Measurement of Circumstellar Disk Sizes in the Upper Scorpius OB
  Association with ALMA
  We present detailed modeling of the spatial distributions of gas and dust in
57 circumstellar disks in the Upper Scorpius OB Association observed with ALMA
at sub-millimeter wavelengths. We fit power-law models to the dust surface
density and CO $J$ = 3-2 surface brightness to measure the radial extent of
dust and gas in these disks. We found that these disks are extremely compact:
the 25 highest signal-to-noise disks have a median dust outer radius of 21 au,
assuming an $R^{-1}$ dust surface density profile. Our lack of CO detections in
the majority of our sample is consistent with these small disk sizes assuming
the dust and CO share the same spatial distribution. Of seven disks in our
sample with well-constrained dust and CO radii, four appear to be more extended
in CO, although this may simply be due to higher optical depth of the CO.
Comparison of the Upper Sco results with recent analyses of disks in Taurus,
Ophiuchus, and Lupus suggests that the dust disks in Upper Sco may be $\sim3$
times smaller in size than their younger counterparts, although we caution that
a more uniform analysis of the data across all regions is needed. We discuss
the implications of these results for disk evolution.
