Context :

Aims : 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 .

Methods : We present our new ALMA cycle 3 observational results of HL~Tau in the ^ { 13 } CO ( 2–1 ) and C ^ { 18 } O ( 2–1 ) emission at resolutions of 0 \aas@@fstack { \prime \prime } 8 ( 110 au ) , and we compare the observed velocity pattern with models of different kinds of gas motions .

Results : Both the ^ { 13 } CO and C ^ { 18 } O emission lines show a central compact component with a size of 2 \arcsec ( 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 \pm 0.3 M _ { \sun } , assuming the inclination angle of the disk to be 47 \degr from the plane of the sky .
The ^ { 13 } CO emission shows two arc structures with sizes of 1000–2000 au and masses of 3 \times 10 ^ { -3 } M _ { \sun } 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 can not 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 \sim 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 .

Conclusions :