OPEN CLUSTERS AS PROBES OF THE GALACTIC MAGNETIC FIELD. I. CLUSTER PROPERTIES

Abstract

Stars in open clusters are powerful probes of the intervening Galactic magnetic field, via background starlight polarimetry, because they provide constraints on the magnetic field distances. We use 2MASS photometric data for a sample of 31 clusters in the outer Galaxy, for which near-IR polarimetric data were obtained, to determine the cluster distances, ages, and reddenings via fitting theoretical isochrones to cluster color-magnitude diagrams. The fitting approach uses an objective chi^2 minimization technique to derive the cluster properties and their uncertainties. We found the ages, distances, and reddenings for 24 of the clusters, and the distances and reddenings for six additional clusters that were either sparse or faint in the near-IR. The derived ranges of log(age), distance, and E(B-V) were 7.25-9.63, ~670-6160 pc, and 0.02-1.46 mag, respectively. The distance uncertainties ranged from ~8 to 20%. The derived parameters were compared to previous studies, and most cluster parameters agree within our uncertainties. To test the accuracy of the fitting technique, synthetic clusters with 50, 100, or 200 cluster members and a wide range of ages were fit. These tests recovered the input parameters within their uncertainties for more than 90% of the individual synthetic cluster parameters. These results indicate that the fitting technique likely provides reliable estimates of cluster properties. The distances derived will be used in an upcoming study of the Galactic magnetic field in the outer Galaxy.