ISO-SWS OBSERVATIONS OF THE TIME VARIABILITY OF OXYGEN-RICH MIRA VARIABLES

We present the first observation of Variation of the infrared spectrum of circumstellar dust emission around an oxygen-rich Mira variable st ar, Z Cyg, over an entire light variation cycle, based on the periodic SWS01 observations. The 10 mu m and 20 mu m 'silicate features' becom e stronger relative to the photospheric emission at maximum than at mi nimum. In addition, the relative intensity of the 10 mu m to the 20 mu m features increases at maximum, indicating an increase of the temper ature of circumstellar dust grains. A simple model analysis suggests e nhanced dust formation near the photosphere around maximum, leading to a scenario that dust nucleation may have occurred near minimum and th e dust grains may have subsequently grown till maximum. The strong obs erved variation suggests that the variability must be taken into accou nt in the interpretation of the infrared spectra of oxygen-rich Mira v ariables.