Measurement of Circumstellar Disk Sizes in the Upper Scorpius OB Association with ALMA

TL;DR

This study uses ALMA observations to measure the sizes of gas and dust in 57 circumstellar disks in Upper Scorpius, revealing they are extremely compact and possibly smaller than disks in younger regions, informing disk evolution.

Contribution

First detailed modeling of gas and dust spatial distributions in Upper Scorpius disks, showing their small sizes and potential evolutionary differences from younger disks.

Findings

Median dust outer radius of 21 au in the sample

Most disks show no CO detection, consistent with small sizes

Upper Sco disks are possibly three times smaller than younger disks

Abstract

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.