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

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.

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