Unveiling the structural content of NGC 6357 via kinematics and NIR variability

TL;DR

This study combines infrared kinematic and variability data to analyze the structure and stellar populations of NGC 6357, revealing asymmetrical stellar motions and cataloging 774 young star light curves despite high extinction.

Contribution

It introduces a novel combined infrared kinematic and variability approach to study NGC 6357, uncovering asymmetrical stellar motions and compiling a large catalog of young star light curves.

Findings

Discovered asymmetrical distribution in stellar motions.

Identified young stars with high variability linked to accretion and spots.

Compiled 774 infrared light curves of young stars.

Abstract

NGC 6357, a star-forming complex at $\sim 1.7$ kpc from the Sun, contains giant molecular clouds and three prominent star clusters alongside with HII regions, very massive stars and thousands of young stellar objects in different evolutionary stages. We present a combined infrared kinematic and time domain study of the line of sight towards this region enabled by the VVVX survey. In terms of kinematics, a novel discovery emerges: an asymmetrical distribution in the vector point diagram. Some stars in the sample exhibit spatial proximity to dusty regions, with their proper motions aligned with filament projections, hinting at a younger population linked to triggered star formation. However, this distribution could also stem from an asymmetric stellar expansion event within NGC 6357, warranting further investigation. Comparing this data with Gaia revealed inconsistencies likely due to high extinction levels in the region. Additionally, owing to accretion episodes and surface cool spots, young stars display high variability. Using the $K_s$-band time series data, we overcome the extreme levels of extinction towards the region, and compile a catalogue of $774$ infrared light curves of young stars. Each light curve has been characterized in terms of asymmetry and periodicity, to infer the dominant underlying physical mechanism. These findings are then correlated with evolutionary stages, aiming to uncover potential age disparities among the observed stars. This study contributes to our understanding the intricate dynamics and evolutionary processes within NGC 6357, offering valuable insights into the formation and development of stellar populations within such complex environments.