(a) Longitude–velocity diagram of the ¹²CO J = 2–1 data integrated from b = |${1.\!\!^{\circ}425}$| to |${1.\!\!^{\circ}5}$| (same as figure 4d). The white thick dashed line represents the inverted v-shaped structure which connects the peaks of the blue and red clouds. The white thin dashed line indicates the position of the O star. (b) Position–velocity diagram obtained by a synthetic ¹²CO J = 1–0 observation (Haworth et al. 2015a, 2015b) using the collision model provided by Takahira, Tasker, and Habe (2014). The image is smoothed to be the size of the spatial and velocity resolutions in the ¹²CO J = 2–1 observation. The integrated Y-axis is from −1 pc to 1 pc with reference to the collision axis. The velocity axis is normalized to the initial colliding velocity of 10 km s⁻¹. The white dashed line indicates an auxiliary line to represent the inverted v-shaped structure.