IRAS and ISO were used to show that, even at the relatively high level of ~ 10-4 fractional luminosity, debris disks occur frequently (a general summary of debris

disk studies after the completion of these two missions can be found in Caroff et al. (2004)). Initial efforts were made with these data to trace the time evolution of debris production (e.g., Habing et al. (2001); Spangler et al. (2001)). A major advance with Spitzer is that debris disks can be detected in sufficient numbers and in complete samples, so the evolution of debris generation can be traced accurately. Strong excesses, suggesting active terrestrial planet building (and destruction) are common around young stars, less than 100 Myr in age (Rieke et al., 2005; Chen et al., 2005). A-type stars are attractive targets to track disk evolution beyond this initial stage both because of their high luminosity and because their main sequence lifetimes span the key period of disk evolution (discovered after the fact, of course). They have been studied most extensively at 24¡m, which tracks the roughly terrestrial planet zone. At this wavelength, they show an envelope to the infrared excess that decays roughly as r1 (Rieke et al., 2005; Su et al., 2006; Rhee et al., 2007), with a characteristic time of about 150 Myr: see Fig. 4.5. However, as many as half of the sample have no detectable excess, even at the youngest age range (5 to 20 Myr) (Rieke et al., 2005). The behaviour at 70 ¡m appears to be similar in both regards (Su et al., 2006), except that the decay of the excesses is much slower (but still consistent with tr 1).

The broad range of excesses around young stars can be explained consistently as arising from the broad range of protoplanetary disk mass apparent from submm observations (Wyatt et al., 2007a) (see Fig. 4.5). That is, to first order, the fate of a planetary system as measured by its debris content is probably determined by the mass of its protoplanetary disk. Given this conclusion, the upper envelope of the excesses should be a true indication of the time dependence of their decay (since it traces the most massive systems at each age). Thus, the inverse time dependence is a confirmation of the theoretical prediction for collisional cascade generation of debris. The difference in decay rates at 24 and 70 ¡m shows that planetary systems evolve from the inside outward, that is, collisional activity dies down in the terrestrial planet zone more quickly than in the Kuiper Belt one.

About 17% of the stars more than 1 Gyr in age have significant 70 ¡m excesses (Trilling et al., 2007a). The persistence of debris systems indicates that the tr1 decay must slow substantially beyond 1 Gyr. Kim et al. (2005) and Bryden et al. (2006) make two different types of comparison of external debris systems with the Solar System, and each concludes that the Kuiper Belt is likely to have a far infrared output within the distribution of that from similar stars. COBE data have been used to place an upper limit of 10r6 for the fractional luminosity of the Kuiper Belt in the far infrared (Backman et al., 1995), so our system would fall slightly below the current detection limits for exo-solar systems at ~ 10r5 fractional luminosity. That is, the detected systems represent the bright end of a distribution that includes the behaviour of the Solar System.


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