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Circumstellar Dust Properties and Periodicity in the Near-IR Light Curves of Seven Symbiotic Miras

Circumstellar dust is an important component of circumstellar medium which dominates infrared spectrum. Determination of dust properties is essential for understanding stellar evolution and interaction of stellar environment with its atmosphere. In this work, dust properties around symbiotic Miras have been determined. Symbiotic binaries with a cool Mira component are characterized by the presence of substantial dust and by the appearance of near-IR obscuration intervals. Publicly available near-IR long-term ground- based photometry in J, H, K, L and M bands from SAAO and Crimean observatory were used in order to analyze 7 symbiotic systems that consist of a Mira cool component and of a hot compact white dwarf. Pulsation periods of Miras were determined by use of Phase dispersion minimization (PDM) and Discrete Fourier transform (DFT) techniques. The same methods have been applied for period determinations of long-term photometric variations which were probably caused by material that was blown from atmospheres of cool components due to their pulsation, and later condensed in the form of surrounding circumstellar dust envelope. Properties of circumstellar dust envelopes around symbiotic Miras have been determined by solving radiative transfer equations by numerical code DUSTY, using the advantage of self-similarity and scale invariance of equations of radiative transfer. Fitting of the near-IR color indices with theoretical values determined from the models of circumstellar regions in symbiotic Mira environments allowed determinations of dust condensation temperature, maximum grain size, optical depth, ratio of condensation radius and cool component radius, terminal dust outflow velocity and mass loss rate. Circumstellar dust around symbiotic Miras is of similar properties, condensation temperatures and grain sizes, which implies similar condensation conditions and processes of dust grain formation around these stars. Condensation temperature of about 1100 K, typical for silicate and carbon dust, has been found. Carbon rich symbiotic Miras show smaller grain sizes (of about 0.5 - 1.0 μm) than oxygen-rich symbiotic Miras with typical grain sizes of up to 4 μm. The study shows that all obscuration events can be explained by variable dust optical depth only, while other dust properties do not change significantly. Obtained pulsation periods of the cool components agree well with other authors, while 5 studied symbiotic Miras show long-term periodic variations which can be explained by possible periodic dust condensation. Active symbiotic Miras, which underwent nova outbursts recently, stimulate the growth of larger dust grains and the condensation of dust starts at greater distances than it is the case for quiescent Miras.

Radiative transfer ; Methods: numerical ; Binaries: symbiotics ; Circumstellar matter ; Late- type ; Winds, outows ; Infrared: stars

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