The time variation in infrared water-vapour bands in Mira variables

The time variation in the water-vapour bands in oxygen-rich Mira variables has been investigated using multi-epoch ISO/SWS spectra of four Mira variables in the 2.5-4.0 mum region. All four stars show H2O bands in absorption around minimum in the visual light curve. At maximum, H2O emission features appear in the similar to3.5-4.0 mum region, while the features at shorter wavelengths remain in absorption. These H2O bands in the 2.5-4.0 mum region originate from the extended atmosphere. The analysis has been carried out with a disk shape, slab geometry model. The observed H2O bands are reproduced by two layers; a "hot" layer with an excitation temperature of 2000 K and a "cool" layer with an excitation temperature of 1000-1400 K. The column densities of the "hot" layer are 6 x 10(20)-3 x 10(22) cm(-2), and exceed 3 x 10(21) cm(-2) when the features are observed in emission. The radii of the "hot" layer (R-hot) are similar to1 R-* at visual minimum and 2 R-* at maximum, where R-* is a radius of background source of the model, in practical, the radius of a 3000 K black body. The "cool" layer has the column density (N-cool) of 7 x 10(20)-5 x 10(22) cm(-2), and is located at 2.5-4.0 R-*. N-cool depends on the object rather than the variability phase. The time variation of R-hot/R-* from 1 to 2 is attributed to the actual variation in the radius of the H2O layer, since the variation in R-hot far exceeds the variation in the "continuum" stellar radius. A high H2O density shell occurs near the surface of the star around minimum, and moves out with the stellar pulsation. This shell gradually fades away after maximum, and a new high H2O density shell is formed in the inner region again at the next minimum. Due to large optical depth of H2O, the near-infrared variability is dominated by the H2O layer, and the L'-band flux correlates with the area of the H2O shell. The infrared molecular bands trace the structure of the extended atmosphere and impose appreciable effects on near-infrared light curve of Mira variables.