Protoplanetary Disk Sizes and Angular Momentum Transport

TL;DR

This paper investigates how measurements of protoplanetary disk sizes can shed light on the mechanisms of angular momentum transport, comparing viscous spreading signatures with alternative processes like winds or truncation.

Contribution

It provides observational evidence supporting viscous spreading in Class II disks and discusses the diversity of disk sizes in relation to different angular momentum transport mechanisms.

Findings

Large sizes of Class II disks are consistent with viscous spreading.

Significant size variation suggests multiple processes influence disk evolution.

Small disks may result from photoevaporation, winds, or truncation.

Abstract

In young circumstellar disks, accretion--the inspiral of disk material onto the central star--is important for both the buildup of stellar masses and the outcome of planet formation. Although the existence of accretion is well documented, understanding the angular momentum transport mechanism that enables disk accretion has proven to be an enduring challenge. The leading theory to date, the magnetorotational instability, which redistributes angular momentum within the disk, is increasingly questioned, and magnetothermal disk winds, which remove angular momentum from the disk, have emerged as an alternative theoretical solution. Here we investigate whether measurements of disk radii can provide useful insights into which, if either, of these mechanisms drive disk accretion, by searching for evidence of viscous spreading in gaseous disks, a potential signature of "in disk" angular momentum transport. We find that the large sizes of most Class II (T Tauri) gas disks compared to those of their earlier evolutionary counterparts, Class I gas disks, are consistent with expectations for viscous spreading in the Class II phase. There is, however, a large spread in the sizes of Class II gas disks at any age, including a population of very small Class II gas disks. Their small sizes may result from processes such as photoevaporation, disk winds, or truncation by orbiting low mass companions.