The Hot Stars of Old Open Clusters: M67, NGC 188, and NGC 6791

Abstract

We analyze ultraviolet (∼ 1500Å) images of the old open clusters M67, NGC 188, and NGC 6791 obtained with Ultraviolet Imaging Telescope (UIT) during the second flight of the Astro observatory in March 1995. Twenty stars are detected on the UIT image of M67, including 11 blue stragglers, seven white dwarf candidates, and the yellow giant -white dwarf binary S1040. The ultraviolet photometry of the blue stragglers F90 (S975) and F131 (S1082) suggests that these stars have hot subluminous companions. We present a semi-empirical integrated ultraviolet spectrum of M67, and show that the blue stragglers dominate the integrated spectrum at wavelengths shorter than 2600Å. The number of white dwarfs in M67 is roughly consistent with the number expected from white dwarf cooling models. Eight candidate sdB/sdO stars are detected in NGC 6791, and two are detected in NGC 188. The luminosity range 1.10 < log L/L ⊙ < 1.27, derived from the ultraviolet photometry of the six sdB candidates, is consistent with theoretical models of metal-rich hot horizontal branch (HB) stars. The fraction of hot HB stars in both NGC 6791 and NGC 188 is about 30%, implying that the integrated spectra of both clusters should show a UV turnup at least as strong as that observed in any elliptical galaxy.

A Hot Companion to a Blue Straggler in NGC 188 as Revealed by the Ultra-Violet Imaging Telescope (Uvit) on Astrosat

“…It is considered to be a BSS based on its location in the CMD. Landsman et al(1998) mentioned that this may have a composite spectrum, as suggested by its colors, with a possibility of a very hot star and a BSS as components. Siegel et al (2014) suspected this to be a red giant and a pre-WD binary.…”

“…A total of 24 BSS candidates were cataloged by Ahumada & Lapasset (2007), with Geller et al (2008) showing 20 BSSs to be confirmed members based on their radial velocities. In their UIT study, Landsman et al (1998) identified three stars with exceptionally blue colors.…”

“…Such clusters are well studied in optical and near-IR pass bands, but only a handful have been studied in the Ultraviolet (UV). In the old open clusters M67, NGC188 and NGC6791, populations of hot stars were studied by Landsman et al (1998) using images from the Ultraviolet Imaging Telescope (UIT). Analysis of wide-field UV photometry of nearby clusters M67, NGC188, NGC2539 and M79 obtained with purni@iiap.res.in the Swift Ultraviolet Optical Telescope (UVOT) showed that UV bright stars can be readily identified using the UV color magnitude diagram (CMD; Siegel et al 2014).…”

In-orbit Performance of UVIT and First Results

“…Note that the figure shows fluxes in the FUV from all the filters, and fluxes in the NUV only from two filters. These flux distributions can be used to compare with models to estimate the temperature, mass and radius of the source, which is known to be a single lined binary (Geller et al 2008), with a suspected hot companion (Landsman et al 1998). b) The globular cluster, NGC 1851 has many UV-bright stars in its core and has been observed by Galex.…”

Horizontal Branch Stars and the Ultraviolet Universe

“…Evidence in favor of the high-metallicity scenario is provided by the presence of EHB stars in high-Z open clusters, including the ∼ 7 Gyr-old ( [81]) NGC 188 ( [48]) and the ∼ 8 Gyr-old ( [12]) NGC 6791 ( [46,45,48,9,12,33]). Both of these clusters are well known for their supersolar metallicities (e.g., [89]).…”