Variability in the UV resonance lines of the cataclysmic variable V795 Herculis (PG 1711 + 336)⋆

Abstract

Results are presented from a study of variability in the UV resonance lines from the cataclysmic variable V795 Her, based on a total of 28 IUE SWP low-resolution spectra. Significant line profile fluctuations are evident in the C IVλ1549 and Si IVλ1400 doublets on time-scales down to ≲ 30 min. There is little or no evidence for changes in N v λ1240. Only C IVλ1549 is (occasionally) seen in emission; the morphology of this profile changes from an absorption-dominated line to an emission-dominated one in ∼ a few hours. The observed C IV and Si IV absorption strengths vary in concert. Although no correlations are evident between changes in the Si IV and C IV line strengths and the mooted orbital period of V795 Her (∼ 2.6 hr), our time-series analyses of the UV data suggest a ∼ 4.8-hr periodic modulation in the line profile variability. Constraints on the origin and geometry of the line formation are discussed, based on these UV data. We consider in particular whether: (i) a continuum-emitting hotspot on the disc, or (ii) wind outflow asymmetry could account for the regular line profile variability. There is freedom in specifying both types of model to ensure that both can reproduce the extent of the observed profile changes, but neither yields the appropriate phase dependence. Alternative specifications for departures from axial symmetry are suggested.