2018ApJ...859...21A

We present Atacama Large Millimeter/Sub-Millimeter Array (ALMA) Band 6 observations of a complete sample of protoplanetary disks in the young (∼1-3 Myr) Lupus star-forming region, covering the 1.33 mm continuum and the ¹²CO, ¹³CO, and C¹⁸O J = 2-1 lines. The spatial resolution is ∼0.''25 with a medium 3σ continuum sensitivity of 0.30 mJy, corresponding to M_dust ∼ 0.2 M_⊕. We apply Keplerian masking to enhance the signal-to-noise ratios of our ¹²CO zero-moment maps, enabling measurements of gas disk radii for 22 Lupus disks; we find that gas disks are universally larger than millimeter dust disks by a factor of two on average, likely due to a combination of the optically thick gas emission and the growth and inward drift of the dust. Using the gas disk radii, we calculate the dimensionless viscosity parameter, α_visc, finding a broad distribution and no correlations with other disk or stellar parameters, suggesting that viscous processes have not yet established quasi-steady states in Lupus disks. By combining our 1.33 mm continuum fluxes with our previous 890 µm continuum observations, we also calculate the millimeter spectral index, α_mm, for 70 Lupus disks; we find an anticorrelation between α_mm and millimeter flux for low-mass disks (M_dust <= 5), followed by a flattening as disks approach α_mm ≃ 2, which could indicate faster grain growth in higher-mass disks, but may also reflect their larger optically thick components. In sum, this work demonstrates the continuous stream of new insights into disk evolution and planet formation that can be gleaned from unbiased ALMA disk surveys.