– 28 – 5.2. The Origin of the Opacity Holes The four different mechanisms that have been proposed to explain the inner holes of transition disks can be distinguished when disk masses, accretion rates and multiplicity information are available (Najita et al., 2007; Cieza, 2008; Alexander, 2008). In this section, we compare the properties of our transition disk sample to those expected for objects whose opacity holes have been produced by these four mechanisms: photoevaporation, tidal truncation in close binaries, grain-growth, and giant planet formation. As shown in Figure 8, our transition disks span the range λ$_{turn}$$_{−}$$_{off}$ = 2.2 to 8.0 µ m and α$_{excess}$ ∼ -2.5 to 1, while typical CTTSs occupy a much more restricted region of the parameter space. Based on the median and quartile SEDs presented by Furlan et al (2006), 50% of late-type (K5–M2) CTTSs have λ$_{turn}$$_{−}$$_{off}$ = 1.25 µ m, -1.0 > α$_{excess}$ > -0.64, and fractional disk luminosities in the 0.07 to 0.15 range. As discussed by Cieza et al. (2005), CTTS are likely to have J-band excesses. However, since this excess is at the ≲ 30-40% level, the observed J-band fluxes are still dominated by the stellar photospheres, satisfying our λ$_{turn}$$_{−}$$_{off}$ definition. Figure 8 also shows that, as expected, transition disks with different L$_{D}$/L$_{∗}$ values occupy different regions of the α$_{excess}$ vs. λ$_{turn}$$_{−}$$_{off}$ space. Objects with L$_{D}$/L$_{∗}$ > 10 − 2 lay close to the CTTSs loci, while objects with L$_{D}$/L$_{∗}$ < 10 − 3 all have λ$_{turn}$$_{−}$$_{off}$ = 8.0 µ m and α$_{excess}$ ≲ 0.0. The objects in the latter group are all non-accreting. Their very low fractional disk luminosities and lack of accretion suggest they are already in a debris disk stage (see § 5.2.1).