VVV analysis of star clusters towards the W31 star-forming complex

TL;DR

This study uses VVV and 2MASS photometry to analyze the physical properties and distances of star clusters in the W31 complex, revealing that SGR1806-20 is more distant and not physically related to W31.

Contribution

The paper provides new distance, age, and structural measurements of clusters W31-CL, BDS 113, and SGR1806-20 using deep infrared photometry and field decontamination techniques.

Findings

W31-CL at 4.5 kpc, 0.5 Myr old

BDS 113 at 4.8 kpc, 1 Myr old

SGR1806-20 at 8.0 kpc, 10±4 Myr old, highly absorbed

Abstract

The giant HII region W31 hosts the populous star cluster W31-CL and others projected on or in the surroundings. The most intriguing object is the stellar cluster SGR1806-20, which appears to be related to a Luminous Blue Variable (LBV) - a luminous supergiant star. We used the deep VVV J-,H-and K$_s$-bands photometry combined with 2MASS data in order to address the distance andother physical and structural properties of the clusters W31-CL, BDS 113 and SGR1806-20. Field-decontaminated photometry was used to analyse colour-magnitude diagrams and stellar radial density profiles, using procedures that our group has developed and employed in previous studies. We concludethat the clusters W31-CL and BDS113 are located at 4.5kpc and 4.8kpc and have ages of 0.5Myr and 1Myr, respectively. This result, together with the pre-main sequence (PMS) distribution in the colour-magnitude diagram, characterises them as members of the W31 complex. The present photometry detects the stellar content, addressed in previous spectroscopic classifications, in the direction of thecluster SGR1806-20, including the LBV, WRs, and foreground stars. We derive an age of 10$\pm$4Myr and a distance of d=8.0$\pm$1.95kpc. The cluster is extremely absorbed, with AV= 25mag. Thepresent results indicate that SGR1806-20 is more distant by a factor 1.8 with respect to the W31 complex, and thus not physically related to it.