2022MNRAS.509.5809S

Kinematics of the molecular clouds in the star-forming complex M17 is studied using the high-resolution CO-line mapping data at resolution (20 arcsec ∼ 0.2 pc) with the Nobeyama 45-m telescope. The northern molecular cloud of M17, which we call the molecular 'lobe', is shown to have an elongated shell structure around a top-covered cylindrical cavity. The lobe is expanding at ∼5 km s^–1 in the minor axis direction, and at ∼3/cos i km s^–1 in the major axis direction, where i is the inclination of the major axis. The kinetic energy of the expanding motion is on the order of ∼3 x 10⁴⁹ erg. We show that the lobe is a backyard structure having the common origin to the denser molecular 'horn' flowing out from NGC 6618 toward the south, so that the lobe and horn compose a bipolar outflow. Intensity distributions across the lobe and horn show a double-peak profile typical for a cylinder around a cavity. Position-velocity diagrams (PVD) across the lobe and horn exhibit open-ring structure with the higher and/or lower-velocity side(s) being lacking or faded. This particular PVD behaviour can be attributed to outflow in a conical cylinder with the flow velocity increasing toward the lobe and horn axes.