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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 isionized predominantly by an O7.5 star HD164492. The study has revealed thatthere are two molecular components at separate velocities peaked toward thecenter of M20 and that their temperatures - 30-50 K as derived by an LVGanalysis - are significantly higher than the 10 K of their surroundings. Weidentify that the two clouds as the parent clouds of the first generation starsin M20. The mass of each cloud is estimated to be ~10^3 Msun and theirseparation velocity is ~8 km/s over ~1-2 pc. We find the total mass of starsand molecular gas in M20 is less than ~3.2 \times 103 Msun, which is too smallby an order of magnitude to gravitationally bind the system. We argue that theformation of the first generation stars, including the main ionizing O7.5 star, was triggered by the collision between the two clouds in a short timescale of ~1 Myrs, a second example alongside Westerlund 2, where a super starcluster may have been formed due to cloud-cloud collision triggering.

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