Properties of Star Clusters - I: Automatic Distance and Extinction Estimates

TL;DR

This paper introduces an automatic, purely photometric method to estimate distances and extinction of star clusters using NIR data, calibrated for accuracy and applied to a large sample of clusters.

Contribution

The authors develop and calibrate a novel method for estimating star cluster distances and reddening from NIR photometry without isochrone fitting, suitable for large surveys.

Findings

Achieved distance estimates with better than 40% accuracy.

Applied the method to 378 known and 397 candidate clusters.

Discovered a systematic variation of extinction per kpc with Galactic longitude.

Abstract

Determining star cluster distances is essential to analyse their properties and distribution in the Galaxy. In particular it is desirable to have a reliable, purely photometric distance estimation method for large samples of newly discovered cluster candidates e.g. from 2MASS, UKIDSS-GPS and VISTA-VVV. Here, we establish an automatic method to estimate distances and reddening from NIR photometry alone, without the use of isochrone fitting. We employ a decontamination procedure of JHK photometry to determine the density of stars foreground to clusters and a galactic model to estimate distances. We then calibrate the method using clusters with known properties. This allows us to establish distance estimates with better than 40% accuracy. We apply our method to determine the extinction and distance values to 378 known open clusters and 397 cluster candidates from the list of Froebrich, Scholz and Raftery (2003). We find that the sample is biased towards clusters of a distance of approximately 3kpc, with typical distances between 2 and 6kpc. Using the cluster distances and extinction values, we investigate how the average extinction per kiloparsec distance changes as a function of Galactic longitude. We find a systematic dependence that can be approximated by A_H(l)[mag/kpc]=0.10+0.001*|l-180deg|/deg for regions more than 60deg from the Galactic Centre.