Old Star Clusters in the FSR catalogue

TL;DR

This study identifies and characterizes 269 old star clusters from the FSR catalogue using 2MASS data, revealing their distribution, ages, and spatial properties in the Milky Way.

Contribution

It provides a homogeneous analysis of old star clusters in the FSR catalogue, including new classifications and detailed spatial and physical properties.

Findings

80% of old open clusters are beyond 7 kpc from the Galactic center

Old open clusters have a scale height of 375 pc, larger than younger clusters

Mean optical extinction towards these clusters is 0.70 mag/kpc

Abstract

We investigate the old star clusters in the sample of cluster candidates from Froebrich, Scholz & Raftery 2007 -- the FSR list. Based on photometry from the 2-Micron All Sky Survey we generated decontaminated colour-magnitude and colour-colour diagrams to select a sample of 269 old stellar clusters. This sample contains 63 known globular clusters, 174 known open clusters and 32 so far unclassified objects. Isochrone fitting has been used to homogeneously calculate the age, distance and reddening to all clusters. The mean age of the open clusters in our sample is 1Gyr. The positions of these clusters in the Galactic Plane show that 80% of open clusters older than 1Gyr have a Galactocentric distance of more than 7kpc. The scale height for the old open clusters above the Plane is 375pc, more than three times as large as the 115pc which we obtain for the younger open clusters in our sample. We find that the mean optical extinction towards the open clusters in the disk of the Galaxy is 0.70mag/kpc. The FSR sample has a strong selection bias towards objects with an apparent core radius of 30" to 50" and there is an unexplained paucity of old open clusters in the Galactic Longitude range of 120deg < l < 180deg.