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 (UV) interstellar line data obtained with STIS on board the Hubble Space Telescope. The detection of a component of interstellar absorption at a mean LSR velocity of $-$14 km s${}^{-1}$ indicates that the distance to QU Car may be $\sim$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 $10^{37}$ ergs s${}^{-1}$. This would place this binary in the luminosity domain occupied by known compact-binary supersoft X-ray sources. Even at a 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 Åis presented. These data yield the discovery that C{\sc iv} $\lambda \lambda$5801,12 is present as an unusually prominent emission line in an otherwise low-contrast line spectrum. Using measurements of this and other lines in a recombination line analysis, it is shown that the C/He abundance as proxied by the n(C${}^{4+}$)/n(He${}^{2+}$) 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 $\sim$2 kpc or more.

12 pages, latex (mn2e style file), accepted for publication in MNRAS original gzipped postscript at ftp://astro.ic.ac.uk/pub/janet/qucar_jed.ps.gz

References in corpus (5)

• The optical afterglow and host galaxy of GRB000926
• Reprocessing the Hipparcos data for evolved giant stars II. Absolute magnitudes for the R-type carbon stars
• Testing the line-driven disk wind model: time-resolved UV spectroscopy of IX Vel and V3885 Sgr
• X-Ray Off States and Optical Variability in CAL 83
• Implications of the HST/FGS parallax of SS Cygni on the disc instability model

Cited by in corpus (11)

• Newly Discovered Cataclysmic Variables from the INT/WFC Photometric H alpha Survey of the Northern Galactic Plane
• A Preliminary Observational Search for Circumbinary Disks Around Cataclysmic Variables
• Rapidly accreting white dwarfs as supernova type Ia progenitors
• Distances of CVs and related objects derived from Gaia Data Release 1
• Life after eruption - III. Orbital periods of the old novae V365 Car, AR Cir, V972 Oph, HS Pup, V909 Sgr, V373 Sct and CN Vel
• Synthetic Spectrum Constraints on a Model of the Cataclysmic Variable QU Carinae
• Novalike Cataclysmic Variables in the Infrared
• Recovery of the old nova V840 Ophiuchi: A carbon rich system
• QU Carinae: Supernova Ia in the making?
• QU Carinae: a SNeIa progenitor?
• The orbital period of the V Sge star candidate QU Carinae