The DA+dMe eclipsing binary EC13471-1258: its cup runneth over ... just

Abstract

The optical spectrum and light curve of EC13471–1258 show that it is an eclipsing binary with an orbital period of 3h 37m comprising a DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf are observed lasting 14 min, with the partial phases lasting 54 s. On one occasion, two pre‐eclipse dips were seen. Timings of the eclipses over 10 yr show jitter of up to 12 s. Flares from the M dwarf are regularly observed. The M dwarf also shows a large‐amplitude ellipsoidal modulation in the V‐band light curve. The component stars emit almost equal amounts of light at 5500 Å. Hubble Space Telescope (HST) STIS spectra show strong Lyman‐α absorption with weak metal lines of C i,ii and Si ii superimposed. Model atmosphere analysis yielded an effective temperature of 14 220 ± 300 K and log g of 8.34 ± 0.20 for the white dwarf with these errors being strongly correlated. Its metal abundance is 1/30th solar with an uncertainty of 0.5 dex, and it is rapidly rotating with V1 sin i= 400 ± 100 km s−1. The white dwarf also shows radial velocity variations with a semi‐amplitude of 138 ± 10 km s−1. The gravitational redshift of the white dwarf was measured as 62 km s−1. From optical spectroscopy the spectral type of the M dwarf was found to be M3.5–M4, its temperature 3100 ± 75 K, its rotational velocity 140 ± 10 km s−1, its radial velocity semi‐amplitude 266 ± 5 km s−1, its mean V−I colour 2.86 and its absolute V magnitude 11.82. Intriguingly, its metal abundance is normal solar. The Hα emission line shows at least two distinct components, one of which is uniformly distributed around the centre of mass of the M dwarf and provided the estimate of the rotational velocity of the M dwarf. The other arises from the other side of the binary centre of mass, well within the white dwarf Roche lobe. This behaviour is confirmed by Doppler tomography, which shows the presence of two distinct velocity components within the primary Roche lobe. The interpretation of these features is uncertain. Variations in strength of the components with binary phase can be attributed to optical thickness in the Balmer lines. Similar behaviour is seen in the observations of the other Balmer emission lines, although with a poorer signal‐to‐noise ratio. Flares in Hα were observed and are consistent with arising from the vicinity of the M dwarf. Dynamical solutions for the binary are discussed and yield an inclination of 75.5°± 2.0o, a white dwarf mass and radius of 0.78 ± 0.04 M⊙ and 0.011 ± 0.01 R⊙, and an M dwarf mass and radius of 0.43 ± 0.04 M⊙ and 0.42 ± 0.02 R⊙. These parameters are consistent with the Wood mass–radius relation for white dwarfs and the Clemens et al. mass–radius relation for M dwarfs; we argue that the M dwarf just fills its Roche lobe. The radius of the white dwarf and the model fit imply a distance of 48 ± 5 pc and an absolute V magnitude of 11.74. The rapid rotation of the white dwarf strongly suggests that the system has undergone mass transfer in the past, and implies that it is a hibernating cataclysmic variable. The...

The crowded magnetosphere of the post-common-envelope binary QS Virginis

“…The results from all four bands were consistent and the final stellar and binary parameters are detailed in Table 2. The results are consistent with those of O'Donoghue et al (2003), but a factor of 4 more precise in mass and almost 10 in radius. In fact, the radius measurement of the white dwarf in QS Vir is the most precise, direct measurement of a white dwarf radius ever.…”

“…Similar absorption features were detected in ultraviolet spectra of V471 Tau just before and after the eclipse of the white dwarf ), which they attributed to cool coronal loops. Interestingly, O'Donoghue et al (2003) detected a sharp pre-eclipse dip in the light curve of QS Vir (see their Figure 8) which could be from similar coronal loops, implying that this material can sometimes be dense enough to block out continuum light. However, the lack of a comparison star during this observation means that this dip should be interpreted with some caution.…”

“…The spectrum of QS Vir was first described by O'Donoghue et al (2003). Ribeiro et al (2010) and Parsons et al (2011) also presented higher resolution spectra.…”

“…O'Donoghue et al (2003) suggested that the ±12 s spread on ephemerides over their 10 years of eclipse timings may be due to such a cycle. This appears unlikely, since their Figure 3 (b) shows a ∼ 0.12 per cent period change in ∼ 600 d -One would expect spot migration to occur over the course of a magnetic activity cycle, as seen on the Sun, and as we find spot features are stable over a ∼ 1 yr period, such a cycle cannot be responsible for this rapid change in period.…”

“…Discovered as an eclipsing PCEB by O'Donoghue et al (2003), QS Vir (EC 13471−1258) has been intensely studied since it shows signs of being both a standard pre-CV, detached white dwarf plus main-sequence binary (Ribeiro et al 2010;Parsons et al 2011), as well as indications of accretion of material on to the white dwarf well above standard rates for detached systems (Matranga et al 2012); the M star is also very close to filling its Roche lobe, leading to the initial interpretation that it could in fact be a hibernating CV that has recently detached due to a nova eruption (O'Donoghue et al 2003). Furthermore, the arrival times of its eclipse show substantial variations (Parsons et al 2010b), which some authors have claimed may be due to the gravitational effects of circumbinary planets (Almeida & Jablonski 2011), although the proposed orbital configuration has recently been shown to be highly unstable (Horner et al 2013), meaning that the true origin of these variations remains unexplained.…”