HST SPATIALLY-RESOLVED SPECTRA OF THE ACCRETION DISC AND GAS-STREAM OF THE NOVA-LIKE VARIABLE UX-URSAE-MAJORIS

Time-resolved eclipse spectroscopy of the nova-like variable UXUMa obt ained with the HST/FOS on 1994 August and November is analysed with ec lipse mapping techniques to produce spatially resolved spectra of its accretion disc and gas stream as a function of distance from the disc centre. The inner accretion disc is characterized by a blue continuum filled with absorption bands and lines, which cross over to emission w ith increasing disc radius, similar to that reported by Rutten et al. at optical wavelengths. The comparison of spatially resolved spectra a t different azimuths reveals a significant asymmetry in the disc emiss ion at ultraviolet (UV) wavelengths, with the disc side closest to the secondary star showing pronounced absorption by an 'iron curtain' and a Balmer jump in absorption. These results suggest the existence of a n absorbing ring of cold gas whose density and/or vertical scale incre ase with disc radius. The spectrum of the infalling gas stream is noti ceably different from the disc spectrum at the same radius suggesting that gas overflows through the impactpoint at the disc rim and continu es along the stream trajectory, producing distinct emission down to 0. 1 R-L1. The spectrum of the uneclipsed Light shows prominent emission lines of Ly alpha; N V lambda 1241, Si IV lambda 1400, C IV lambda 155 0, He II lambda 1640, and Mg II lambda 2800, and a UV continuum rising towards longer wavelengths. The Balmer jump appears clearly in emissi on indicating that the uneclipsed light has an important contribution from optically thin gas. The lines and optically thin continuum emissi on are most probably emitted in a vertically extended disc chromospher e + wind. The radial temperature profiles of the continuum maps are we ll described by a steady-state disc model in the inner and intermediat e disc regions (R less than or equal to 0.3 R-L1) There is evidence of an increase in the mass accretion rate from August to November (from (M) over dot = 10(-8.3+/-0.1) to 10(-8.1+/-0.1) M. yr(-1)), in accorda nce with the observed increase in brightness. Since the UX UMa disc se ems to be in a high mass accretion, high-viscosity regime in both epoc hs, this result suggests that the mass transfer rate of UX UMa varies substantially (similar or equal to 50 per cent) on time-scales of a fe w months. It is suggested that the reason for the discrepancies betwee n the prediction of the standard disc model and observations is not an inadequate treatment of radiative transfer in the disc atmosphere, bu t rather the presence of additional important sources of light in the system besides the accretion disc (e.g. optically thin continuum emiss ion from the disc wind and possible absorption by circumstellar cool g as).