Measuring the ratio of the gas and dust emission radii of protoplanetary
  disks in the Lupus star-forming region
  We perform a comprehensive demographic study of the CO extent relative to
dust of the disk population in the Lupus clouds, in order to find indications
of dust evolution and possible correlations with other properties. We increase
up to 42 the number of disks of the region with measured CO and dust sizes
($R_{\mathrm{CO}}$, $R_{\mathrm{dust}}$) from observations with the Atacama
Large Millimeter/submillimeter Array (ALMA). The sizes are obtained from
modeling the ${^{12}}$CO $J = 2-1$ line emission and continuum emission at
$\sim 0.89$ mm with an empirical function (Nuker profile or Gaussian function).
The CO emission is more extended than the dust continuum, with a
$R_{68\%}^{\mathrm{CO}}$/$R_{68\%}^{\mathrm{dust}}$ median value of 2.5, for
the entire population and for a sub-sample with high completeness. 6 disks,
around $15\%$ of the Lupus disk population have a size ratio above 4. Based on
thermo-chemical modeling, this value can only be explained if the disk has
undergone grain growth and radial drift. These disks do not have unusual
properties in terms of stellar mass ($M_{\star}$), disk mass
($M_{\mathrm{disk}}$), CO and dust sizes ($R_{\mathrm{CO}}$,
$R_{\mathrm{dust}}$), and mass accretion. We search for correlations between
the size ratio and $M_{\star}$, $M_{\mathrm{disk}}$, $R_{\mathrm{CO}}$ and
$R_{\mathrm{dust}}$: only a weak monotonic anti-correlation with the
$R_{\mathrm{dust}}$ is found. The lack of strong correlations is remarkable and
suggests that the bulk of the population may be in a similar evolutionary
stage, independent of the stellar and disk properties. These results should be
further investigated, since the optical depth difference between CO and dust
continuum may play a role in the inferred size ratios. Lastly, the CO emission
for the majority of the disks is consistent with optically thick emission and
an average CO temperature of around 30 K.
