Rotationally Broadened Synthetic Spectra for HST and IUE Observations of Exposed White Dwarfs in Cataclysmic Variables11Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5‐26555.

Abstract

ABSTRACTIn many dwarf novae during quiescence, the underlying white dwarf dominates the flux in the far ultraviolet. By observing these exposed degenerate stars with the Hubble Space Telescope Goddard High Resolution Spectrograph and Faint Object Spectrograph (HST GHRS and FOS), the white‐dwarf rotation rates can be determined or constrained by fitting synthetic spectra broadened by rotation. Using TLUSTY and SYNSPEC, we present a grid of rotationally broadened white‐dwarf synthetic spectra, convolved with the resolution of the HST GHRS (G140L, G160M), FOS (G130H), and IUE low‐resolution short‐wavelength prime (SWP) spectra, computed for log g = 8, solar composition, Teff = 15,000, 25,000, and 50,000 K, and rotational velocities of 0–5000 km s−1. We examine the predicted appearance of accreted solar‐composition white‐dwarf atmospheres in the context of determining or constraining the white‐dwarf rotational velocities. We present model grids that demonstrate this capability. We present the first application of these grids to the SWP low‐resolution spectra of the white dwarf in the nova‐like variable TT Ari, exposed during the very low brightness state of that system. We also rule out the possibility that the absence of Lyα absorption in many cataclysmic variables in quiescence (specifically those containing white dwarfs with Teff = 15,000–20,000 K) could be due to rotational velocities of the white dwarf near breakup.