ALMA Survey of Lupus Protoplanetary Disks. II. Gas Disk Radii

We present Atacama Large Millimeter/Sub-Millimeter Array (ALMA) Band 6 observations of a complete sample of protoplanetary disks in the young (similar to 1-3Myr) Lupus star-forming region, covering the 1.33mm continuum and the (CO)-C-12, (CO)-C-13, and (CO)-O-18 J = 2-1 lines. The spatial resolution is similar to 0 ''.25 with a medium 3 sigma continuum sensitivity of 0.30 mJy, corresponding to M-dust similar to 0.2M(circle plus) We apply Keplerian masking to enhance the signal-to-noise ratios of our (CO)-C-12 zeromoment 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, avisc, 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 1.33mm continuum fluxes with our previous 890 mu m continuum observations, we also calculate the millimeter spectral index, alpha(mm), for 70 Lupus disks; we find an anticorrelation between amm and millimeter flux for low-mass disks (M-dust less than or similar to 5), followed by a flattening as disks approach alpha(mm) approximate to 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.