R. Stehle

Structure and properties of transition fronts in accretion discs

ABSTRA C T We use high-resolution time-dependent numerical simulations of accretion discs around white dwarfs to study the structure and properties of transition fronts in the context of the thermal‐ viscous disc instability model. The thermal structure of cooling and heating fronts is dominated by radiative cooling and viscous heating, respectively, except in a very narrow precursor region in heating fronts where advection and radial transport of energy dominate. Cooling fronts are much broader than heating fronts, but the widths of both types of fronts scale with the local vertical scaleheight of the disc. We confirm that during a fair fraction of the propagation time of a cooling front, the structure of the inner disc is close to self-similar. The speed of heating fronts is , a few km s ˇ1 , while the speed of cooling fronts is , a fraction of ak m s ˇ1 . We show that direct measurements of the speed of transition fronts probably cannot discriminate between various prescriptions proposed for the viscosity parameter a. A natural prediction of the disc instability model is that fronts decelerate as they propagate in the disc, independent of the prescription for a. Observation of this effect would confirm that dwarf nova outbursts are driven by the thermal‐viscous instability. Most of our results also apply to lowmass X-ray binaries in which the accreting object is a neutron star or a black hole.

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.

Stability of accretion discs threaded by a strong magnetic field

We study the stability of poloidal magnetic fields anchored in a thin accretion disc. The two-dimensional hydrodynamics in the disc plane is followed by a grid-based numerical simulation including the vertically integrated magnetic forces. The three-dimensional magnetic field outside the disc is calculated in a potential field approximation from the magnetic flux density distribution in the disc. For uniformly rotating discs we confirm numerically the existence of the interchange instability as predicted by Spruit, Stehle & Papaloizou. In agreement with predictions from the shearing sheet model, discs with Keplerian rotation are found to be stabilized by the shear, as long as the contribution of magnetic forces to support against gravity is small. When this support becomes significant, we find a global instability which transports angular momentum outwardly and allows mass to accrete inwardly. The instability takes the form of a m=1 rotating ‘crescent’, reminiscent of the purely hydrodynamic non-linear instability previously found in pressure-supported discs. A model where the initial surface mass density Σ(r) and Bz(r) decrease with radius as power laws shows transient mass accretion during about six orbital periods, and settles into a state with surface density and field strength decreasing approximately exponentially with radius. We argue that this instability is likely to be the main angular momentum transport mechanism in discs with a poloidal magnetic field sufficiently strong to suppress magnetic turbulence. It may be especially relevant in jet-producing discs.

The standstill luminosity in Z Cam systems

We consider accretion discs in close binary systems. We show that the heating of a disc at the impact point of the accretion stream contributes significantly to the local energy budget at its outer edge. As a result, the thermal balance relation between local accretion rate and surface density (the ‘S-curve’) changes; the critical mass transfer rate above which no dwarf nova outbursts occur can be up to 40 per cent smaller than without impact heating. Standstills in Z Cam systems thus occur at smaller mass transfer rates than otherwise expected, and are fainter than the peak luminosity during the dwarf nova phase as a result.

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.