The spectroscopic orbits and other parameters of the symbiotic binary FN Sgr

¹ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina e-mail: ebrandi@fcaglp.unlp.edu.ar
² Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CIC), Argentina
³ Copernicus Astronomical Center, Warsaw, Poland
⁴ Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
⁵ Department of Physics and Astronomy, Northwestern University, 2131 Sheridan Road, Evanston, IL 60208 – 2900, USA
⁶ Observatório Nacional – Rua Gen. José Cristino, 77, Sao Cristovao – CEP 20921-400, Rio de Janeiro, Brazil

Received: 15 December 2004
Accepted: 28 April 2005

Abstract

We present a study of the eclipsing symbiotic binary FN Sgr with a period of 568.3 days determined photometrically and confirmed spectroscopically. The hot component underwent a 2.5 mag eruption covered by most of our spectroscopic observations. In particular, we have determined for the first time spectroscopic orbits based on the radial velocity curves for both components. A set of blue absorption lines resembling an A-F type star is present in all our spectra and they seem to be associated with the hot component. Based on the light curve, we derive the red giant's radius () and the orbital inclination (i = 80°). We find that FN Sgr is similar to other S-type symbiotic binaries, composed by an M 5-type giant () and a hot white dwarf (, ) with a binary separation of ~1.6 AU. The red giant is just filling its Roche lobe and a geometrically and optically thick accretion disk is likely to be present around the low-mass accretor. The evolution of T_h and L_h along the active phase argues in favour of accretion disk instabilities similar to those of Z And. We have also studied spectral changes and photometric variations as a function of both the hot component activity and the orbital motion.