Bipolar Molecular Outflow from M17

TL;DR

This paper presents high-resolution CO-line observations revealing a bipolar molecular outflow in M17, characterized by an expanding shell structure and velocity patterns consistent with a conical outflow from the star-forming region.

Contribution

It provides detailed kinematic analysis of the bipolar outflow in M17, highlighting the shell structure and velocity distribution, which advances understanding of molecular outflows in star-forming complexes.

Findings

The northern molecular cloud ('lobe') has an elongated shell structure.

The lobe is expanding at approximately 5 km/s.

The outflow exhibits a conical velocity structure with increasing flow velocity toward the axes.

Abstract

Kinematics of the molecular clouds in the star forming complex M17 is studied using the high-resolution CO-line mapping data at resolution ($20" \sim 0.2$ 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 $\sim 5$ \kms in the minor axis direction, and at $ \sim 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 $\sim 3\times 10^{49}$ ergs. 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 behavior can be attributed to outflow in a conical cylinder with the flow velocity increasing toward the lobe and horn axes.