2002A&A...383..972M

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 2.5-4.0µm region. All four stars show H₂O bands in absorption around minimum in the visual light curve. At maximum, H₂O emission features appear in the ∼3.5-4.0µm region, while the features at shorter wavelengths remain in absorption. These H₂O bands in the 2.5-4.0µm 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 2000K and a ``cool'' layer with an excitation temperature of 1000-1400K. The column densities of the ``hot'' layer are 6x10<sup>20</sup>-3x10<sup>22</sup>cm<sup>–2</sup>, and exceed 3x10<sup>21</sup>cm<sup>–2</sup> when the features are observed in emission. The radii of the ``hot'' layer (R_hot) are ∼1R_* at visual minimum and 2R_* at maximum, where R_* is a radius of background source of the model, in practical, the radius of a 3000K black body. The ``cool'' layer has the column density (N<sub>cool</sub>) of 7x10<sup>20</sup>-5x10<sup>22</sup>cm<sup>–2</sup>, and is located at 2.5-4.0R<sub>*</sub>. N<sub>cool</sub> depends on the object rather than the variability phase. The time variation of R<sub>hot</sub>/R<sub>*</sub> from 1 to 2 is attributed to the actual variation in the radius of the H<sub>2</sub>O layer, since the variation in R<sub>hot</sub> 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.