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