The Swift/UVOT Stars Survey. III. Photometry and Color-Magnitude
  Diagrams of 103 Galactic Open Clusters
  As part of the Swift/UVOT Stars Survey, we present near-ultraviolet
point-source photometry for 103 Galactic open clusters. These data, taken over
the span of the mission, provide a unique and unprecedented set of
near-ultraviolet point-source photometry on simple stellar populations. After
applying membership analysis fueled mostly by GAIA DR2 proper motions, we find
that 49 of these 103 have clear precise CMDs amenable to investigation. We
compare the CMDs to theoretical isochrones and find good agreement between the
theoretical isochrones and the CMDs. The exceptions are the fainter parts of
the main sequence and the red giant branch in the uvw2-uvw1 CMDs, which is most
likely due either to the difficulty of correcting for the red leak in the uvw2
filter or limitations in our understanding of UV opacities for cool stars. For
the most part, our derived cluster parameters -- age, distance and reddening --
agree with the consensus literature but we find a few clusters that warrant
substantial revision from literature values, notably NGC~2304, NGC~2343,
NGC~2360, NGC~2396, NGC~2428, NGC~2509, NGC~2533, NGC~2571, NGC~2818,
Collinder~220 and NGC~6939. A number of clusters also show white dwarf and blue
straggler sequences. We confirm the presence of extended main sequence turnoffs
in NGC~2360 and NGC~2818 and show hints of it in a number of other clusters
which may warrant future spectroscopic study. Most of the clusters in the study
have low extinction and the rest are well fit by a "Milky-Way-like" extinction
law. However, Collinder~220 hints at a possible "LMC-like" extinction law. We
finally provide a comprehensive point-source catalog to the community as a tool
for future investigation.
