ISO-SWS Observations of the Time Variability of Oxygen-righ Mira Variables

Abstract

We present the first observation of variation of the infrared spectrum of circumstellar dust emission around an oxygen-rich Mira variable star, Z Cyg, over an entire light variation cycle, based on the periodic SWS01 observations. The 10 µm and 20 µm ‘silicate features’ become stronger relative to the photospheric emission at maximum that at minimum. In addition, the relative intensity of the 10 µm to the 20 µm features increases at maximum, indicating an increase of the temperature of circumstellar dust grain. A simple model analysis suggests enhanced dust formation near the photosphere around maximum, leading to a scenario that dust nucleation may have occurred near minimum and the dust grains may have subsequently grown till maximum. The strong observed variation suggests that the variability must be taken into account in the interpretation of the infrared spectra of oxygen-rich Mira variables.