Multi-wavelength bispectrum speckle interferometry of R Cas and comparison of the observations with Mira star models

We present diffraction-limited (30 mas) bispectrum speckle interferometry observations of the Mira star R Cas with the Russian 6 m SAO telescope. The speckle interferograms were recorded through narrow-band interference filters with centre wavelength/bandwidth of 671 nm/6 nm and 714 nm/6 nm (strong TiO absorption band), 700 nm/6 nm (moderate TiO absorption), and 1045 nm/9 nm (continuum). The reconstructed images show that the average uniform-disk diameters of R Cas are 43.6 mas ± 2.0 mas at 671 nm, 49.2 mas ± 2.0 mas at 714 nm, 37.2 mas ±2.0 mas at 700 nm, and 29.9 mas ±3.0 mas at 1045 nm. In the 671 nm, 714 nm and 700 nm images the disks of R Cas are non-uniform and elongated with position angles of the long axis of 52◦, 57◦ and 54◦, and axis ratios of 0.70, 0.76 and 0.87, respectively. For example, at 671 nm the size (elliptical uniform disk fit) of the elongated R Cas disk is 51.0 mas × 35.6 mas. The 1045 nm image shows no significant asymmetry. We compare our observations with theoretical Mira star models and systematically check the capacity of monochromatic diameter ratios and linear diameters for discriminating between model representations of the observed star. Monochromatic τλ=1 radii were derived from the observed visibilities by application of model-predicted center-to-limb variations of the intensity. Adopting the HIPPARCOS parallax we obtained a photospheric radius (Rosseland τRoss=1 radius) of R Cas of 377 R ± 60 R (32.9 mas ± 3.3 mas). The derived photospheric radius and the large period (∼ 430 days) suggest first overtone pulsation for R Cas. We also determined the effective temperature of R Cas at near-minimum phase to 1880 K ± 130 K and propose Teff ∼ 1900 K for spectral type M10 in the temperature calibration of very late M giant spectral classes.