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

¹ Universidade Federal do Rio Grande do Sul, Departamento de Astronomia CP 15051 Porto Alegre 91501-970 Brazil
e-mail: eliade.lima@ufrgs.br, bica@if.ufrgs.br; charles@if.ufrgs.br
² Departamento de Física, CCNE, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
³ Departamento de Física, Universidade Federal de Sergipe, Av. Marechal Rondon s/n, 49100-000, São Cristóvão, Brazil

Received: 14 November 2013
Accepted: 10 June 2014

Abstract

Context. NGC 6357 is an active star-forming region located in the Sagittarius arm that displays several star clusters, which makes it a very interesting target to investigate star formation and early cluster evolution.

Aims. We explore NGC 6357 with the VISTA Variables in the Vía Láctea (VVV) photometry of seven embedded clusters (ECs), and one open cluster projected in the outskirts of the complex.

Methods. Photometric and structural properties (age, reddening, distance, core, and total radii) of the star clusters were derived. VVV saturated stars were replaced by their 2MASS counterparts. Field-decontaminated VVV photometry was used to analyse colour–magnitude diagrams (CMDs) and stellar radial density profiles and determine astrophysical parameters.

Results. 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 ± 0.1 kpc based on the cluster CMD morphologies. Of the ECs, one contains the binary star WR 93, while the remaining ones are dominated by pre-main sequence stars, young stellar objects, 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.