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

¹ The Institute of Space and Astronautical Science (ISAS), Yoshino-dai 3-1-1, Sagamihara, Kanagawa 229-8510, Japan
² Department of Astronomy, School of Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
³ Department of Physics, UMIST, PO Box 88, Manchester M60 1QD, UK
⁴ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
⁵ SRON-Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

Corresponding author: M. Matsuura, m.matsuura@umist.ac.uk

Received: 21 May 2001
Accepted: 17 December 2001

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 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 – cm^-2, and exceed  cm^-2 when the features are observed in emission. The radii of the “hot” layer () are ~1  at visual minimum and 2  at maximum, where 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 () of – cm^-2, and is located at 2.5–4.0 .  depends on the object rather than the variability phase. The time variation of R_hot from 1 to 2 is attributed to the actual variation in the radius of the H₂O layer, since the variation in  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.