C. K. J. Moran

Six detached white-dwarf close binaries

We determine the orbits of four double-degenerate systems (DDs), composed of two white dwarfs and two white-dwarf-M-dwarf binaries. The four DDs, WD1022+050, WD1428+373, WD1824+040 and WD2032+188, show orbital periods of 1.157155(5), 1.15674(2), 6.26602(6) and 5.0846(3) d, respectively. These periods combined with estimates for the masses of the brighter component, based on their effective temperatures, allow us to constrain the masses of the unseen companions. We estimate that the upper limit for the contribution of the unseen companions to the total luminosity in the four DDs ranges between 10 and 20 per cent. In the case of the two white-dwarf-M-dwarf binaries, WD 1042-690 and WD2009+622, we calculate the orbital parameters by fitting simultaneously the absorption line from the white dwarf and the emission core from the M dwarf. Their orbital periods are 0.337083(1) and 0.741 226(2) d, respectively. We find signatures of irradiation on the inner face of the companion to WD2009+622. We calculate the masses of both components from the gravitational redshift and the mass-radius relationship for white dwarfs and find masses of 0.75-0.78 and 0.61-0.64 M ○. for WD 1042-690 and WD2009+622, respectively. This indicates that the stars probably reached the asymptotic giant branch in their evolution before entering a common envelope phase. These two white-dwarf-M-dwarf binaries will become cataclysmic variables, although not within a Hubble time, with orbital periods below the period gap.

A detached double degenerate with a 1.4-h orbital period

We have discovered that the detached double degenerate binary WD 0957-666 has an orbital period of 1.46h, rather than the 1.15-d orbital period reported earlier. This is the shortest period example of such a system yet discovered. We obtain a unique period, which fits both our data and earlier data. At this period the emission of gravitational radiation will cause the binary to merge within approximately 2.0x10^8yr. This system represents a population of short orbital period binaries which will merge within a Hubble time, and so could account for type Ia supernovae, although, due to the low mass of both stars (0.3 to 0.4 Msolar), it is unlikely to become a supernova itself. We have detected the companion star and have measured a mass ratio of q=1.15+/-0.10. This is the third double degenerate for which q has been measured, and all three have q~=1, which is in conflict with the predicted mass ratio distribution, which peaks at 0.7. This system is viewed close to edge-on, and we estimate that the probability of this system undergoing eclipses is 15 per cent.

The triple degenerate star WD 1704+481

WD 1704+481 is a visual binary in which both components are white dwarfs. We present spectra of the H alpha line of both stars which show that one component (WD 1704+481.2=Sanduleak B=GR 577) is a close binary with two white dwarf components. Thus, WD 1704+481 is the first known triple degenerate star. From radial velocity measurements of the close binary we find an orbital period of 0.1448 d, a mass ratio, qM(bright)M(faint), of 0.70 +/- 0.03 and a difference in the gravitational redshifts of 11.5 +/- 2.3 km s(-1). The masses of the close pair of white dwarfs predicted by the mass ratio and gravitational redshift difference combined with theoretical cooling curves are 0.39 +/- 0.05 and 0.56 +/- 0.07 M-.. WD 1704+481 is therefore also likely to be the first example of a double degenerate in which the less massive white dwarf is composed of helium and the other white dwarf is composed of carbon and oxygen.

The mass ratio distribution of short-period double degenerate stars

Short-period double degenerates (DDs) are close white dwarf-white dwarf binary stars which are the result of the evolution of interacting binary stars. We present the first definitive measurements of the mass ratio for two DDs, WD 0136+768 and WD 1204+450, and an improved measurement of the mass ratio for WD 0957 - 666. We compare the properties of the six known DDs with measured mass ratios to the predictions of various theoretical models. We confirm the result that standard models for the formation of DDs do not predict sufficient DDs with mass ratios close to 1. We also show that the observed difference in cooling ages between white dwarfs in DDs is a useful constraint on the initial mass ratio of the binary. A more careful analysis of the properties of the white dwarf pair WD 1704+481.2 leads us to conclude that the brighter white dwarf is older than its fainter companion. This is the opposite of the usual case for DDs and is caused by the more massive white dwarf being smaller and cooling faster. The mass ratio in the sense (mass of younger star)/(mass of older star) is then 1.43 ′ 0.06 rather than the value of 0.70 ′ 0.03 given previously.

A mystery solved: the mass ratio of the dwarf nova EM Cygni

We have discovered that the spectrum of the well-known dwarf nova EM Cyg is contaminated by light from a K2–5V star (in addition to the K-type mass donor star). The K2–5V star contributes approximately 16 per cent of the light from the system and if not taken into account has a considerable effect upon radial velocity measurements of the mass donor star. We obtain a new radial velocity amplitude for the mass donor star of K2 = 202 ± 3kms −1 , which compares with the value of K2 = 135 ± 3kms −1 obtained in Stover, Robinson & Nather’s classic 1981 study of EM Cyg. The revised value of the amplitude combined with a measurement of rotational broadening of the mass donor v sini = 140 ± 6kms −1 , leads to a new mass ratio of q = M2/M1 = 0.88 ± 0.05. This solves a long standing problem with EM Cyg because Stover et al.’s measurements indicated a mass ratio q > 1, a value which should have led to dynamically unstable mass transfer for the secondary mass deduced by Stover et al. The revised value of the mass ratio combined with the orbital inclination i = 67±2 ◦ leads to masses of 0.99±0.12M⊙ and 1.12±0.08M⊙ for the mass donor and white dwarf respectively. The mass donor is evolved, since it has a later spectral type (K3) than its mass would imply. We discuss whether the K star could be physically associated with EM Cyg or not, and present the results of the spectroscopic study.

The systemic velocities of four long-period cataclysmic variable stars

Although a large number of orbital periods of cataclysmic variable (CV) stars have been measured, comparison of period and luminosity distributions with evolutionary theory is affected by strong selection effects. A test has been discovered that is independent of these selection effects and is based upon the kinematics of CVs. If the standard models of evolution are correct then long-period (Porb > 5 h) CVs should be typically less than 1.5 Gyr old, and their line-of-sight velocity dispersion (σ) should be small. We present results from a pilot study, which indicate that this postulate is indeed true. Four long-period dwarf novae (EM Cyg, V426 Oph, SS Cyg and AH Her) were observed over a complete orbit, in order for accurate radial velocities to be obtained. We find values of −1.7, 5.4, 15.4 and 1.8 km s1 with uncertainties of the order of 3 km s1, referred to the dynamical local standard of rest, leading to a dispersion of 8 km s1. Calculation of a 95 per cent confidence interval gives the result 4 <σ < 28 km s1 compared with a prediction of 15 km s1. We also have an improved determination of mass donor spectral type, K2 and q for the four systems.

A new magnetic white dwarf: PG 2329+267

We have discovered that the white dwarf PG 2329+267 is magnetic, and, assuming a centred dipole structure, has a dipole magnetic field strength of approximately 2.3 MG. This makes it one of only approximately 4 per cent of isolated white dwarfs with a detectable magnetic field. Linear Zeeman splitting, as well as quadratic Zeeman shifts, is evident in the hydrogen Balmer sequence and circular spectropolarimetry reveals ,10 per cent circular polarization in the two displaced j components of Ha. We suggest from comparison with spectra of white dwarfs of known mass that PG 2329+267 is more massive than typical isolated white dwarfs, in agreement with the hypothesis that magnetic white dwarfs evolve from magnetic chemically peculiar Ap and Bp type main-sequence stars.

Hα-Emission Doppler Tomography of Long-Period Cataclysmic Variable Stars

We present Doppler maps of Hα (6562.76A) emission of 4 well-known dwarf novae, SS Cyg, AH Her, EM Cyg and V426 Oph. All 4 systems were in quiescence during our observations. All of them have visible mass donor stars allowing us to establish precise orbital phases. None of them show what is often thought of as the classic pattern of symmetric disc plus bright-spot at the gas stream/disc impact. Instead they have regions of emission at low velocities in the area below the point representing the velocity of the white dwarf. In addition, emission with a velocity consistent with an origin on the heated face of the mass donor can be seen. We consider possible explanations for these peculiar images.