engleski

Modelling of circumstellar dust around symbiotic Mira HM Sge

The Mira component of symbiotic binaries is characterized by a presence of substantial dust and by appearance of a strong near-IR obscuration. Determination of the properties of circumstellar dust and their temporal evolution is an essential step in understanding the evolution of symbiotic Miras, mutual interactions between the components and the circumstellar matter, influence of stellar winds and mass transfer mechanism. HM Sge is one of the best studied symbiotic Miras which underwent a nova eruption in 1976. The spectral energy distribution in the near- and mid-IR spectral range (1-15 microns) of HM Sge was obtained from the publicly available near- and mid-IR observations. We used also the mid-IR spectra from IRAS and ISO, as well as the long-term ground-based near-IR photometry from the Crimean observatory. In order to further constrain the properties and distribution geometry of the dust, interferometric measurements from MIDI VLTI have been included in our research. The DUSTY code was used to solve the radiative transfer through the dust and to determine the circumstellar dust properties of the inner dust regions around the Mira component of HM Sge. The near- and mid-IR SEDs and interferometric visibilities were simultaneously fitted with the theoretical DUSTY spectra and visibilities by use of the chi2– test. Dust temperature, maximum grain size, dust density distribution, mass-loss rate, terminal wind velocity and optical depth have been determined. We have shown that a single silicate dust shell with a sublimation temperature of around 1200 K can explain both spectra and interferometric visibilities. The near-IR obscuration can be fully understood by increased mass loss and by condensation of dust near the sublimation radius, leading to the observed increase in optical depth.

binaries: symbiotic; circumstellar matter; radiative transfer; stars: AGB and post-AGB; infrared: stars; method: interferometry

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