## 4. The method of calculation and resultsHaving the fluxes of the components of the binary AG Dra we can compose a system of two equations with two unknown quantities - its distance and the radius of its primary. The effective temperature of this component was estimated by Smith et al. (1996) and amounts to be 4300 K. The cool giant's continuum can be fitted with a function giving the energy distribution of Boo, since, according to Griffin & Lynas-Grey (1999) the effective temperature of this star is 429030 K the same as that of AG Dra. This function consists of two parts, the first part is related to the radiation of a blackbody with a temperature of 4300 K and the second one to the energy distribution of Boo. At the wavelength of the U band the second part has a value of . Then the first equation is: where Let us now consider the second equation. We suppose that the
emitting circumbinary nebula is formed by a wind with spherical
symmetry and a constant velocity and has an inner boundary the radius
of the giant, as only a small portion occulted by it is not an ionized
region. To give an expression to the flux of the nebula the state of
ionization of helium is need to be known. We calculated the ratio of
the emission measures of the neutral and ionized helium, allowing that
the lines of the are pure
recombination lines. That is really the case when the electron
temperature is 15 000 K (Mikolajewska et al. 1995). We used visual
line fluxes of and the flux of the
He II 4686 line from the paper of Gonzalez-Riestra et
al. (1999) at a phase, close to the maximum light. Since these data
are related to quiescence of the system, we used the sum of the fluxes
of the narrow and broad emission components of the line
He II 4686. In this way we obtained a ratio
HeHe where Our estimates can be compared with the estimates, based on other methods. For example Smith et al. (1996) studying the IR absorption spectrum and performing an abundance analysis of the giant, concluded that its bolometric magnitude and radius are in the intervals and 50 . Mikolajewska et al. (1995) came to the conclusion that the distance is 2.5 kpc. On the other hand the observational data of HIPPARCOS satellite provided a lower limit to the distance of about 1 kpc (Viotti et al. 1997). © European Southern Observatory (ESO) 2000 Online publication: January 29, 2001 |