The Circumstellar Disk and Asymmetric outflow of the EX Lup Outburst System

TL;DR

This study uses ALMA observations to resolve the circumstellar disk and asymmetric outflow of EX Lup, revealing a stable dust disk and evidence of outflow activity that links EXor and FUor objects in star formation.

Contribution

First high-resolution imaging of EX Lup's disk and outflow, showing persistent outflow activity in an EXor, bridging the gap between FUor and EXor star outburst phenomena.

Findings

Disk radius constrained to 23 au with no asymmetries

Asymmetric $^{12}$CO emission indicating outflow activity

Detection of outflow cavity walls similar to FUor sources

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations at 0.3 arcsec-resolution of EX Lup, the prototype of the EXor class of outbursting pre-main sequence stars. The circumstellar disk of EX Lup is resolved for the first time in 1.3mm continuum emission and in the $J$=2--1 spectral line of three isotopologues of CO. At the spatial resolution and sensitivity achieved, the compact dust continuum disk shows no indications of clumps, fragments, or asymmetries above 5-sigma level. Radiative transfer modeling constrains the characteristic radius of the dust disk to 23 au and a total dust mass of 1.0$\times$10$^{-4}$ M$_\odot$ (33 M_earth), similar to other EXor sources. The $^{13}$CO and C$^{18}$O line emission trace the disk rotation and are used to constrain the disk geometry, kinematics, and a total gas disk mass of 5.1$\times$10$^{-4}$ M$_\odot$. The $^{12}$CO emission extends out to a radius of 200 au and is asymmetric, with one side deviating from Keplerian rotation. We detect blue-shifted, $^{12}$CO arc-like emission located 0.8 arcsec to the north-west, and spatially disconnected from the disk emission. We interpret this extended structure as the brightened walls of a cavity excavated by an outflow, which are more commonly seen in FUor sources. Such outflows have also been seen in the borderline FU/EXor object V1647 Ori, but not towards EXor objects. Our detection provides evidence that the outflow phenomenon persists into the EXor phase, suggesting that FUor and EXor objects are a continuous population in which outflow activity declines with age, with transitional objects such as EX Lup and V1647 Ori.