a)¹²CO (1–0) intensity-weighted velocity map toward the 27 km s^-1 cloud. The color bar represents velocities in units of km s^-1. The HII region S287, the stellar association Mon OB3, and the open cluster NGC 2311 are marked with a black dashed ellipse and open pentagrams. The solid black ellipse marks the cavity-like feature. b) Same as Fig. 5a but for the intensity-weighted ¹²CO (1–0) line width map of the 27 km s^-1 cloud. The arrows represent the p-v cuts of the interacting interface I and II, indicated by I and II in the panel. Their corresponding position-velocity maps are shown in Fig. 7.