FM7p.13 — FUSE, STIS, and Keck Observations of the Ultraviolet-Bright Star vZ 1128 in the Globular Cluster M3

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Aug 10th at 6:00 PM until 6:00 PM

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Author(s): William V. Dixon1, Pierre Chayer1

Institution(s): 1. Space Telescope Science Institute

Optical color-magnitude diagrams of Galactic globular clusters generally include one or two UV-bright stars, objects brighter than the horizontal branch and bluer than the red-giant branch (RGB). These stars are evolving from the asymptotic giant branch (AGB) to the hot tip of the white-dwarf cooling curve. We present a spectral analysis of the UV-bright star vZ 1128 in M3 based on observations with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Space Telescope Imaging Spectrograph (STIS), and the Keck HIRES echelle spectrograph. By fitting the H I, He I, and He II lines in the Keck spectrum with non-LTE H-He models, we obtain Teff = 36,600 K, log g = 3.95, and N(He)/N(H) = 0.15. The star's FUSE and STIS spectra show photospheric absorption from C, N, O, Al, Si, P, S, Fe, and Ni. No stellar features from elements beyond the iron peak are observed. Both components of the N V 1240 A doublet exhibit P Cygni profiles, indicating a weak stellar wind, but no other wind features are seen. The star's photospheric abundances appear to have changed little since it left the RGB. Its C, N, O, Al, Si, Fe, and Ni abundances are consistent with published values for the red-giant stars in M3, and the relative abundances of C, N, and O follow the trends seen on the cluster RGB. In particular, its low C abundance suggests that the star left the AGB before the onset of third dredge-up.