Multi-wavelength Diameters of Nearby Miras and Semiregulars

We have used optical interferometry to obtain multi-wavelength visibility curves for eight red giants over the wavelength range 650–1000 nm. The observations consist of wavelengthdispersed fringes recorded with MAPPIT (Masked APerture-Plane Interference Telescope) at the 3.9-m Anglo-Australian Telescope. We present results for four Miras (R Car, o Cet, R Hya, R Leo) and four semi-regular variables (R Dor, W Hya, L2 Pup, γ Cru). All stars except γ Cru show strong variations of angular size with wavelength. A uniform-disk model was found to be a poor fit in most cases, with Gaussian (or other more tapered profiles) preferred. This, together with the fact that most stars showed a systematic increase in apparent size toward the blue and a larger-than-expected linear size, even in the red, all point toward significant scattering by dust in the inner circumstellar environment. Some stars showed evidence for asymmetric brightness profiles, while L2 Pup required a two-component model, indicating an asymmetrical circumstellar dust shell.