Abstract
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{4.0m region. All four stars show H2O bands in absorption around minimum in the visual light curve. At maximum, H2O emission features appear in the 3.5{4.0m region, while the features at shorter wavelengths remain in absorption. These H2O bands in the 2.5{4.0m 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 10 20 {3 10 22 cm 2 , and exceed 3 10 21 cm 2 when the features are observed in emission. The radii of the \hot layer (Rhot )a re1R at visual minimum and 2R 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 (Ncool )o f 7 10 20 {5 10 22 cm 2 , and is located at 2.5{4.0R. Ncool depends on the object rather than the variability phase. The time variation of Rhot=R from 1 to 2 is attributed to the actual variation in the radius of the H2O layer, since the variation in Rhot 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 0 -band flux correlates with the area of the H2O shell. The infrared molecular bands trace the structure of the extended atmosphere and impose appreciable eects on near-infrared light curve of Mira variables.
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