FSR 584 – a new globular cluster in the Galaxy?

Abstract

Aims. We investigate the nature of the recently catalogued star cluster candidate FSR 584, which is projected in the direction of the molecular cloud W 3 and may be the nearest globular cluster to the Sun. Methods. 2MASS colour-magnitude diagrams, the stellar radial density profile, and proper motions are employed to derive fundamental and structural parameters. Results. The colour-magnitude diagram morphology and the radial density profile show that FSR584 is an old star cluster. With proper motions, the properties of FSR 584 are consistent with a metal-poor ([Fe/H] ≈ -2) globular cluster with a well-defined turnoff and evidence of a blue horizontal-branch. FSR 584 might be a Palomar-like halo globular cluster that is moving towards the Galactic plane, currently at ≈20 pc above it. The distance from the Sun is d ⊙ ≈ 1.4 kpc, and it is located at ≈ 1 kpc outside the Solar circle. The radial density profile is characterized by a core radius of R core = 0.3 ± 0.1 pc. However, we cannot exclude the possibility of an old open cluster. Conclusions. Near-infrared photometry coupled to proper motions support the scenario where FSR 584 is a new globular cluster in the Galaxy. The absorption is A v = 9.2 ± 0.6, which makes it a limiting object in the optical and explains why it has so far been overlooked.