The Effect of Molecular Cloud Properties on the Kinematics of Stars
  Formed in the Trifid Region
  The dynamical states of molecular clouds may affect the properties of the
stars they form. In the vicinity of the Trifid Nebula ($d=1180\pm25$ pc), the
main star cluster (Trifid Main) lies within an expanding section of the
molecular cloud; however, ~0.3 deg 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 re-analyzed 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 ($0.6\pm0.2$ km/s) is three times lower than in Trifid Main ($1.9\pm0.2$
km/s). Furthermore, in Trifid Main, proper motions indicate that the portion of
the star cluster superimposed on the optical nebula is expanding. Expansion of
the HII 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.
