R. C. North

Orbital periods of 22 subdwarf B stars

Subdwarf B (sdB) stars are thought to be core helium burning stars with low mass hydrogen envelopes. In recent years it has become clear that many sdB stars lose their hydrogen through interaction with a binary companion and continue to reside in binary systems today. In this paper we present the results of a programme to measure orbital parameters of binary sdB stars. We determine the orbits of 22 binary sdB stars from 424 radial velocity measurements, raising the sample of sdBs with known orbital parameters to 38. We calculate lower limits for the masses of the companions of the sdB stars which, when combined with the orbital periods of the systems, allow us to discuss approximate evolutionary constraints. We find that a formation path for sdB stars consisting of mass transfer at the tip of the red giant branch (RGB) followed by a common envelope phase explains most, but not all of the observed systems. It is particularly difficult to explain both long period systems and short period, massive systems. We present new measurements of the effective temperature, surface density and surface helium abundance for some of the sdB stars by fitting their blue spectra. We find that two of them (PG 0839 + 399 and KPD 1946 + 4340) do not lie in the extreme horizontal branch (EHB) band, indicating that they are post-EHB stars.

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.

New Subdwarf B Star Periods

Subdwarf B (sdB) stars are thought to be helium burning stars with low mass hydrogen envelopes. Several evolutionary paths have been proposed to explain the formation of these systems. One of these scenarios is the evolution of the sdB progenitor within a binary system. In fact [3] found that out of a sample of 36 sdBs, 21 of them reside in close binary systems. This result combined with conclusions reached by other authors [1] suggest that two-thirds of sdBs are in binary systems.

Mapping the Peculiar Binary GP Com

We present high resolution spectra of the AM CVn helium binary GP Com at two different wavelength ranges. The spectra show the same flaring behaviour observed in previous UV and optical data. We find that the central spike contributes to the flare spectra indicating that its origin is probably the compact object. We also detect that the central spike moves with orbital phase following an S-wave pattern. The radial velocity semiamplitude of the S-wave is 10km s-1 which lies very close to the white dwarf. The Stark effect seems to significantly affect the central spike of some of the lines, suggesting that it forms in a high electron density region. This again favours the idea that the central spike originates in the white dwarf. We present Doppler maps obtained for the emission lines which show three clear emission regions.

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.