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

We present diffraction-limited (30 mas) bispectrum speckle interferometry o bservations of the Mira star R Cas with the Russian 6 m SAO telescope. The speckle interferograms were recorded through narrow-band interference filte rs 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 Car; are non-uni form and elongated with position angles of the long axis of 52 degrees, 57 degrees and 54 degrees, and axis ratios of 0.70, 0.76 and 0.87, respectivel y. For example, at 671 nm die size (elliptical uniform disk fit) of the elo ngated R Cas disk is 51.0 mas x 35.6 mas. The 1045 nm image shows no signif icant asymmetry. We compare our observations with theoretical Mira star models and systemati cally check the capacity of monochromatic diameter ratios and linear diamet ers for discriminating between model representations of the: observed star. Monochromatic tau(lambda)=1 radii were derived from the observed visibilit ies by application of model-predicted center-to-limb variations of the inte nsity. Adopting the HIPPARCOS parallax we obtained a photospheric radius (R osseland tau(Ross)=1 radius) of R Cas of 377 R. +/- 60 R. (32.9 mas +/- 3.3 mas). The derived photospheric radius and the large period (similar to 430 days) suggest first overtone pulsation for R Gas. We also determined the e ffective temperature of R Cas at near-minimum phase to 1880 K +/- 130 K and propose T-eff similar to 1900 K for spectral type M10 in the temperature c alibration of very late M giant spectral classes.