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