Open AccessQU Car: a very high luminosity nova‐like binary with a carbon‐enriched companion
Author(s) -
Drew Janet E.,
Hartley Louise E.,
Long Knox S.,
Walt Johan van der
Publication year - 2003
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2003.06059.x
Subject(s) - physics , astrophysics , luminosity , astronomy , line (geometry) , atomic carbon , galaxy , geometry , mathematics , quantum mechanics , hydrogen
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 10 37 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 C iv λλ5801,12 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 ∼ 2 kpc or more.