Large-scale CO spiral arms and complex kinematics associated with the T Tauri star RU Lup

TL;DR

Deep ALMA observations of RU Lup reveal complex large-scale CO structures, including spiral arms and non-Keplerian features, indicating dynamic processes like gravitational instability or external perturbations in this young stellar system.

Contribution

This study provides the first detailed high-resolution mapping of large-scale CO structures around RU Lup, uncovering complex morphology and kinematics not seen in typical protoplanetary disks.

Findings

Identification of four major CO components: a compact Keplerian disk, an envelope-like structure, spiral arms, and external clumps.

Detection of at least five spiral arms extending up to 1000 au from the star.

Evidence suggesting gravitational instability or external perturbation as possible causes for the observed structures.

Abstract

While protoplanetary disks often appear to be compact and well-organized in millimeter continuum emission, CO spectral line observations are increasingly revealing complex behavior at large distances from the host star. We present deep ALMA maps of the $J=2-1$ transition of $^{12}$CO, $^{13}$CO, and C$^{18}$O, as well as the $J=3-2$ transition of DCO$^+$, toward the T Tauri star RU Lup at a resolution of $\sim0.3''$ ($\sim50$ au). The CO isotopologue emission traces four major components of the RU Lup system: a compact Keplerian disk with a radius of $\sim120$ au, a non-Keplerian ``envelope-like'' structure surrounding the disk and extending to $\sim260$ au from the star, at least five blueshifted spiral arms stretching up to 1000 au, and clumps outside the spiral arms located up to 1500 au in projection from RU Lup. We comment on potential explanations for RU Lup's peculiar gas morphology, including gravitational instability, accretion of material onto the disk, or perturbation by another star. RU Lup's extended non-Keplerian CO emission, elevated stellar accretion rate, and unusual photometric variability suggest that it could be a scaled-down Class II analog of the outbursting FU Ori systems.