2000A&A...353.1016H

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 671nm/6nm and 714nm/6nm (strong TiO absorption band), 700nm/6nm (moderate TiO absorption), and 1045nm/9nm (continuum). The reconstructed images show that the average uniform-disk diameters of R Cas are 43.6mas±2.0mas at 671nm, 49.2mas±2.0mas at 714nm, 37.2mas±2.0mas at 700nm, and 29.9mas±3.0mas at 1045nm. In the 671nm, 714nm and 700nm 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 671nm the size (elliptical uniform disk fit) of the elongated R Cas disk is 51.0masx35.6mas. The 1045nm 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 τ_λ=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 τ_Ross=1 radius) of R Cas of 377R_☉±60R_☉ (32.9mas±3.3mas). The derived photospheric radius and the large period (∼430days) suggest first overtone pulsation for R Cas. We also determined the effective temperature of R Cas at near-minimum phase to 1880K±130K and propose T_eff∼1900K for spectral type M10 in the temperature calibration of very late M giant spectral classes.