The Rapidly Rotating, Hydrogen Deficient, Hot Post-Asymptotic Giant Branch Star ZNG 1 in the Globular Cluster M5

Abstract

We report observations of the hot post-asymptotic giant branch star ZNG 1 in the globular cluster M5 (NGC 5904) with the Far Ultraviolet Spectroscopic Explorer (FUSE). From the resulting spectrum, we derive an effective temperature T_eff = 44300 +/- 300 K, a surface gravity log g = 4.3 +/- 0.1, a rotational velocity v sin i = 170 +/- 20 km/s, and a luminosity log (L/L_sun) = 3.52 +/- 0.04. The atmosphere is helium-rich (Y = 0.93), with enhanced carbon (2.6% by mass), nitrogen (0.51%) and oxygen (0.37%) abundances. The spectrum shows evidence for a wind with terminal velocity near 1000 km/s and an expanding shell of carbon- and nitrogen-rich material around the star. The abundance pattern of ZNG 1 is suggestive of the ``born-again'' scenario, whereby a star on the white-dwarf cooling curve undergoes a very late shell flash and returns to the AGB, but the star's rapid rotation is more easily explained by a previous interaction with a binary companion.