  Recent ALMA surveys of protoplanetary disks have shown that for most disks
the extent of the gas emission is greater than the extent of the thermal
emission of the millimeter-sized dust. Both line optical depth and the combined
effect of radially dependent grain growth and radial drift may contribute to
this observed effect. For a sample of 10 disks from the Lupus survey we
investigate how well dust-based models without radial dust evolution reproduce
the observed 12CO outer radius, and determine whether radial dust evolution is
required to match the observed gas-dust size difference. We used the
thermochemical code DALI to obtain 12CO synthetic emission maps and measure gas
and dust outer radii (Rco, Rmm) using the same methods as applied to the
observations, which were compared to observations on a source-by-source basis.
For 5 disks we find that the observed gas-dust size difference is larger than
the gas-dust size difference due to optical depth, indicating that we need both
dust evolution and optical depth effects to explain the observed gas-dust size
difference. For the other 5 disks the observed gas-dust size difference can be
explained using only line optical depth effects. We also identify 6 disks not
included in our initial sample but part of a survey of the same star-forming
region that show significant 12CO emission beyond 4 x Rmm. These disks, for
which no Rco is available, likely have gas-dust size differences greater than 4
and are difficult to explain without substantial dust evolution. Our results
suggest that radial drift and grain growth are common features among both
bright and fain disks. The effects of radial drift and grain growth can be
observed in disks where the dust and gas radii are significantly different,
while more detailed models and deeper observations are needed to see this
effect in disks with smaller differences.
