T. Onaka 1, T. de Jong 2,3, I. Yamamura 3, J. Cami 4, & T. Tanabe 5
1 Department of Astronomy, University of Tokyo, Japan
2 SRON-Utrecht, The Netherlands
3 Astronomical Institute, University of Amsterdam, The Netherlands
4 SRON-Groningen, The Netherlands
5 Institute of Astronomy, University of Tokyo, Japan
Two M-type Mira variable stars, Z Cyg and T Cep, have been observed with SWS01 over an entire light variability cycle to investigate the dust formation process and the dust properties around late-type stars. Both stars have been observed for 7 times with a variability phase separation of approximately 0.2.
Z Cyg has strong ``silicate'' emission feature in its spectrum and shows large variation in the feature. The 10 micron and 20 micron features became larger relative to the photospheric emission at maximum than at minimum, while the intensity ratio of the 10 micron to 20 micron band increased at maximum, indicating the increase of the temperature of circumstellar dust grains at maximum. All the spectra of Z Cyg can be fitted fairly well with a simple optically-thin dust shell model with the same dust emissivity empirically derived. However, the variation in the derived dust temperature seems to be large compared to what is expected from the variation in the luminosity. To account for the observed variations, we propose two possibilities: (i) the inner dust shell temperature is quite low (< 500K) as claimed by Onaka et al (1989) and thus a small variation in temperatures caused by the luminosity change could affect the mid-infrared spectrum appreciably; or (ii) the dust emissivity changes with the temperature, in such a way that the 10 micron band strength increases relative to the 20 micron band with temperature as indicated by Day (1976) and Bedijn (1987).
T Cep shows an atypical 10 micron band feature. It also shows the ``13 micron feature'', which has been proposed to be aluminum oxide (e.g. Sloan et al. 1996) as well as CO2 bands in the 13-16 micron region (Justtanont et al. 1998) and SO2 band around 7.3 micron (Yamamura et al. 1998). The variation in the mid-infrared spectrum of T Cep is not large compared to that of Z Cyg, but it does not synchronize with the visual light variation and seems to have longer variability period than the visual light variation. The comparison of the variation in the mid-infrared spectrum with the dust shell luminosity change indicates that there may be dust production at the first minimum phase. The 13 micron feature shows the variation in its shape and the peak shifts to longer wavelengths at maximum, indicating that more than one component involved in this feature.
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