1000 AU Exterior Arcs Connected to the Protoplanetary Disk around HL Tau

TL;DR

This study uses ALMA observations to analyze the gas kinematics around HL Tau, revealing arc structures connected to the disk that suggest complex infalling or outflowing motions, challenging simple models of envelope dynamics.

Contribution

It provides detailed ALMA observations of HL Tau's envelope and disk, identifying arc structures and proposing new interpretations of their kinematics and origins.

Findings

The disk shows Keplerian rotation with a mass estimate of 1.8 solar masses.

Arc structures of 1000-2000 au are observed with complex velocity patterns.

Simple infall or rotation models cannot fully explain the observed gas motions.

Abstract

The protoplanetary disk around HL Tau is so far the youngest candidate of planet formation, and it is still embedded in a protostellar envelope with a size of thousands of au. In this work, we study the gas kinematics in the envelope and its possible influence on the embedded disk. We present our new ALMA cycle 3 observational results of HL Tau in the 13CO (2-1) and C18O (2-1) emission at resolutions of 0.8" (110 au), and we compare the observed velocity pattern with models of different kinds of gas motions. Both the 13CO and C18O emission lines show a central compact component with a size of 2" (280 au), which traces the protoplanetary disk. The disk is clearly resolved and shows a Keplerian motion, from which the protostellar mass of HL Tau is estimated to be 1.8+/-0.3 M$_\odot$, assuming the inclination angle of the disk to be 47 deg from the plane of the sky. The 13CO emission shows two arc structures with sizes of 1000-2000 au and masses of 3E-3 M$_\odot$ connected to the central disk. One is blueshifted and stretches from the northeast to the northwest, and the other is redshifted and stretches from the southwest to the southeast. We find that simple kinematical models of infalling and (counter-)rotating flattened envelopes cannot fully explain the observed velocity patterns in the arc structures. The gas kinematics of the arc structures can be better explained with three-dimensional infalling or outflowing motions. Nevertheless, the observed velocity in the northwestern part of the blueshifted arc structure is ~60-70% higher than the expected free-fall velocity. We discuss two possible origins of the arc structures: (1) infalling flows externally compressed by an expanding shell driven by XZ Tau and (2) outflowing gas clumps caused by gravitational instabilities in the protoplanetary disk around HL Tau.