Abstract
We present new results from the Disks@EVLA program for two young stars: CY Tau and DoAr 25. We trace continuum emission arising from their circusmtellar disks 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 whose origin is different from thermal dust emission from 5 cm observations. Directly from interferometric data, we find that observations at 7 mm and 1 cm trace emission from a compact disk while millimeter-wave observations trace an extended disk structure. From a physical disk model, where we characterize the disk structure of CY Tau and DoAr 25 at wavelengths shorter than 5 cm, we find that (1) dust continuum emission is optically thin at the observed wavelengths and over the spatial scales studied, (2) a constant value of the dust opacity is not warranted by our observations, and (3) a high-significance radial gradient of the dust opacity spectral index, {\beta}, is consistent with the observed dust emission in both disks, with low-{\beta} in the inner disk and high-{\beta} in the outer disk. Assuming that changes in dust properties arise solely due to changes in the maximum particle size (amax), we constrain radial variations 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 observational constraints agree with theoretical predictions of the radial-drift barrier, however, fragmentation of dust grains could explain our amax(R) constraints if these disks have lower turbulence and/or if dust can survive high-velocity collisions.
Highlights
The process of planet formation requires that small dust grains in the interstellar medium (ISM) undergo a dramatic transformation, growing by more than 12 orders of magnitude in order to form terrestrial planets and the cores of gas and ice giants
The resulting image properties and source photometry can be found in Table 3, from these measurements we infer an spectral index for CY Tau from 1.3 to 7.1 mm of α = 2.6
The same approach has been successfully employed to measure radial variations of the dust opacity in the disks surrounding RY Tau, DG Tau, and AS 209 (Isella et al 2010; Pérez et al 2012); we present our results for the circumstellar disks of CY Tau and DoAr 25
Summary
The process of planet formation requires that small dust grains in the interstellar medium (ISM) undergo a dramatic transformation, growing by more than 12 orders of magnitude in order to form terrestrial planets and the cores of gas and ice giants. As a very first step, these μm-sized ISM dust grains must increase their size and become macroscopic (Testi et al 2014), a process that alters the optical properties of the dust grains considerably: as grains reach millimeter or larger sizes, the absolute value of the dust opacity, κλ, decreases, and at the same time the power-law spectral index of the dust opacity, β (where kl μ l-b), becomes smaller (e.g., Miyake & Nakagawa 1993; Henning & Stognienko 1996; Draine 2006). Disk-integrated measurements, from sub-mm to cm wavelengths, have shown that in most protoplanetary disks the value of β tends to be lower than in the ISM (where βISM ∼ 1.5–2.0, consistent with the presence of μm-sized dust grains or smaller in the ISM, Li & Draine 2001) Direct measurements of the dust emission spectrum (i.e., the spectral energy distribution, SED, at long wavelengths) can be used to derive the value of β, a method extensively employed in the literature
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