A Model of the Dust Envelope of the Carbon Mira Star V CrB from Photometry, Infrared Spectroscopy, and Speckle Polarimetry

UBVJHKLM photometry for the carbon Mira star V CrB are presented. The infrared observations were carried out in the time interval 1989–2018, while the U , B , and V data were obtained in 2001–2014. The light and color curves are analyzed. The pulsation period of V CrB has been found to be 355d.2 in the infrared JHKLM bands and 352^d for the optical BV band. In the JHK bands, apart from periodic pulsations, there are distinct sinusoidal variations in the average brightness level with a characteristic period of ∼8300 days. Color–magnitude relationships have been revealed for the infrared and optical bands. The phase curves exhibit the wavelength dependence of the brightness variability amplitude. The light curves for various bands and colors are discussed. We have constructed the model of a spherically symmetric circumstellar dust envelope that allows the observed spectral energy distribution at both maximum and minimum light to be reproduced equally well (within the model assumptions) and is consistent with the observations of V CrB by differential speckle polarimetry. The model is characterized by the following parameters: the optical depth is τ_K=0.33 , the inner and outer radii of the envelope are 8 and 40 000 AU, respectively. The envelope contains spherical carbon dust grains ( 3/4 by mass) and silicon carbide dust grains. Dust grains with a radius of 0.5 μ m account for 90% of the envelope mass. The remaining 10% of the mass is accounted for by finer dust with a grain radius of 0.1 μ m. Based on the observational data, we have estimated the bolometric flux from V CrB: 2.6× 10^-7 and 5.1× 10^-7 erg cm ^-2 s ^-1 at minimum and maximum light, respectively. The effective temperature of the star is T_max=3000 K at maximum light and T_min=2400 K at minimum light.