Spectra of the C¹⁸O (2–1) emission extracted from the combined data. The relative offsets of the positions A–D, where the spectra were extracted, with respect to the position of HL Tau are shown at the upper right corners in the panels. These positions are also labeled in Fig. 5. Dotted vertical lines denote the systemic velocity of HL Tau, V_LSR = 7 km s⁻¹, measured from the Keplerian rotation of its disk (Yen et al. 2017). Blue vertical lines show the expected line-of-sight velocities from the model of a free-falling and rotating envelope at those positions (Yen et al. 2017).