5 Conclusions

The oxygen-rich Mira variable star, Z Cyg, has been observed with the SWS periodically over one and a half period. The SWS spectrum shows quite large variations. The circumstellar emission for $\lambda > 7$ $\mu$m is enhanced and the ratio of the 10 to 18 $\mu$m bands is increased around visual maxima. The variation can be interpreted in terms of the stellar luminosity variation if the proper dust emissivity is adopted. In other words, the dust emissivity can be derived by the spectroscopic variability observations. For optically thin dust shells, this analysis provides a fairly robust means to derive dust optical properties. Possible evidence for dust formation near minimum is discussed. The present analysis relies on the relation between $T_{\rm i}$ and the integrated flux, thus observationally on the spectral shape and the absolute flux level. Further improvement in the absolute calibration will enable a better determination of the dust optical properties. It will also facilitate detailed investigations of the dust formation process and the effects of dynamical motion of the dust shell. Studies of the photospheric emission of Mira variables taking account of the molecular opacities are also important to improve the estimation of the dust emissivity, particularly in the 10 $\mu$m region. Application of this analysis to a large sample would be quite interesting to understand the general characteristics of the variability of oxygen-rich Mira variables in the infrared and the nature of the circumstellar dust grains. The optical properties derived for the Z Cyg dust shell indicate that the highest temperature of the dust shell is $700 \pm 100$ K and the band ratio Q(18 $\mu$m)/Q(10 $\mu$m) is in the range $0.51 \pm 0.08$. The dust around Z Cyg is not representative for silicate dust around all oxygen-rich AGB stars, but it may be representative for a class of them. While dust emissivity variations in 10 $\mu$m region have been indicated by previous studies, the present analysis suggests that such variations also exist in the 20 $\mu$m region.

Acknowledgements

In this research, we have used, and acknowledge with thanks, data from the AAVSO International Database, based on observations submitted to the AAVSO by variable star observers worldwide. We would like to thank the SWS Instrument Dedicated Team (IDT) for their help in the data reduction, the ISO project members for their efforts in scheduling the time critical observations, and K. Kawara, Y. Satoh, T. Tanabé, H. Okuda, T. Tsuji, and other members of the Japanese ISO group for their continuous help and support. This work was supported in part by Grants-in-Aid for Scientific Research from the Japan Society of Promotion of Science (JSPS). I.Y. was supported by Grant-in-Aid for Encouragement of Young Scientists (No. 13740131) from the JSPS.