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Astron. Astrophys. 364, 557-562 (2000)

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1. Introduction

The photometric behaviour of many symbiotic stars is characterized by an alternance of quiescent periods and active phases. In a number of cases cyclic light variations are observed in the quiescent periods which could be caused by occultation of a hot stellar object, an accretion disk and/or a part of the circumbinary gas component. That is why symbiotic stars are interpreted as interacting binary systems possessing dense gaseous nebulae, which are generated during quiescence from the stellar wind of their primary components being in the most cases normal cool giants or Miras. The emission of the symbiotic nebula in many cases predominates in the wavelength region of the U photometric system.

The star AG Dra (BD +67o922) belongs to the group of yellow symbiotics and consists of a cool primary of the spectral type K, probably more luminous than a normal class III giant (Huang et al. 1994; Mikolajewska et al 1995; Greiner et al. 1997) and a hot compact companion, accreting matter from the primary. Its photometric history includes many active phases separated by quiescent periods.

During quiescence the U light of AG Dra displays a [FORMULA] periodicity with an amplitude of about one magnitude. This periodicity is related to the orbital motion of the binary as confirmed by the radial velocity curve of its cool component (Garcia & Kenyon 1988; Mikolajewska et al. 1995; Smith et al. 1996). Important characteristics of the U light curve are the changes in the phases and shapes of the orbital maxima as well as the variations by more than 20% of their fluxes (Hric et al. 1993, 1994; Friedjung et al. 1998). The decrease of the flux at the time of the orbital minimum is supposed to be caused by an occultation of a bright region that surrounds the hot companion (Mikolajewska et al. 1995; Friedjung et al. 1998). It was proposed as well (Friedjung et al. 1998; Gonzalez-Riestra et al. 1999) that the flux variations of the orbital maxima are due to variation of the giant's wind.

The quiescent B and V light variations of AG Dra with a period of about [FORMULA] and an amplitude of [FORMULA] are probably caused by an intrinsic variability of the cool giant, whose radiation becomes more important at longer wavelengths (Bastian 1998; Friedjung et al. 1998). The continuum of the giant also contributes to the U flux, but its variations at the time of the orbital maximum are determined primarily by changes of the emissivity of the nebula, since they are significantly larger than [FORMULA] up to about [FORMULA].

Many times AG Dra has been in a state of increased activity (after 1936, 1951, 1966, 1980 and 1994), characterized by one or more light maxima. The reason for this activity is probably related to the nature of its cool component, which drives the eruptions of the hot companion (Friedjung 1997; Greiner et al. 1997; Galis et al. 1999). These eruptions are induced by variations of the accretion rate which are due to variations of the mass-loss rate of the giant rather than to an elliptical motion, since its radial velocity curve shows no sign of a measurable eccentricity (Mikolajewska et al. 1995; Smith et al. 1996; Galis et al. 1999). Consequently the investigation of the reasons for the brightenings of AG Dra is turned to the study of the nature of its cool component. One task, discussed in this study, is to estimate its absolute size, which from its side is related to obtaining the distance to the system.

At the present time there is no good knowledge of some of the fundamental parameters of the system AG Dra like the distance, radius and luminosity of the cool component (Friedjung et al. 1998; Gonzalez-Riestra et al. 1999; Galis et al. 1999). To derive the radius and the distance we used the radiation at the wavelength of the U photometric system, as the continuum fluxes of the circumbinary nebula and the giant are of the same order at this wavelength. The circumbinary nebula is optically thin in the U region providing possibility its whole continuum emission to be observed. It is considered as formed by the giant's wind. If its ionized portion extends to the giant we will be able to compose one system of two equations with two unknown quantities - the distance to the system and the radius of the cool giant using the nebular and the giant's emission. Moreover the U photometric variations give possibility to make some conclusions about the emitting region(s) of the nebula.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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