2022ApJ...937...46K

The dynamical states of molecular clouds may affect the properties of the stars they form. In the vicinity of the Trifid Nebula (d = 1180 ± 25 pc), the main star cluster (Trifid Main) lies within an expanding section of the molecular cloud; however, ∼0.°3 to the north (Trifid North), the cloud's velocity structure is more tranquil. We acquired a Chandra X-ray observation to identify pre-main-sequence stars in Trifid North, complementing a previous observation of Trifid Main. In Trifid North, we identified 51 candidate pre-main-sequence stars, of which 13 are high-confidence Trifid members based on Gaia EDR3 parallaxes and proper motions. We also reanalyzed the membership of Trifid Main and separated out multiple background stellar associations. Trifid North represents a stellar population ∼10% as rich as Trifid Main that formed in a separate part of the cloud. The 1D stellar velocity dispersion in Trifid North (0.6 ± 0.2 km s^–1) is 3 times lower than that in Trifid Main (1.9 ± 0.2 km s^–1). Furthermore, in Trifid Main, proper motions indicate that the portion of the star cluster superimposed on the optical nebula is expanding. Expansion of the H II region around the O-star HD 164492A, and the resulting gas expulsion, can explain both the motions of the stars and gas in Trifid Main. Contrary to previous studies, we find no evidence that a cloud-cloud collision triggered star formation in the region.