Open Clusters as Probes of the Galactic Magnetic Field: I. Cluster
  Properties
  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.
