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. From the resulting spectrum, we derive an effective temperature Teff = 44300 ± 300 K, a surface gravity log g = 4.3 ± 0.1, a rotational velocity v sin i = 170 ± 20 km s-1, and a luminosity log = 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 of a wind with a terminal velocity near 1000 km s-1 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 asymptotic giant branch, but the star's rapid rotation is more easily explained by a previous interaction with a binary companion.