dramatic exception (Kalas et al., 2005). The HST image reveals a ring that is about 25 AU wide and has a sharp inner edge at a radius from the star of 133 AU (Fig. 4.6). The ring center is offset by 15 AU from the position of the star, a configuration that probably can only be maintained by interaction with a massive planet. The planet presumably also is responsible for the sharp inner edge, either through gravitational resonances or through ejection of material (Kalas et al., 2005). Comparison of the scattered light image with the thermally radiated one from Spitzer (Staplefeldt et al., 2004) shows two interesting effects. First, the asymmetry in the Spitzer images of the ring at 24 and 70 ¡m is likely due to the greater heating closer to the star due to the 15 AU

ring offset. Second, the grains responsible for the central peak in the 24 ¡m image, the asteroidal/zodiacal component of the system, are invisible in scattered light, presumably because the grain population there is very tenuous.

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