Probing embedded star clusters in the HII complex NGC 6357 with VVV

TL;DR

This study investigates star formation in NGC 6357 by analyzing photometric data of embedded and open clusters, discovering new clusters, and deriving their properties to understand early cluster evolution.

Contribution

It provides new photometric analysis of NGC 6357's clusters, discovering four embedded clusters and one intermediate-age cluster, and offers revised distance estimates and structural properties.

Findings

Discovered four new embedded clusters and one intermediate-age cluster.

Revised the distance to NGC 6357 as approximately 1.78 kpc.

Identified substructure within the Pismis 24 cluster.

Abstract

NGC 6357 is an active star-forming region located in the Sagittarius arm displaying several star clusters, which makes it a very interesting target to investigate star formation and early cluster evolution. We explore NGC 6357 with the "VISTA Variables in the V\'ia a L\'actea" (VVV) photometry of seven embedded clusters (ECs), and one open cluster (OC) projected in the outskirts of the complex.Photometric and structural properties (age, reddening, distance, core and total radii) of the star clusters are derived. VVV saturated stars are replaced by their 2MASS counterparts. Field-decontaminated VVV photometry is used to analyse Colour-Magnitude Diagrams (CMDs), stellar radial density profiles (RDPs) and determine astrophysical parameters. We report the discovery of four ECs and one intermediate-age cluster in the complex area. We derive a revised distance estimate for NGC 6357 of 1.78$\pm$0.1 kpc based on the cluster CMD morphologies. Among the ECs, one contains the binary star the WR 93, while the remaining ones are dominated by pre-main sequence (PMS) stars, young-stellar objects (YSO) and/or and have a developed main sequence. These features reflect a significant age spread among the clusters. Evidence is found that the relatively populous cluster Pismis 24 hosts two subclusters.