Multicolour eclipse studies of UU Aquarii — II. The accretion disc

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Abstract
Maximum-entropy eclipse mapping techniques are used to obtain maps of the surface brightness distribution of the UU Aqr accretion disc in its high (V ≃ 13.5 mag) and low (V ≃ 13.8 mag) brightness states. A two-colour diagram of the pixels of the reconstructed disc shows that the inner disc is optically thick, with colours in between the blackbody and main-sequence relationships. The angular scale of the disc and the distance to UU Aqr are estimated from a colour-magnitude diagram by a method similar to cluster main-sequence fitting. We find D = 200 ± 30 pc. Temperatures in the disc range from ∼ 6000 K in the outer regions to ∼16 000 K near the white dwarf at disc centre. The radial temperature profiles in the low-brightness state are consistent with the T∝R− 3/4 law for steady mass accretion at a rate of Ṁ = 10−9.0±0.2 M⊙ yr−1. The radial temperature profiles in the high-brightness state are flatter than the T∝R− 3/4 law. The inferred mass accretion rate in this case is 10−9.2±0.1 M⊙ yr− 1 at R = 0.1RL1 and 10−8.8± 0.1 M⊙ yr−1 at R = 0.3 RL1, where RL1 is the distance from disc centre to the inner Lagrangian point. Together with other observed characteristics, this suggests that UU Aqr is possibly another member of the SW Sex stars. Comparison of narrow-band maps about the H∝ and He i 6678 lines with the N-band map reveals that both lines appear in emission at the inner disc (R≲ 0.2RL1) but disappear into the continuum emission at larger disc radii. The Ha and He i 6678 net emission profiles are noticeably different from the empirical I ∝ R−1.5 law inferred from Doppler tomography, suggesting either that the line-emitting regions on the disc surface are not in Keplerian orbits or that a substantial fraction of the Hot and He i 6678 lines do not arise from the accretion disc. This latter hypothesis is supported by the fact that both lines show significant uneclipsed components of 55 and 20 per cent, respectively. The comparison of eclipse maps of the low and high states reveals that the differences are caused by changes in the structure of the outer parts of the disc, the most noticeable effect being the appearance of a conspicuous red, bright spot at the disc rim. Together with the inferred increase in Ṁ at the outer disc, this result suggests that the observed brightness changes are caused by variations in the mass transfer rate from the companion star.

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