Multi-wavelength Diameters of Nearby Miras and Semiregulars

Abstract

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 wavelength-dispersed 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, L$_2$ Pup, $\gamma$ Cru). All stars except $\gamma$ 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 L$_2$ Pup required a two-component model, indicating an asymmetrical circumstellar dust shell.