The time variation in infrared water-vapour bands in Mira variables
Onaka, T.; Murakami, H.; Yamamura, I.; Matsuura, M.; Cami, J.
Japan, United Kingdom, Netherlands
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 2.5-4.0 mu m 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 mu m region, while the features at shorter wavelengths remain in absorption. These H2O bands in the 2.5-4.0 mu m 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*E20-3*E22 cm-2, and exceed 3*E21 cm-2 when the features are observed in emission. The radii of the ``hot'' layer (Rhot) 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 (Ncool) of 7*E20-5*E22 cm-2, and is located at 2.5-4.0 R*. 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 H_2O 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.