The old metal-poor open cluster ESO 92-SC05: accreted from a dwarf galaxy?

Abstract

The study of old open clusters outside the solar circle can bring constraints on formation scenarios of the outer disk. In particular, accretion of dwarf galaxies has been proposed as a likely mechanism in the area. We use BVI photometry for determining fundamental parameters of the faint open cluster ESO 92-SC05. Colour-Magnitude Diagrams are compared with Padova isochrones, in order to derive age, reddening and distance. We derive a reddening E(B-V)= 0.17, and an old age of $\sim$6.0 Gyr. It is one of the rare open clusters known to be older than 5 Gyr. A metallicity of Z$\sim$0.004 or [M/H]$\sim$-0.7 is found. The rather low metallicity suggests that this cluster might be the result of an accretion episode of a dwarf galaxy.