IUE observations of the high-velocity symbiotic star AG Draconis - III. A compendium of 17 years of UV monitoring, and comparison with optical and X-ray observations

We present the first extensive analysis of the ultraviolet observations wit h the IUE mission of the high velocity symbiotic system AG Draconis, coveri ng the period June 1979-February 1996 which included three active phases of the system with six light maxima. The low resolution IUE line and continuu m fluxes are compared with optical observations and with archival X-ray dat a. 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 m odulation of the Nv, C Iv, He II, and O I lines with respect to the interco mbination lines may indicate that the former are formed in a region near th e line connecting the two stars, probably slightly receding, while the latt er lines originate in an extended ionized nebula surrounding the white dwar f. Large orbit-to-orbit variation are probably associated with fluctuation of the K-star wind density. From the He II line we determine for the WD dur ing quiescence a Zanstra temperature of 109600+5400 degrees K, implying, at a distance of 2.5 kpc, a radius of 0.08+/-0.01 R., and a luminosity of 900 +/-200 L.. During the different outbursts AG Dra displayed a variety of behaviours. Ac cording to the strength of the He II/FUV continuum ratio we have identified cool and hot outbursts. In fact, during the "minor" 1985-1986 outbursts th e peak fluxes of the high ionization emission lines was comparable with tho se during the 1980-83 and 1994-95 major outbursts. The white dwarf temperat ure decreased to about 90000 degrees K during the "cool" outbursts, while i t increased to 120000-130000 degrees 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 antic orrelation between optical/UV and X-ray fluxes. The minimum X-ray flux obse rved also during the minor ("hot") outbursts might be attributed to the inc reased opacity of the WD envelope and wind to photons shortward the N+4 ion ization limit. We also note that the beginning of the last activity phase o f AG Bra was marked by the temporary appearance in July 1994 of strong P Cy gni absorptions in the high ionization resonance lines with quite high term inal velocities of at least 700 km s(-1).