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 Th and Lh 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.
Highlights
Symbiotic stars are interacting binaries in which an evolved giant star transfers material to a much hotter compact companion
In some cases a red giant is replaced by a yellow giant or a carbon star, and a white dwarf by a main sequence or a neutron star
In selecting the southern S-type symbiotic star FN Sgr, we chose to use visual photometry from the Variable Star Section Circulars of The Royal Astronomical Society of New Zealand (RASNZ) over 30 years (1972–2002) and high resolution spectra obtained with the 2.15 m telescope of Complejo Astronómico El Leoncito (CASLEO) (San Juan, Argentina) during the period 1990–2002
Summary
Symbiotic stars are interacting binaries in which an evolved giant star transfers material to a much hotter compact companion. Most symbiotic stars (∼80%) contain a normal giant star, and based on their near-IR colors (showing presence of stellar photospheres of Teff ∼ 3000–4000 K) they are classified S-type systems (stellar). The remaining symbiotics contain a Mira variable, and their near-IR colors give temperatures of ∼1000 K indicating the presence of warm dust shells, and they are classified as D-type (dusty). In selecting the southern S-type symbiotic star FN Sgr, we chose to use visual photometry from the Variable Star Section Circulars of The Royal Astronomical Society of New Zealand (RASNZ) over 30 years (1972–2002) and high resolution spectra obtained with the 2.15 m telescope of CASLEO (San Juan, Argentina) during the period 1990–2002
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