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

Abstract

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 and the Space Telescope Imaging Spectrograph. From these data, we derive an effective temperature T _eff = 43,000 ± 1400 K, a surface gravity $\mathrm{log}g$ = 4.47 ± 0.08, a rotational velocity $v\sin i$ = 157 ± 12 km s ^−1 , and a mass M = 0.92 ± 0.17 M _☉ . 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. 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.

Keywords

• Stellar abundances
• Stellar atmospheres
• Helium-rich stars
• Stellar evolution