Measuring the ratio of the gas and dust emission radii of protoplanetary disks in the Lupus star-forming region

TL;DR

This study analyzes the sizes of gas and dust disks in the Lupus star-forming region, revealing that gas disks are generally more extended than dust disks, with implications for dust evolution and disk dynamics.

Contribution

It provides the largest measurement sample to date of CO and dust sizes in Lupus disks, highlighting the prevalence of gas-dust size ratios and their potential link to grain growth and radial drift.

Findings

Gas disks are typically 2.5 times larger than dust disks.

Approximately 15% of disks have a size ratio above 4, indicating advanced dust evolution.

Weak anti-correlation between size ratio and dust disk size.

Abstract

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.