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

Abstract

We present Hubble Space Telescope STIS (1160-1700 Å) 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, 2600 and 2600 s, allowing us to study the spectral time evolution on time-scales down to ~10 s. We find evidence of a high-state non-magnetic CV at low inclination, with unusually high ionization. We observed narrow absorption lines (approximately a few hundred km s^-1 wide) in NVλ1240, OVλ1371 and SiIVλ1398, and broader (HWZI ~ 1000 km s^-1) emission in CIIIλ1176, CIVλ1549 and HeIIλ1640, all with a superposed absorption component. High ionization is indicated by the HeII emission, which is unusually strong in comparison with CIV, and the relative strength of the OV absorption line. The dereddened ultraviolet 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 time-scale long enough not to repeat within the ~ 2600-s exposures. The absorption superposed on the CIV emission line moves coherently with the NV and SiIV absorption, suggesting the same origin for all absorption lines - most likely to be in the accretion disc atmosphere. Weak blueshifted absorption in NV and CIV provides evidence of an outflow component and we estimate a maximum outflow velocity of ~ 2000 km s^-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 CVs, the wind speeds of which are observed to reach to >~ 4000 km s^-1. Unusually, three ionization stages of carbon (CII, CIII and CIV) are present in emission, with the linewidth increasing with higher ionization. The presence of CII in emission and the positive linewidth/ionization correlation are most easily reconciled with an origin in a disc chromosphere, beyond the influence of the extreme ultraviolet-emitting inner disc.