Astron. Astrophys. 344, 607-613 (1999)

1. Introduction

It is well known that symbiotic stars are binary systems where eclipse effects are best seen in the spectral range covered by the IUE satellite. During the eclipse both line flux and continuum attenuation are observable as the hot companion revolves around the cool giant. The S-type symbiotics, those that have short binary periods, (1-3 years), are best suited for an analysis of eclipse effects. In addition to line and flux continuum attenuation, there is also Rayleigh scattering that introduces a strong wavelength dependent flux reduction immediately preceding and following an eclipse. It is commonly accepted that Rayleigh scattering occurs in the extended atmosphere of the cool giant, which attenuates the continuum radiation of the hot component in the far-UV range. Previous analysis of the eclipses where effects of the Rayleigh scattering are seen were done by Vogel (1991) (hereafter V91) and Vogel et al. (1992) for EG And; Pereira et al. (1995) (hereafter PVN95) for SY Mus and Pereira & Landaberry (1996) and Gonzalez-Riestra et al. (1990) for BF Cyg. The measurement of the phase dependent column densities due to Rayleigh scattering (which accounts typically for values between 10²⁰-10²⁴ cm^-2), thus probe the density structure of the giant's wind.

In this work we concentrate on the eclipse aspect of the symbiotic phenomenon above mentioned, in order to derive, through the variation of the column density of Rayleigh scattering the most suitable empirical velocity law for the giant's cool wind. For the symbiotic star SY Mus our analysis is based on acquisition of several UV spectra obtained by the IUE satellite in the first semester of 1996. All of the data were obtained close to eclipse, just before and after the shortwave continuum attenuation. In the first part of this paper we derive the flux (emission line and continuum) phase relation and in the second part we study the structure of the wind by using velocity laws for the cool wind obtained from inversion techniques. This method was first successfully applied to EG And (V91) and a more refined approach was subsequently done by Knill et al. (1993), hereafter KDV93.

In a previous paper PVN95 analyzed all the SY Mus IUE data up to August 1992. In that work the authors derived the basic properties of the hot component (R=0.12 , T=105 000 K and L=1600), the nebulae as defined by the parameter =3.3, the emission lines and continuum flux-phase relation and the ephemeris of the system which we shall adopt here (see Eq. (1)). They also derived the reddening, the nebular densities as well as the attenuation factor of the eclipse as defined by 1-Q^c(). We shall return to this point later in Sect. 3.

© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999
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