Old open clusters in the inner Galaxy : FSR 1744, FSR 89 and FSR 31

Abstract

Context. We examine the dynamical survival of intermediate-age/old open clusters in the inner Galaxy. Aims. We aim to establish the nature and derive fundamental and structural parameters of the recently catalogued objects FSR 1744, FSR 89 and FSR 31 to constrain the Galactic tidal disruption efficiency, improve statistics of the open cluster parameter space, and better define their age-distribution function inside the Solar circle. The current status of the issue dealing with the small number of detected open clusters in the inner Galaxy is discussed. Methods. Properties of the objects are investigated with 2MASS colour–magnitude diagrams and stellar radial density profiles built with field star decontaminated photometry. Diagnostic diagrams of structural parameters are used to separate dynamical from highbackground effects affecting such centrally projected open clusters. Results. FSR 1744, FSR 89 and FSR 31 are Gyr-class open clusters located at Galactocentric distances 4.0–5.6 kpc. Compared to nearby open clusters, they have small core and limiting radii. Conclusions. With respect to the small number of open clusters observed in the inner Galaxy, the emerging scenario in the nearinfrared favours disruption driven by dynamical evolution rather than observational limitations associated with absorption and/or high background levels. Internally, the main processes associated with the dynamical evolution are mass loss by stellar evolution, mass segregation and evaporation. Externally they are tidal stress from the disk and bulge, and interactions with giant molecular clouds. FSR 1744, FSR 89 and FSR 31 have structural parameters consistent with their Galactocentric distances, in the sense that tidally induced effects may have accelerated the dynamical evolution.