HST spatially-resolved spectra of the accretion disc and gas stream of the nova-like variable UX Ursae Majoris

摘要

Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtainedwith the HST/FOS on 1994 August and November is analyzed with eclipse mappingtechniques to produce spatially resolved spectra of its accretion disc and gasstream as a function of distance from disc centre. The inner accretion disc is characterized by a blue continuum filled withabsorption bands and lines which cross over to emission with increasing discradius, similar to that reported by Rutten et al (1994) at optical wavelengths.The comparison of spatially resolved spectra at different azimuths reveals asignificant asymmetry in the disc emission at UV wavelengths, with the discside closest to the secondary star showing pronounced absorption by an `ironcurtain' and a Balmer jump in absorption. These results suggest the existenceof an absorbing ring of cold gas whose density and/or vertical scale increasewith disc radius. The spectrum of the infalling gas stream is noticeablydifferent from the disc spectrum at the same radius suggesting that gasoverflows through the impact point at disc rim and continues along the streamtrajectory, producing distinct emission down to 0.1 RL1. The radial temperature profiles of the continuum maps are well described by asteady-state disc model in the inner and intermediate disc regions. There isevidence of an increase in the mass accretion rate from August to November(from Mdot= 10^{-8.3 +/- 0.1} to 10^{-8.1 +/- 0.1} Msun/yr), in accordance withthe observed increase in brightness. Since the UX UMa disc seems to be in ahigh mass accretion, high-viscosity regime in both epochs, this result suggeststhat the mass transfer rate of UX UMa varies substantially (~ 50 per cent) ontime scales of a few months.