# The Swift/UVOT Stars Survey. III. Photometry and Color-Magnitude Diagrams of 103 Galactic Open Clusters

## TL;DR

This study provides near-ultraviolet photometry and detailed color-magnitude diagrams for 103 Galactic open clusters, comparing observations with models, revising cluster parameters, and offering a valuable catalog for future research.

## Contribution

It presents the first extensive near-ultraviolet photometry for a large sample of open clusters, with analysis of their properties and implications for stellar evolution models.

## Key findings

- 49 clusters have clear, precise CMDs suitable for analysis.
- Good agreement between observed CMDs and theoretical isochrones, with some discrepancies.
- Identification of white dwarf and blue straggler sequences, and evidence of extended main sequence turnoffs.

## Abstract

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.