Far‐Ultraviolet Observations of the Dwarf Nova VW Hydri in Quiescence

Abstract

We present a 904-1183 A spectrum of the dwarf nova VW Hyi taken with the Far Ultraviolet Spectroscopic Explorer during quiescence, 11 days after a normal outburst, when the underlying white dwarf accreter is clearly exposed in the far-ultraviolet. However, model fitting shows that a uniform-temperature white dwarf does not reproduce the overall spectrum, especially at the shortest wavelengths. A better approximation to the spectrum is obtained with a model consisting of a white dwarf and a rapidly rotating belt. The white dwarf component accounts for 83% of the total flux, has a temperature of 23,000 K, a v sin i = 400 km s-1, and a low carbon abundance. The best-fit accretion belt component accounts for 17% of the total flux, with a temperature of about 48,000-50,000 K and a rotation rate Vrot sin i around 3000-4000 km s-1. The requirement of two components in the modeling of the spectrum of VW Hyi in quiescence helps to resolve some of the differences in interpretation of ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of a second component (and its exact nature) in VW Hyi itself is still relatively uncertain, given the lack of better models for spectra of the inner disk in a quiescent dwarf nova.