Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...347..478g&link_type=abstract
Astronomy and Astrophysics, v.347, p.478-493 (1999)
Astronomy and Astrophysics
Astrophysics
22
Stars: Binaries: Symbiotic, Stars: Individual: Ag Dra, Stars: Population Ii, Stars: White Dwarfs, Ultraviolet: Stars, X-Rays: Stars
Scientific paper
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 109600+/-5400çK, implying, at a distance of 2.5 kpc, a radius of 0.08+/-0.01 Rsun, and a luminosity of 900+/-200 Lsun. 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 90000çK during the ``cool'' outbursts, while it increased to 120000-130000çK 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 700 km s(-1) . 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.
González-Riestra Rosario
Greiner Jochen
Iijima Toru
Viotti Roberto
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