Misaligned Circumstellar Disks and Orbital Motion of the Young Binary XZ Tau

TL;DR

This study uses multi-epoch ALMA data to analyze the misaligned circumstellar disks and orbital motion of the young binary system XZ Tau, revealing complex disk orientations and orbital characteristics.

Contribution

First detailed multi-epoch ALMA analysis of XZ Tau revealing misaligned disks and elliptical orbit, demonstrating time-domain capabilities of ALMA.

Findings

Disks around XZ Tau A and B are compact and unresolved.

The disks' rotational axes are significantly misaligned.

The binary's orbit is elliptical with high eccentricity.

Abstract

We report our analyses of the multi-epoch (2015-2017) ALMA archival data of the Class II binary system XZ Tau at Bands 3, 4 and 6. The millimeter dust continuum images show compact, unresolved (r <~ 15 au) circumstellar disks (CSDs) around the individual binary stars; XZ Tau A and B, with a projected separation of ~ 39 au. The 12CO (2-1) emission associated with those CSDs traces the Keplerian rotations, whose rotational axes are misaligned with each other (P.A. ~ -5 deg for XZ Tau A and ~ 130 deg for XZ Tau B). The similar systemic velocities of the two CSDs (VLSR ~ 6.0 km s-1) suggest that the orbital plane of the binary stars is close to the plane of the sky. From the multi-epoch ALMA data, we have also identified the relative orbital motion of the binary. Along with the previous NIR data, we found that the elliptical orbit (e = 0.742+0.025-0.034, a = 0''.172+0''.002-0''.003, and {\omega} = -54.2+2.0-4.7 deg) is preferable to the circular orbit. Our results suggest that the two CSDs and the orbital plane of the XZ Tau system are all misaligned with each other, and possible mechanisms to produce such a configuration are discussed. Our analyses of the multi-epoch ALMA archival data demonstrate the feasibility of time-domain science with ALMA.