Detached white-dwarf close-binary stars -- CV's extended family

I review detached binaries consisting of white dwarfs with either other white dwarfs or low mass main-sequence stars in tight orbits around them. Orbital periods have been measured for 15 white dwarf/white dwarf systems and 22 white dwarf/M dwarf systems. While small compared to the number of periods known for CVs (>300), I argue that each variety of detached system has a space density an order of magnitude higher that of CVs. While theory matches the observed distribution of orbital periods of the white dwarf/white dwarf binaries, it predicts white dwarfs of much lower mass than observed. Amongst both types of binary are clear examples of helium core white dwarfs, as opposed to the usual CO composition; similar systems must exist amongst the CVs. White dwarf/M dwarf binaries suffer from selection effects which diminish the numbers seen at long and short periods. They are useful for the study of irradiation; I discuss evidence to suggest that Balmer emission is broadened by optical depth effects to an extent which limits its usefulness for imaging the secondary stars in CVs.Comment: 6 pages, to appear in proceedings of Warner symposium on Cataclysmic Variable star

Kinematics of the helium accretor GP Com

We present time-resolved spectra of the double-degenerate binary star, GP Com. The spectra confirm the presence and period (46.5 min) of the `S'-wave feature found by Nather, Robinson and Stover. GP Com is erratically variable at X-ray and UV wavelengths. We have found the equivalent variability in our data, which, as also seen in UV data, is mostly confined to the emission lines. The HeII 4686 changes by the largest amount, consistent with X-ray driven photo-ionisation. The flaring part of the line profiles is broader than the average, as expected if they are dominated by the inner disc. The HeII 4686 profile is especially remarkable in that its blue-shifted peak is 1400 km/s from line centre compared to 700 km/s for the HeI lines (the red-shifted peak is blended with HeI 4713). We deduce that HeII 4686 emission is confined to the inner 1/4 of the disc. We suggest that the activity of the inner disc indicates that accretion is significant (and unstable) there, in contrast to quiescent dwarf novae, in support of models in which GP Com is in a (quasi-)steady-state of low mass transfer rate. GP Com shows triple-peaked lines profiles which consist of the usual double-peaked profiles from a disc plus a narrow component at line centre. We find evidence for both radial velocity and flux variability in this component, inconsistent with a nebula origin. The radial velocity amplitude and its phase relative to the `S'-wave are consistent with an origin on the accreting white dwarf, if the mass ratio, q = M2/M1, is of order 0.02, as expected on evolutionary grounds. However this explanation is still not satisfactory as the systemic velocity of the narrow component shows significant variation from line to line, and we have no explanation for this.Comment: 9 pages Latex, 9 figures. Accepted for publication in Monthly Notice

Using HINODE/Extreme-Ultraviolet Imaging Spectrometer to confirm a seismologically inferred coronal temperature

The Extreme-Ultraviolet Imaging Spectrometer on board the HINODE satellite is used to examine the loop system described in Marsh et al. (2009) by applying spectroscopic diagnostic methods. A simple isothermal mapping algorithm is applied to determine where the assumption of isothermal plasma may be valid, and the emission measure locii technique is used to determine the temperature profile along the base of the loop system. It is found that, along the base, the loop has a uniform temperature profile with a mean temperature of 0.89 +- 0.09 MK which is in agreement with the temperature determined seismologically in Marsh et al. (2009), using observations interpreted as the slow magnetoacoustic mode. The results further strengthen the slow mode interpretation, propagation at a uniform sound speed, and the analysis method applied in Marsh et al. (2009). It is found that it is not possible to discriminate between the slow mode phase speed and the sound speed within the precision of the present observations

Period Changes in Ultra-compact Double White Dwarfs

In recent years there has been much interest in the nature of two stars, V407 Vul and RXJ0806+1527, which are widely thought to be binary white dwarfs of very short orbital period, 570 and 321 seconds respectively. As such they should be strong sources of gravitational waves and possible ancestors of the accreting AM CVn stars. Monitoring at X-ray and optical wavelengths has established that the period of each star is decreasing, at rates compatible with that expected from gravitational radiation. This has been taken to support the ``unipolar inductor'' model in which the white dwarfs are detached and the X-rays produced by the dissipation of magnetically-induced electric currents. In this paper we show that this interpretation is incorrect because it ignores associated torques which transfer angular momentum between the spin of the magnetic white dwarf and the orbit. We show that this torque is 10^5 times larger than the GR term in the case of V407 Vul, and 10 times larger for RXJ0806+1527. For V407 Vul, the unipolar inductor model can only survive if the white dwarf spins 100 times faster than the orbit. Since this could only come about through accretion, the validity of the unipolar inductor appears questionable for this star. We also consider whether accretion models can fit the observed spin-up, concluding that they can, provided that a mechanism exists for driving the mass transfer rate away from its equilibrium value.Comment: 6 pages, 1 figure, accepted for publication in MNRA

A double-frequency dwarf nova oscillation

We have detected coherent oscillations (``dwarf nova oscillations'') in Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were seen towards the end of a superoutburst of OY Car. The oscillations are extraordinary compared to the many other examples in the literature for two reasons. First, their amplitude is large, with a peak-to-peak variation of 8 to 20% of the total flux over the range 1100 to 2500A. However, most remarkably we find that there are two components present simultaneously. Both have periods close to 18sec (equivalent to 4800 cycles/day) but they are separated by 57.7+/-0.5 cycles/day. The lower frequency component of the pair has a strong second harmonic while its companion, which has about twice its amplitude, does not. The oscillation spectra appear hotter than the mean spectrum and approximately follow the continuum distribution of a black-body with a temperature in the range 30,000 to 50,000K. We tentatively suggest that the weaker non-sinusoidal component could represent the rotation of the white dwarf, although we have been unable to recover any such signal in quiescent data.Comment: 9 pages, 9 figure

On gravity from SST, geoid from Seasat, and plate age and fracture zones in the Pacific

A composite map produced by combining 90 passes of SST data show good agreement with conventional GEM models. The SEASAT altimeter data were deduced and found to agree with both the SST and GEM fields. The maps are dominated (especially in the east) by a pattern of roughly east-west anomalies with a transverse wavelength of about 2000 km. Comparison with regional bathymetric data shows a remarkedly close correlation with plate age. Most anomalies in the east half of the Pacific could be partly caused by regional differences in plate age. The amplitude of these geoid or gravity anomalies caused by age differences should decrease with absolute plate age, and large anomalies (approximately 3 m) over old, smooth sea floor may indicate a further deeper source within or perhaps below the lithosphere. The possible plume size and ascent velocity necessary to supply deep mantle material to the upper mantle without complete thermal equilibration was considered. A plume emanating from a buoyant layer 100 km thick and 10,000 times less viscous than the surrounding mantle should have a diameter of about 400 km and must ascend at about 10 cm/yr to arrive still anomalously hot in the uppermost mantle