The structure of H2O shells in Mira atmospheres - Correlation with disk brightness distributions and a spectrophotometric signature

Dynamic models of M-type Mira variables predict the occurrence of water "shells", i.e. of zones of high H 2O density and high H 2O absorption inside the stellar atmosphere. The density, position and width of these shells is closely correlated with different types of two-component shapes of the intensity distribution on the disk in the H, K and L near-continuum bandpasses. We investigate these correlations and highlight the role of a spectrophotometric H 2O index that warns against serious complications in diameter measurements in the case of substantial water contamination of the bandpass of observation. Simultaneous spectrophotometric and interferometric measurements may allow observers to estimate real continuum diameters more precisely.