Structure and Large-Scale Kinematics of Young Stellar Populations in the NGC 6357 and NGC 6334 Giant Molecular Cloud Complex

TL;DR

This study maps the 3D structure and kinematics of young stars in the G352 giant molecular cloud complex, revealing its filamentary structure, stellar group motions, and its status as a proto-OB association.

Contribution

It provides a comprehensive 3D map of stellar populations in G352, including new distance measurements and kinematic analysis of its subgroups.

Findings

G352 is a ~150 pc filament connecting NGC 6357 and NGC 6334.

The stellar groups show diverse motions, with some deviating from Galactic rotation.

G352 is a proto-OB association with unbound stellar groups.

Abstract

We map the three-dimensional structure and large-scale kinematics of the young stellar populations in the G352 giant molecular cloud (GMC) complex. In radio and infrared images, G352 appears as long filament extending ~$3^{\circ}$ (~150 pc) parallel to the Galactic midplane. It connects the NGC 6357 and NGC 6334 giant H II regions and the GM1-24 compact H II region. We identify 1727 stellar members of G352 via matching large catalogs of Chandra X-ray point sources and Spitzer mid-infrared excess sources to the Gaia DR3 astrometric catalog. Our catalog of 11,470 X-ray point sources ranks among the three largest contiguous X-ray survey datasets ever assembled for a massive star-forming complex. We revise the mean heliocentric distance of G352 to $1670\pm 80$ pc, with the median parallaxes of seven constituent groups exhibiting a trend toward increasing distance with decreasing Galactic longitude. We identify two foreground stellar groups superimposed on NGC 6357 that may belong to the Sag OB4 association. The three massive clusters in NGC 6357 exhibit peculiar velocities that trail Galactic circular motion by ${\sim}8$ km/s, while the stars associated with NGC 6334 are more consistent with a circular orbit. GM1-24 has a distinct proper motion and smaller parallax compared to NGC 6334. The steep pitch angle of the GMC filament into the sky appears inconsistent with a spiral arm. The various stellar groups are not gravitationally bound to each other, making G352 a proto-OB association.