QU Car: a very high luminosity nova-like binary with a carbon-enriched companion

Abstract

QU Car is listed in cataclysmic variable star catalogues as a nova-like variable. This little-studied, yet bright interacting binary is re-appraised here in the light of new high-quality ultraviolet interstellar line data obtained with STIS on-board the Hubble Space Telescope. The detection of a component of interstellar absorption at a mean local standard of rest velocity of -14 km s^-1 indicates that the distance to QU Car may be ~2 kpc or more - a considerable increase on the previous lower-limiting distance of 500 pc. If so, the bolometric luminosity of QU Car could exceed 10³⁷ erg s^-1. This would place this binary in the luminosity domain occupied by known compact-binary supersoft X-ray sources. Even at 500 pc, QU Car appears to be the most luminous nova-like variable known. New intermediate dispersion optical spectroscopy of QU Car spanning 3800-7000 Å is presented. These data yield the discovery that CIVλλ5801,12 is present as an unusually prominent emission line in an otherwise low-contrast line spectrum. Using measurements of this and other lines in a recombination line analysis, it is shown that the C/He abundance as proxied by the n(C⁴⁺)/n(He²⁺) ratio may be as high as 0.06 (an order of magnitude higher than the solar ratio). Furthermore, the C/O abundance ratio is estimated to be greater than 1. These findings suggest that the companion in QU Car is a carbon star. If so, it would be the first example of a carbon star in such a binary. An early-type R star best matches the required abundance pattern and could escape detection at optical wavelengths provided the distance to QU Car is ~2 kpc or more.