UV observations of TT Arietis and the magnetic rotator hypothesis

Abstract

TT Arietis, one of the brightest cataclysmic variables, m_v∼10.2→11.0, is usually classed as a novalike variable, but after a recent dip in brightness to m_v∼14.5, Krautter et al.¹ suggested it to be a dwarf nova in an extended high state. Photometric light curves^2–4 generally show a sinusoidal light variation with a period of 0.1329 days and no eclipses⁵. Cowley et al.⁶ have measured radial velocity curves and find a period of 0.13755 days. We have made⁷ IR measurements of the system from which we deduce a distance for TT Ari of 100±20 pc based on a colour change for the system from V to K of +0.4 while the system is faint to V − K = −0.17 when brighter. The red colour is caused by dilution of the primary 2.2-µm light by the light of the secondary, giving m_K = 11.75^+0.45_−0.25 for the secondary star; Bailey's method⁸ then leads to the distance. The sinusoidal light variations have generally been interpreted as due to a hot spot on the edge of an accretion disk. Since good quality IUE spectra can be made with exposure times of about 15 min, we decided to observe as many IUE spectra as possible round the binary period with a view to studying the behaviour of the hot spot at UV wavelengths. We now report that the variations at these and X-ray wavelengths round the orbital cycle rule out a hot spot/accretion disk model for TT Ari. Instead we propose that it has a magnetized white dwarf primary rotating with the photometric period of 0.1329 days.