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

Abstract

As part of the Swift/UVOT Stars Survey, we present near-ultraviolet (3000-1700 A) 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 these are clusters in the third Galactic quadrant where previous studies have mistaken the disk sequence for the cluster. However, the GAIA DR2 proper motions clearly favor a different sequence. 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. arXiv:1905.04359v1 [astro-ph.SR] 10 May 2019Siegel et al.

A multiwavelength view of the open cluster NGC 2527: discovery of active stars

“…In Figure 15 the stars have been differentially coloured based on their Teff obtained by comparing the optical magnitudes with the best fitting PARSEC isochrone. We notice that the deviation of these cool stars from isochrones begins around 6500 K, which is similar to that identified by Siegel, et al (2019). These deviations from isochrones have also been noted in NUV photometry of cool stars by Barker & Paust (2018).…”

“…Several cool stars deviate from the single star isochrones in the UVW1-UVW2 CMD as shown in the Figure 15. Siegel, et al (2019) had also identified this strange feature in the UVW2-UVW1 CMDs of a few star clusters. They had suggested that this feature could be due to red leak in UVW1 and UVW2 filters or deficiencies in the underlying atmosphere models.…”

“…We obtained the Swift UVOT photometric data of NGC 2527 from Siegel, et al (2019). These data had been processed using the reduction pipeline outlined in Siegel, et al (2014).…”

“…UV emission from stars in open clusters, on the other hand, is dominated by luminous hot stars and/or a white dwarfs (WDs) hidden in binary systems as WDs are less prominent in the optical bands wherein a large number of late-type MS and red giant branch (RGB) stars dominate. Recent UV studies of OCs (Siegel, et al 2014;Sindhu, et al 2018;Siegel, et al 2019) have shown that UV Color-Magnitude Diagrams (CMDs) are essential tools for identifying and studying the properties of UV bright stellar population such as spatially unresolved MS stars with cool WD companions, post-interaction binaries with hot WD companions, MS and RGB stars with high chromospheric activity, and blue straggler stars (BSS). Though, there have been extensive studies of OCs in the optical and X-ray passbands, detailed studies in UV passbands have been rare, exceptions being old open clusters ( M67 Sindhu, et al 2018;NGC 188 Gosnell, et al 2014).…”

“…Though, there have been extensive studies of OCs in the optical and X-ray passbands, detailed studies in UV passbands have been rare, exceptions being old open clusters ( M67 Sindhu, et al 2018;NGC 188 Gosnell, et al 2014). Another effort in this direction has been the Swift/UVOT Stars Survey Siegel, et al (2019) which led to a point-source photometry catalogue for 103 Galactic OC with isochrones fitted to the UV CMDs of stars.…”