Experimental study of amorphous silicate formation

Broad infrared bands at 10 and 18 mum have often been observed around oxygen-rich evolved stars, and these are thought to arise from amorphous silicate dust grains. In order to study the formation mechanism of silicate dust grains, we have analyzed synthesized silicate dust analogs by reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). The dust analog was produced from SiO and Mg vapors in O-2. The analyses showed that an amorphous silicate is formed at about temperature of 650 K through O-deficient silicate formation. However, when the substrate temperature was below 650 K, the main product is a mixture of SiOx and MgO, where 1 < x <= 2. Our experimental results indicate that temperature is an important factor for amorphous silicate formation in the circumstellar environment of oxygen-rich evolved stars.