Five old open clusters more in the outer Galactic disk

TL;DR

This study presents new photometric data for six outer Galactic star clusters, revealing that four are older than 1 Gyr and identifying Berkeley 76 as one of the most distant old open clusters, enhancing understanding of the Galaxy's outer disk.

Contribution

First detailed photometric analysis of six outer disk clusters, identifying four as older than 1 Gyr and discovering Berkeley 76's extreme galactocentric distance.

Findings

Four clusters are older than 1 Gyr.

Berkeley 76 is over 17 kpc from the Galactic center.

Haffner 15 is a young, reddened cluster in the Perseus arm.

Abstract

New photometric material is presented for 6 outer disk supposedly old, Galact ic star clusters: Berkeley 76, Haffner 4, Ruprecht 10, Haffner 7, Haffner 11, and Haffner 15, that are projected against the rich and complex Canis Major overde nsity at $225^o \leq l \leq 248^o $, $-7^o \leq b \leq -2^o$. This CCD data-set, in the UBVI pass-bands, is used to derive their fundamental parameters, in particular age and distance. Four of the program clusters turn out to be older than 1 Gyr. This fact makes them ideal targets for future spectroscopic campaigns aiming at deriving their metal abundances. This, in turn, contributes to increase the number of well-studied outer disk o ld open clusters. Only Haffner 15, previously considered an old cluster, is found to be a young, significantly reddened cluster, member of the Perseus arm in the third Galactic quadrant. As for Haffner~4, we suggest an age of about half a Gyr. The most interesting result we found is that Berkeley~76 is probably located at more than 17 kpc from the Galactic center, and therefore is among the most peripherical old open clusters so far detected. Besides, for Ruprecht~10 and Haffner~7, which were never studied before, we pr opose ages larger than 1 Gyr. All the old clusters of this sample are scarcely populated and show evidence o f tidal interaction with the Milky Way, and are therefore most probably in advanced st ages of dynamical dissolution.