Vol. 608
In section 6. Interstellar and circumstellar matter

1000 au exterior arcs connected to the protoplanetary disk around HL Tauri

The authors present ALMA observational results of HL Tau in the 13 CO (2–1) and C18 O (2–1) emission at resolution 0" . 8 (110 au), and compare the observed velocity pattern with models of different kinds of gas motions. They find that both 13CO and C18O emission lines show a central compact component with a size of 2′′ (280 au) that 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⊙, assuming the inclination angle of the disk to be 47◦ from the plane of the sky. The 13CO emission shows two arc structures with sizes of 1000–2000 au and masses of 3 × 10−3 M⊙ connected to the central disk. One is blueshifted and stretches from the northeast to the northwest, the other is redshifted and stretches from the southwest to the southeast. The gas kinematics of the arc structures can be best explained by three-dimensional infalling or outflowing motions, although the observed velocity in the northwestern part of the blueshifted arc structure is ∼60–70% higher than the expected free-fall velocity. The authors 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.