Time-resolved ultraviolet spectroscopy of the compact interacting binary QU Car

We present HST/STIS (1160–1700Å) echelle spectra of the cataclysmic variable (CV) star, QU Car, at three epochs. In catalogues this binary is classified as a nova-like variable. QU Car was observed three times in time-tag mode for 2300 sec, 2600 sec and 2600 sec, allowing us to study the spectral time evolution on timescales down to ∼10 sec. We find evidence of a high-state non-magnetic CV at low inclination, with unusually high ionisation. We observed narrow absorption lines (∼ few hundred km s wide) in Nvλ 1240, Ovλ 1371 and Si ivλ 1398, as well as broader (HWZI ∼ 1000kms ) emission in C iiiλ 1176, C ivλ 1549 and He iiλ 1640, all with a superposed absorption component. High ionisation is indicated by the He ii emission, which is unusually strong in comparison with C iv, and the relative strength of the Ov absorption line. The dereddened UV continuum spectral index of, on average, −2.3 suggests that disc accretion dominates the spectral energy distribution. In two observations velocity shifting is noted in the absorption lines on a timescale long enough not to repeat within the ∼ 2600-sec exposures. The absorption superposed on the C iv emission line moves coherently with the Nv and Si iv absorption, suggesting the same origin for all absorption lines – most likely to be in the accretion disc atmosphere. Weak blueshifted absorption in Nv and C iv provides evidence of an outflow component and we estimate a maximum outflow velocity of ∼ 2000km s . This may be linked to a wind launched from further out in the disc than is typically seen in those high-state non-magnetic CV whose wind speeds are observed to reach to > ∼ 4000km s . Unusually, three ionisation stages of carbon – C ii, C iii and C iv – are present in emission, with line width increasing with higher ionisation. The presence of C ii in emission and the positive line-width/ionisation correlation is most easily reconciled with an origin in a disc chromosphere, beyond the influence of the EUVemitting inner disc.