Spatially resolved spectra of the accretion disc of the nova-like variable UU Aquarii

Abstract

A B S T R A C TTime-resolved spectroscopy of the nova-like variable UU Aquarii is analysed 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 A Ê . 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 H i and He i lines appear as deep, narrow absorption features in the inner disc regions, transitioning to emission with P Cyg profiles for intermediate and large disc radii. The spectrum of the uneclipsed component has strong H i and He i emission lines plus a Balmer jump in emission, and is explained as optically thin emission from a vertically extended disc chromosphere 1 wind. Most of the line emission probably arises from the wind. The spatially resolved spectra also suggest the existence of gas stream`disc-skimming' overflow in UU Aqr, which can be seen down to R . 0X2R L1 X The comparison of our eclipse maps with those of Baptista, Steiner & Horne 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.

What can we learn from accretion disc eclipse mapping experiments?

“…The lines and optically thin continuum emission are most probably emitted in a vertically extended disc chromosphere + wind (e.g., Baptista et al 2000b). The uneclipsed spectrum of UX UMa at long wavelengths is dominated by a late-type spectrum that matches the expected contribution from the secondary star (Rutten et al 1994).…”

The impact of accretion disc winds on the optical spectra of cataclysmic variables

“…In fact, multiple observations of eclipsing NLs show that the Hα line is only moderately eclipsed compared to the continuum (e.g. Baptista et al 2000;Groot et al 2004; see also section 5.4), implying a significant vertical extent for the line-forming region. This type of model should therefore not be ruled out a priori, but this specific model was not adopted as our optically optimized model due to its unrealistically high continuum level in eclipse.…”

“…There, the single-peaked lines are often only weakly eclipsed, and a significant fraction of the line flux remains visible even near mid-eclipse (e.g. Baptista et al 2000;Groot et al 2004). This points to line formation in a spatially extended region, such as a disk wind (see Fig.…”

Eclipse Mapping: Astrotomography of Accretion Discs

“…These high massaccretion (∼ 10 −8 − 10 −9 M ⊙ yr −1 ) discs seem hot and optically thick in their inner regions and cool and optically thin in their outer parts. The spectrum of the infalling gas stream in UX UMa [10] and UU Aqr [13] is noticeably different from the disc spectrum at the same radius, suggesting the existence of gas stream "discskimming" overflow that can be seen down to R ≃ (0.1 − 0.2) R L1 .…”