The Effect of Molecular Cloud Properties on the Kinematics of Stars Formed in the Trifid Region

TL;DR

This study investigates how the properties of molecular clouds influence star kinematics, revealing that different regions within the Trifid cloud exhibit distinct stellar velocity dispersions and expansion behaviors, with no evidence of cloud-cloud collision triggering star formation.

Contribution

It provides new measurements of stellar velocities and motions in the Trifid region, highlighting the impact of local cloud conditions on star formation and kinematics, and challenges previous cloud-collision hypotheses.

Findings

Trifid North has lower stellar velocity dispersion than Trifid Main.

Stars in Trifid Main are expanding due to gas expulsion from the HII region.

No evidence supports cloud-cloud collision as the star formation trigger.

Abstract

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.