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 µm 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– 4.0 µm region, while the features at shorter wavelengths remain in absorption. These H2O bands in the 2.5–4.0 µm 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 (R hot) are ∼1 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 × 10 20 –5 × 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. Key words. stars: AGB and post-AGB – stars: atmospheres – stars: variables:general – infrared: stars – stars: late-type