1999A&A...347..478G

We present the first extensive analysis of the ultraviolet observations with the IUE mission of the high velocity symbiotic system AG Draconis, covering the period June 1979-February 1996 which included three active phases of the system with six light maxima. The low resolution IUE line and continuum fluxes are compared with optical observations and with archival X-ray data. The analysis of the IUE observations near minimum (quiescence) led us to find that during the orbital motion the hot WD component is not eclipsed, in agreement with a non large inclination of the binary orbit. The larger modulation of the NV, CIV, HeII, and Oi lines with respect to the intercombination lines may indicate that the former are formed in a region near the line connecting the two stars, probably slightly receding, while the latter lines originate in an extended ionized nebula surrounding the white dwarf. Large orbit-to-orbit variation are probably associated with fluctuation of the K-star wind density. From the HeII line we determine for the WD during quiescence a Zanstra temperature of 109600±5400°K, implying, at a distance of 2.5kpc, a radius of 0.08±0.01R_☉, and a luminosity of 900±200L_☉. During the different outbursts AG Dra displayed a variety of behaviours. According to the strength of the HeII/FUV continuum ratio we have identified cool and hot outbursts. In fact, during the ``minor'' 1985-1986 outbursts the peak fluxes of the high ionization emission lines was comparable with those during the 1980-83 and 1994-95 major outbursts. The white dwarf temperature decreased to about 90000°K during the ``cool'' outbursts, while it increased to 120000-130000°K during the 1985-86 ``hot'' outbursts. The behaviour during the major (``cool'') outbursts is explained by expansion and cooling of the white dwarf atmosphere, which explains the marked anticorrelation between optical/UV and X-ray fluxes. The minimum X-ray flux observed also during the minor (``hot'') outbursts might be attributed to the increased opacity of the WD envelope and wind to photons shortward the N⁺⁴ ionization limit. We also note that the beginning of the last activity phase of AG Dra was marked by the temporary appearance in July 1994 of strong P Cygni absorptions in the high ionization resonance lines with quite high terminal velocities of at least 700km/s.