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

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 a large velocity gradient analysis - are significantly higher than the 10 K of their surroundings. We identify the two clouds as the parent clouds of the first generation stars in M20. The mass of each cloud is estimated to be ∼10³ M _∞ and their separation velocity is ∼8kms^-1 over ∼1-2 pc. We find that the total mass of stars and molecular gas in M20 is less than ∼3.2 × 10³ M _∞, 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 star, was triggered by the collision between the two clouds in a short timescale of ∼1 Myr, a second example alongside Westerlund 2, where a super-star cluster may have been formed due to cloud-cloud collision triggering.