Speaker
Description
We investigated a population of evolved member stars in NGC 6791, a Galactic open cluster known to be old, with an age rivaling those of Pop II globular clusters but rich in metallicity as Pop I. Using Gaia DR3 proper motion and parallax, we identified member candidates against field stars, from which cluster parameters are derived. We confirm that NGC 6791 has a heliocentric distance of about 4.4 kpc, and an age of about 8 Gyr but, contrary to most open clusters, it still hosts more than a few thousand member stars. In the Gaia color-magnitude diagram, there is an anomalous group of members located between the main sequence and the hot subdwarf B stars, which are thought to be extreme horizontal-branch stars undergoing helium core fusion. These "warm" subdwarfs have Gaia (BP-RP) color between 0.0 and 0.5, corresponding to early A-type (10,000 K) to mid-F (6800 K) main sequence stars but are much fainter. Their nature is uncertain. One possibility is binarity, for which the combined luminosity of two nearly equal brightness stellar components results in intermediate colors/temperatures. We test the hypothesis by comparison of the observed spectral energy distribution with synthetic composite single star spectra. As an equivalent diagnosis, we present archival images to examine whether there is nearby companion to contaminate the flux, or as unresolved pairs there is any brightness change with wavelengths from UV, optical, to infrared.
| Participate the oral/poster presentation award competition | Yes |
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