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We present new results from the Disks@EVLA program for two young stars: CYTau and DoAr 25. We trace continuum emission arising from their circusmtellardisks from spatially resolved observations, down to tens of AU scales, at{\lambda} = 0.9, 2.8, 8.0, and 9.8 mm for DoAr25 and at {\lambda} = 1.3, 2.8,and 7.1 mm for CY Tau. Additionally, we constrain the amount of emission whoseorigin is different from thermal dust emission from 5 cm observations. Directlyfrom interferometric data, we find that observations at 7 mm and 1 cm traceemission from a compact disk while millimeter-wave observations trace anextended disk structure. From a physical disk model, where we characterize thedisk structure of CY Tau and DoAr 25 at wavelengths shorter than 5 cm, we findthat (1) dust continuum emission is optically thin at the observed wavelengthsand over the spatial scales studied, (2) a constant value of the dust opacityis not warranted by our observations, and (3) a high-significance radialgradient of the dust opacity spectral index, {\beta}, is consistent with theobserved dust emission in both disks, with low-{\beta} in the inner disk andhigh-{\beta} in the outer disk. Assuming that changes in dust properties arisesolely due to changes in the maximum particle size (amax), we constrain radialvariations of amax in both disks, from cm-sized particles in the inner disk (R< 40 AU) to millimeter sizes in the outer disk (R > 80 AU). These observationalconstraints agree with theoretical predictions of the radial-drift barrier,however, fragmentation of dust grains could explain our amax(R) constraints ifthese disks have lower turbulence and/or if dust can survive high-velocitycollisions.

Grain Growth in the Circumstellar Disks of the Young Stars CY Tau and DoAr 25