M. S. Catalán

Multi-epoch Doppler tomography and polarimetry of QQ Vul ⋆

ABSTRA C T We present multi-epoch high-resolution spectroscopy and photoelectric polarimetry of the long-period polar (AM Herculis star) QQ Vul. The blue emission lines show several distinct components, the sharpest of which can unequivocally be assigned to the illuminated hemisphere of the secondary star and used to trace its orbital motion. This narrow emission line can be used in combination with Na i absorption lines from the photosphere of the companion to build a stable long-term ephemeris for the star: inferior conjunction of the companion occurs at HJDa 244 8446:4710O5UaE 0: 154 520 11O11U: The polarization curves are dissimilar at different epochs, thus supporting the idea of fundamental changes of the accretion geometry, e.g., between one- and two-pole accretion modes. The linear polarization pulses display a random scatter by 0.2 phase units and are not suitable for the determination of the binary period. The polarization data suggest that the magnetic (dipolar) axis has a colatitude of 238, an azimuth of 2508, and an orbital inclination between 508 and 708. Doppler images of blue emission and red absorption lines show a clear separation between the illuminated and non-illuminated hemispheres of the secondary star. The absorption lines on their own can be used to determine the mass ratio of the binary by Doppler tomography with an accuracy of 15‐20 per cent. The narrow emission lines of different atomic species show remarkably different radial velocity amplitudes: Ka 85‐130 km s 21 : Emission lines from the most highly ionized species, He ii, originate closest to the inner Lagrangian point L1. We can discern two kinematic components within the accretion stream; one is associated with the ballistic part, and the other with the magnetically threaded part of the stream. The location of the emission component associated with the ballistic accretion stream appears displaced between different epochs. Whether this displacement indicates a dislocation of the ballistic stream, e.g. by a magnetic drag, or emission from the magnetically threaded part of the stream with near-ballistic velocities, remains unsolved.

Changes in the structure of the accretion disc of EX Draconis through the outburst cycle

ABSTRA C T We report on the analysis of high-speed photometry of the dwarf nova EX Draconis through its outburst cycle with eclipse mapping techniques. The eclipse maps show evidence of the formation of a one-armed spiral structure in the disc at the early stages of the outburst, and reveal how the disc expands during the rise phase until it fills most of the primary Roche lobe at maximum light. During the decline phase the disc becomes progressively fainter until only a small bright region around the white dwarf is left at minimum light. The eclipse maps also suggest the presence of an inward- and an outward-moving heating wave during the rise and an inward-moving cooling wave in the decline. The inferred speed of the outward-moving heating wave is of the order of 1 km s 21 , while the speed of the cooling wave is a fraction of that. Our results suggest a systematic deceleration of both the heating and the cooling waves as they travel across the disc, in agreement with predictions of the disc instability model. The analysis of the brightness temperature profiles indicates that most of the disc appears to be in steady state during quiescence and at outburst maximum, but not during the intermediate stages. As a general trend, the mass accretion rate in the outer regions is larger than in the inner disc on the rising branch, while the opposite holds during the declining branch. We estimate a mass accretion rate of _ Ma 10 28 M( yr 21 at outburst maximum and _ Ma 10 29:1 M( yr 21 in quiescence. The brightness temperature profile in quiescence also suggests that the viscosity parameter is high at this stage, acool * 0:25, which favours the masstransfer instability model. The uneclipsed light has a steady component, understood in terms of emission from the red secondary star, and a variable component that is proportional to the out-of-eclipse flux and corresponds to about 3 per cent of the total brightness of the system. The variable component is interpreted as arising in a disc wind.

Cyclical period changes in Z Chamaeleontis

We report the identification of cyclical changes in the orbital period of the eclipsing dwarf nova Z Cha. We used times of mid-eclipse collected from the literature and our new eclipse timings to construct an observed-minus-calculated diagram covering 30 yr of observations (1972-2002). The data present cyclical variations that can be fitted by a linear plus sinusoidal function with period 28 ′ 2 yr and amplitude 1.0 ′ 0.2 min. The statistical significance of this period by an F-test is larger than 99.9 per cent. The derived fractional period change, ΔP/P = 4.4 x 10 - 7 , is comparable to that of other short-period cataclysmic variables (CVs), but is one order of magnitude smaller than those of the long-period CVs. Separate fits to the first and second halves of the data lead to ephemerides with quite different cycle periods and amplitudes, indicating that the variation is not sinusoidal or, most probably, is not strictly periodic. The observed cyclical period change is possibly caused by a solar-type magnetic activity cycle in the secondary star. An incremental variation in the Roche lobe of the secondary star of ΔR L 2 /R L 2 ≃ 1.7 x 10 - 4 is required in order to explain both the observed period change and the modulation of the quiescent brightness previously reported by Ak, Ozkan & Mattei.

A Spiral Structure in the Disk of EX Draconis on the Rise to Outburst Maximum

We report on the R-band eclipse mapping analysis of high-speed photometry of the dwarf nova EX Dra on the rise to the maximum of the 1995 November outburst. The eclipse map shows a one-armed spiral structure of ~180° in azimuth, extending in radius from R 0.2RL1 to 0.43RL1 (where RL1 is the distance from the disk center to the inner Lagrangian point), that contributes about 22% of the total flux of the eclipse map. The spiral structure is stationary in a reference frame corotating with the binary and is stable for a timescale of at least five binary orbits. The comparison of the eclipse maps on the rise and in quiescence suggests that the outbursts of EX Dra may be driven by episodes of enhanced mass transfer from the secondary star. Possible explanations for the nature of the spiral structure are discussed.