Testing the line-driven disc wind model: time-resolved ultraviolet spectroscopy of IX Vel and V3885Sgr

Abstract

To confront the predictions of the most recent line-driven disc wind models with observational evidence, we have obtained Hubble Space Telescope (HST ) STIS (1180-1700Å) echelle spectra of the nova-like variables IX Vel and V3885Sgr at three epochs. The targets were observed in time-tag mode for ~2000s on each occasion, allowing us to study the spectral time evolution on time-scales down to ~10s. The mean ultraviolet (UV) spectra are characterized by the wind signature of broad blueshifted absorption in Lyα , Nvλ 1240, Siivλ 1398, Cvλ 1549 and Heiiλ 1640. There is very little redshifted emission other than in Civ. Narrow blueshifted absorption dips, superposed on the broad absorption at around -900kms^-1 , accompany periods of well-developed wind activity. The continuum level and mean line profiles vary markedly from observation to observation - with the wind signatures almost disappearing in one epoch of observation of IX Vel. The strong positive correlation between UV brightness and wind activity predicted by line-driven disc wind models is disobeyed by both binaries. The wind signatures in the UV spectrum of IX Vel are revealed to be remarkably steady on time-scales ranging from ~10 to ~1000s. More variability is seen in V3885Sgr, the binary with the lower opacity outflow. However, there is only one epoch in which the line profile changes significantly in ~100s or less. Narrow absorption dips, when present, show only smooth, small changes in velocity. We surmise that these may trace the orbital motion of the white dwarf. The near-absence of line profile variability on the shorter 10- to 100-s time-scales, and the lack of correlation between wind activity and luminosity, could both arise if a non-radiative factor such as the magnetic field geometry controls the mass loss rate in these binaries.