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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
R. González-Riestra 1,
R. Viotti 2,
T. Iijima 3 and
J. Greiner 4
Received 8 April 1999 / Accepted 29 April 1999
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 N V , C IV , He II , and O I 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 He II line we determine for the WD during quiescence a Zanstra temperature of 1096005400o K, implying, at a distance of 2.5 kpc, a radius of 0.080.01 , and a luminosity of 900200 .
During the different outbursts AG Dra displayed a variety of behaviours. According 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 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 90000o K during the "cool" outbursts, while it increased to 120000-130000o 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 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 700 km s-1.
Key words: stars: binaries: symbiotic stars: individual: AG Dra stars: Population II stars: white dwarfs ultraviolet: stars X-rays: stars
* Based on observations made with the International Ultraviolet Explorer collected at the Villafranca Satellite Tracking Station and retrieved from the IUE-INES Archive, on ROSAT observations, and on optical observations collected at the Asiago Observatory of the Padova Astronomical Observatory.
Send offprint requests to: R. González-Riestra
Correspondence to: R. González-Riestra
Online publication: June 30, 1999