Position–velocity (PV) diagrams of the C¹⁸O (2–1) emission obtained with the IRAM 30m observations. The PV diagrams were extracted along different position angles (labeled at the upper left corner in each panel) and passing through the position of XZ Tau, which is shown as zero offset and denoted with horizontal dotted lines. Vertical dotted lines present the systemic velocity of HL Tau measured from the Keplerian rotation of the HL Tau disk. The position of HL Tau is at the offset of −20′′, denoted as a horizontal dashed line, in the PV diagram along PA of 90°. Red dashed curves from right to left in each panel present the expected velocity profiles of the expanding shell with expanding velocities of 0.3, 0.6, and 0.9 km s⁻¹, respectively, as labeled in the PV diagram along PA of 10°.