Spatially-resolved spectra of the accretion disc of the novalike UU Aquarii

Abstract

Time-resolved spectroscopy of the novalike variable UU Aquarii is analyzed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disc and gas stream as a function of distance from disc centre in the range 3600-6900 Å. The spatially-resolved spectra show that the continuum emission becomes progressively fainter and redder for increasing disc radius -- reflecting the radial temperature gradient -- and reveal that the HI and HeI lines appear as deep, narrow absorption features in the inner disc regions transitioning to emission with P Cygni profiles for intermediate and large disc radii. The spectrum of the uneclipsed component has strong HI and HeI emission lines plus a Balmer jump in emission and is explained as optically thin emission from a vertically extended disc chromosphere + wind. Most of the line emission probably arises from the wind. The spatially-resolved spectra also suggest the existence of gas stream ``disk-skimming'' overflow in UU Aqr, which can be seen down to R \simeq 0.2 R_{L1}. The comparison of our eclipse maps with those of Baptista, Steiner & Horne (1996) suggests that the asymmetric structure in the outer disc previously identified as the bright spot may be the signature of an elliptical disc similar to those possibly present in SU UMa stars during superoutbursts.