Time resolved ultraviolet spectroscopy of the compact interacting binary QU Car

We present HST/STIS (1160–1700u 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 2300sec, 2600sec and 2600sec, allowing us to study the spectral time evolution on timescales down to �10sec. We find evidence of a high-state non-magnetic CV at low inclination, with unusually high ionisation. We observed narrow absorption lines (� few hundred kms 1 wide) in N v�1240, O v�1371 and Si iv�1398, as well as broader (HWZI � 1000kms 1 ) emission in C iii�1176, C iv�1549 and He ii�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 O v 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 N v 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 N v and C iv provides evidence of an outflow component and we estimate a maximum outflow velocity of � 2000kms 1 . 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 > 4000kms 1 . 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.