Observations of the Ultraviolet-Bright Star ZNG 1 in the Globular Cluster M5 (NGC 5904)

We have analyzed archival spectra of the hot UV-bright star ZNG 1 in the globular cluster M5 (NGC 5904) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Space Telescope Imaging Spectrograph (STIS). From these data, we derive an effective temperature $T_{\mathrm{e}\mathrm{f}\mathrm{f}}=\mathrm{43,000}\pm 1400$ K, a surface gravity $\log g=4.47\pm 0.08$, a rotational velocity $v\sin i=157\pm 12$ km s${}^{-1}$, and a mass $M=0.92\pm 0.17M_{\odot }$. The atmosphere is helium-rich ($Y=0.99$) and enhanced in CNO (relative to the cluster). The spectrum exhibits wind features with a terminal velocity near 1500 km s${}^{-1}$ and strong discrete absorption components (DACs). The high helium abundance, stellar mass, and rotational velocity suggest that the star is a merger remnant, and its parameters are consistent with models of a pair of merging He-core white dwarfs.