Astron. Astrophys. 347, 478-493 (1999)

1. Introduction

The high-velocity, high-galactic latitude object AG Dra (BD+67^o922) is the brightest symbiotic system in the ultraviolet which made it a good target for the IUE experiment. In addition, during the lifetime of IUE AG Dra underwent six light maxima (outbursts ) in the visual which have been a unique opportunity for monitoring in the ultraviolet many outbursts of the same object. The IUE observations of AG Dra started in June 1979 and continued until January 1996, and have therefore covered a very long period of the stellar activity. Because of the largely varied activity of this object during this period, the IUE database is representing at the moment the unique and best way to study the activity of this interacting variable.

AG Dra is also known for being the most intense X-ray source among symbiotic stars (Anderson et al. 1981), and one of the best representatives of the category of the supersoft X-ray objects (Greiner 1996). Since the early HEAO-2 detection (Anderson et al. 1981), the star has been also observed with EXOSAT and ROSAT satellites. Actually, it was good luck that AG Dra underwent two minor outbursts (in 1985 and 1986) just during the lifetime of EXOSAT, which gave us the opportunity of observing for the first time the outburst of a symbiotic star in X-rays (Viotti et al. 1995). More recently, the ROSAT satellite monitored AG Dra during a long period of quiescence (1990-1993), and followed in greater detail the 1994 and 1995 outbursts (Greiner et al. 1997). ROSAT observations have been performed until 1998, and will hopefully give more interesting results on this X-ray symbiotic object. AG Dra was also observed by the HIPPARCOS satellite which has provided a lower limit of about 1 kpc to its distance, in agreement for a luminosity of the K star probably larger than that of a normal K-giant star (Viotti et al. 1997). It was also found that the astrometric position of AG Dra is coincident with the barycentre of the main radio source observed by Torbett & Campbell (1987), within the uncertainty of the radio observations.

In the previous papers we have analyzed the ultraviolet spectrum of AG Dra during its first major outburst (Viotti et al. 1983, Paper I), and have investigated the UV variation for the period from 1979 to 1983 which brackets the 1980-1983 active phase (Viotti et al. 1984, Paper II). More recently, we have studied the ultraviolet (IUE) and X-ray (ROSAT) emission of AG Dra during the 1994 and 1995 outbursts and during the previous quiescent phase (Greiner et al. 1997). We found that the hot component of the system is a white dwarf, which during its quiescent phase is burning hydrogen-rich matter on its surface at a rate of 3.2 y^-1 (for a distance of 2.5 kpc), which provides a luminosity of about 2500  and a surface temperature of 85000^o K. The high accretion rate might be provided by a Roche lobe filling cool companion, though other mechanisms can be considered. The remarkable decrease of the X-ray flux during the 1994-95 optical maxima is very likely due to a temperature decrease of the hot component, as the result of a mass transfer increase causing an expansion of the compact star to about twice its original size. Extensive studies of the IUE observations of AG Dra have also been made among others, by Lutz et al. (1987), Kafatos et al. (1993) and Mikolajewska et al. (1995).

In this paper we make the first complete analysis of the full set of IUE low resolution images of AG Dra collected between June 1979 and February 1996. In addition, IUE data are complemented by optical spectrophotometry made at the Asiago Astrophysical Observatory during 1985-86 and 1993-97, and compared with the results of the X-ray observations with EXOSAT (1985-86) and ROSAT (1990-97) satellites. We finally discuss the results in the light of the interacting binary model of AG Dra.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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