Time variation of SWS spectra of M-type Mira variables and its dust optical properties. I. Z Cyg

Abstract

The M-type Mira variable star, Z Cyg, was observed with the Short-Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) 7 times at roughly 60 day intervals over one and a half period. The infrared spectrum (2.38-45.2 mu m) of Z Cyg shows prominent silicate emission bands at 10 mu m and 18 mu m and displays quite large variations over the observed period. The variation in the infrared spectrum of Z Cyg is synchronized with the visual light curve. The circumstellar emission and the 10 mu m to 18 mu m silicate band ratio increases at maximum and decreases at minimum, indicating a variation in the dust temperature with phase. Apart from minor emission features which may be partly due to oxide dust, the observed spectra can be fitted by optically thin dust shell models with one single silicate dust emissivity profile. Thus silicate is the dominant dust component in the circumstellar shell of Z Cyg. The variation in the integrated infrared flux and in the dust temperature derived from the observed spectra can be interpreted in terms of the variation in the luminosity of the central star if proper dust optical properties are adopted. Conversely, the dust emissivity can be estimated from the variations in the infrared spectrum. The derived optical properties are relatively insensitive to the assumptions made in the model analysis because of the optically thin nature of the dust shell. Possible evidence for dust formation near minimum is discussed. The observed variation of the dust shell spectrum of Z Cyg is fitted most consistently with a model in which the inner dust shell temperature is 700 +/- 100 K at maximum. From the model fits we derive a ratio of the 18 mu m and 10 mu m emission efficiencies of Q(18 mu m)/Q(10 mu m) = 0.51 +/- 0.08. The Z Cyg dust also has a broad feature in the 20-25 mu m region in addition to the 18 mu m silicate band. The optical properties of dust grains around Z Cyg are compared with those in circumstellar shells of other oxygen-rich late-type stars and it is shown that there are variations in the 20 mu m emissivity of circumstellar dust, possibly related to the presence of another dust component. 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 United Kingdom) and with the participation of ISAS and NASA.