Symbiotic variable star

A Symbiotic Variable star is a variable binary star system in which one star has expanded its outer envelope and is shedding mass quickly, and another hot star (often a white dwarf) is ionizing the gas [ [http://www.daviddarling.info/encyclopedia/S/symbiotic_star.html David Darling site symbiotic star description] ] .

Periodic variations in the light curves of symbiotic stars

" "A group of classical symbiotic binaries exhibits a periodic variation in the optical continuum due to orbital motion. This variation is usually ascribed to a reflection effect. However, we demonstrate that the reflection effect is not the correct interpretation of the light curves for some of those symbiotic stars. Our approach in solving this problem is based on the assumption of the presence of circumstellar matter located mostly between the components of a symbiotic binary. We assume that the observed wave-like variation of the brightness can be produced by a different projection of the circumstellar nebulosity into the line of sight along the orbital cycle. " " - SKOPAL A.

The evolution of the nebular environment in symbiotic systems

" "Colliding stellar winds can be the determining factor for the evolution of the nebular environment in symbiotic systems. They may also be at the origin of jet-like features seen in symbiotic stars. The nebula in symbiotic binaries is primarily due to mass lost by the cool star. However, there are indications that during some periods in the evolution of symbiotic systems both stars suffer mass loss. The two stellar winds possess highly different physical characteristics. We study the evolution of the colliding zone of the two winds in cylindrically symmetrical models in the initial phase after the beginning of the outburst. The numerical hydrodynamical calculations show that already on timescales of years the structure of symbiotic nebulae changes substantially, due to the impact of the colliding winds." " - NUSSBAUMER H.; WALDER R

A new approach to symbiotic stars

" "It is shown with detailed model calculations that a typical outbreak of a symbiotic star can be induced by an increase in the mass loss of the cool star. The onset of that stellar wind will provide a fast increasing target for the photons emitted by the hot star. The cool stellar wind is ionized by the radiation of the hot companion. Depending on the temperature of the hot star, the mass loss, and the stellar separation, the emission of the low- and medium-ionized atoms may originate at various regions between the cool and the hot star. Differences in the combination of mass loss, stellar separation, and radiation temperature may also explain the observationally established division of symbiotic stars into s-type and d-type." " NUSSBAUMER H.; VOGEL M.

Models of symbiotic stars

" "One of the most important features of symbiotic stars is the coexistence of a cool spectral component that is apparently very similar to the spectrum of a cool giant, with at least one hot continuum, and emission lines from very different stages of ionization. The cool component dominates the infrared spectrum of S-type symbiotics; it tends to be veiled in this wavelength range by what appears to be excess emission in D-type symbiotics, this excess usually being attributed to circumstellar dust. The hot continuum (or continua) dominates the ultraviolet. X-rays have sometimes also been observed. Another important feature of symbiotic stars that needs to be explained is the variability. Different forms occur, some variability being periodic. This type of variability can, in a few cases, strongly suggest the presence of eclipses of a binary system. One of the most characteristic forms of variability is that characterizing the active phases. This basic form of variation is traditionally associated in the optical with the veiling of the cool spectrum and the disappearance of high-ionization emission lines, the latter progressively appearing (in classical cases, reappearing) later. Such spectral changes recall those of novae, but spectroscopic signatures of the high-ejection velocities observed for novae are not usually detected in symbiotic stars. However, the light curves of the 'symbiotic nova' subclass recall those of novae. We may also mention in this connection that radio observations (or, in a few cases, optical observations) of nebulae indicate ejection from symbiotic stars, with deviations from spherical symmetry. " "- Friedjung, Michael

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