Present assessment of dust clearing trends with radius

A single sentence summary of the above set of results on inner disks, mid-range disks, and outer disks is that the often quoted "10 Myr disk lifetime" is a gross generalization. While there are some clear declining trends at several wavelengths in measured disk strength and disk frequency with time, the simple fact that we can consider the quantity "disk frequency" implies that at any given age, some stars have disks while others do not, and thus a range in disk evolutionary times. The dispersion in disk lifetimes is at least factors of a few, if not an order of magnitude.

There is a some evidence that disk clearing times may be shorter in the near-infrared than in the mid-infrared, though this conclusion is not strong at present. The most conservative statement is that dust disk dissipation appears to occur within 3-8 Myr for the vast majority of stars, with minor evidence for more rapid time scales at smaller radii. The dissipation time for the mean disk may be <2-3 Myr. Sensitive observations with Spitzer of statistically significant samples of young stars spanning an appropriate age range are needed before such conclusions are robust, however. Such are beginning to emerge.

It should also be noted that the methods employed to date for statistical study of disks and disk lifetimes largely detect the presence or absence of a disk and do not tell us much about the detailed disk properties (radial/vertical structure, total mass, composition, etc.). This is another area in which the improved sensitivity and the spectroscopic capa bilities of Spitzer, along with the spatially resolved imaging capabilities of ground-based facilities, will improve our understanding, though only for selected individual objects.

Finally, we reiterate that a complication in developing our empirical understanding of the timescales and physical processes associated with primordial disk dissipation is that soon after dusty disk material begins agglomerating to form planetesimals, the proto-planets likely collide and re-form the dust. When does a particular system go from being primordial (dominated by growth of smaller bodies into larger ones) to debris (dominated by destruction of larger bodies into smaller ones which are then removed from the system via Poynting-Robertson drag and stellar wind effects)? For disks surrounding stars with ages in the 5-15 Myr age range, there is some ambiguity as to whether they are primordial or debris disks. Several prominent examples are TW Hya, which is still accreting (Muzerolle et al. 2000), Beta Pic and AU Mic, both of which are nearby and spatially resolved, and new spatially unresolved detections in the 5-15 Myr age range emerging from Spitzer (e.g., Chen et al. 2005; Low et al. 2005; Silverstone et al. 2006).

As in the above discussion of primordial optically thick disks, spatial resolution is the key element for advances in debris disk studies with, for example, the color of scattered light providing critical information about the radial distribution of grain sizes (e.g., Metchev et al. 2005). Our main diagnostic for observationally distinguishing primordial disks from debris disks is the presence of gas, discussed in this proceedings in more detail by Najita.

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