Molecular clouds in the Trifid nebula M20; Possible evidence for a cloud-cloud collision in triggering the formation of the first generation stars

TL;DR

This study investigates molecular clouds in the Trifid nebula M20, providing evidence that a cloud-cloud collision likely triggered the formation of its first-generation stars, including an O7.5 star, within about 1 million years.

Contribution

It presents new observational evidence supporting cloud-cloud collision as a star formation trigger in M20, a scenario previously suggested for other regions.

Findings

Two molecular clouds at different velocities are identified as star-forming parent clouds.

The clouds' temperatures are significantly higher than their surroundings, indicating interaction.

The collision likely triggered star formation within ~1 million years.

Abstract

A large-scale study of the molecular clouds toward the Trifid nebula, M20, has been made in the J=2-1 and J=1-0 transitions of 12CO and 13CO. M20 is ionized predominantly by an O7.5 star HD164492. The study has revealed that there are two molecular components at separate velocities peaked toward the center of M20 and that their temperatures - 30-50 K as derived by an LVG analysis - are significantly higher than the 10 K of their surroundings. We identify that the two clouds as the parent clouds of the first generation stars in M20. The mass of each cloud is estimated to be ~10^3 Msun and their separation velocity is ~8 km/s over ~1-2 pc. We find the total mass of stars and molecular gas in M20 is less than ~3.2 \times 103 Msun, which is too small by an order of magnitude to gravitationally bind the system. We argue that the formation of the first generation stars, including the main ionizing O7.5 s tar, was triggered by the collision between the two clouds in a short time scale of ~1 Myrs, a second example alongside Westerlund 2, where a super star cluster may have been formed due to cloud-cloud collision triggering.