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Time Variation of SWS Spectra of Oxygen-rich Mira Variables
T. Onaka 1 - T. de Jong 2, 3 - I. Yamamura 1, 2 - T. Tanabe 4 - O. Hashimoto 5 - H. Izumiura 6
1 Department of Astronomy, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan -2 SRON Laboratory for Space Research Groningen, 9700 AV Groningen, The Netherlands -
3 Astronomical Institute `Anton Pannekoek', University of Amsterdam, 1098 SJ Amsterdam, The Netherlands -
4 Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181, Japan -
5 Gumma Observatory, Ohtomo, Maebashi, Gumma 371, Japan -
6 Okayama Astrophysical Observatory, National Astronomical Observatory, Asakuchi, Okayama 719-02, Japan
Two M-type Mira variables have been observed by SWS01 over an entire light
variation cycle. One of the target stars, Z Cyg, shows
strong variation of the circumstellar emission. The 10 
m and 20 m
`silicate features' become stronger relative to the photospheric emission
around maximum and the relative intensity of the 10 m
to the 20 m features becomes larger also around maximum, indicating
the increase of the temperatures of circumstellar dust grains. These
variations have been confirmed by the PHT observations simultaneously
made. We attribute the variation to the grain nucleation and subsequent
grain growth
processes in the circumstellar shell and make simple model analysis
(cf. Onaka et al. 1989). Changes in the dust
properties (Le Bertre 1993), changes in the size distribution of
circumstellar grains (Little-Marenin et al. 1996), or changes
in the photosperic emission (Aringer et al. 1997) may also contribute
to the variation. The other
star, T Cep, shows a different type of variation. This star shows
the `13 m' emission feature (Sloan et al. 1996)
as well as the bands of CO2. The
time variation of these features in the spectrum of T Cep are also presented.
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Postscript version