A study of the absorption lines from the donor star in the exotic cataclysmic variable AE Aquarii

Abstract

We estimate the fundamental system parameters (e.g. mass ratio, inclination, stellar masses) of the unusual cataclysmic variable AE Aquarii by characterizing the behaviour of the mass-donor star. Our analysis focuses on the absorption lines from the donor star, avoiding biases induced by use of the poorly understood emission lines. Using simultaneous high-speed photometry and high-resolution H α spectroscopy, we observed weak absorption lines in AE Aquarii’s spectrum. By modelling these lines we estimate the spectral type of the mass-donor star (K3–K5), its orbital and rotation velocities (K1 ~ 138 ± 6 km s–1, K2 = 157.9 ± 0.8 km s–1; Vrot sin i = 85–108 km s–1), and its contribution to the total flux of the system (69–94 per cent). From K2 and Vrot sin i we derive the mass ratio 0.58 < q ≡ M2/M1 < 0.89, the range depending upon the spectral type and limb darkening. By using K2 and the amplitude of the pulse arrival-time delay curve of Eracleous et al., we estimate the mass ratio of the binary system (q = 0.646 ± 0.013). We also observe variations in the absorption-line flux deficit, and, if interpreted as ellipsoidal variations, their amplitudes suggest an inclination i = 49° ± 15°. However, the deviations from the expected ellipsoidal variations are significant, suggesting a non-uniform distribution of absorption-line strength across the donor star (perhaps indicative of star-spot regions). Realistic probability distributions for the inclination and the stellar masses are derived using a Monte Carlo simulator assuming M1 < 1.4 Mʘ, but with no assumptions about the mass of the donor star. We find M1 = 0.89 ± 0.23 Mʘ, M2 = 0.57 ± 0.15 Mʘ, and further constrain i to 55° ± 7°. We also report the serendipitous discovery of a (previously unknown) eclipsing variable star near AE Aquarii