Sounding-rocket microgravity experiments on alumina dust

Alumina (Al2O3) is believed to be the first major condensate to form in the gas outflow from oxygen-rich evolved stars because of the refractoriness and that α-Al2O3 (corundum, most stable polymorph) is a potential origin of a 13 μm feature that appears close to stars. However, no one has directly reproduced the 13 μm feature experimentally, and it has remained as a noteworthy unidentified infrared band. Here, we report nucleation experiments on Al2O3 nanoparticles monitored by a specially designed infrared spectrometer in the microgravity environment of a sounding rocket. The conditions approximate to those around asymptotic giant branch (AGB) stars. The measured spectra of the nucleated Al2O3 show a sharp feature at a wavelength of 13.55 μm and comparable in width to that observed near oxygen-rich AGB stars. Our finding that α-Al2O3 nucleates under certain condition provides a solid basis to elaborate condensation models of dust around oxygen-rich evolved stars.


Supplementary Note 1
The spherical tantalum oxide particles have peak positions at 13.24 m and 14.27 m for amorphous and nanocrystalline, respectively, at room temperature ( Supplementary Fig. 10), which is close to the peak at 13.55 m from the experiment, shown in Figure 3c. Nevertheless, tantalum oxide does not become a candidate of the origin of the experimental 13.55 m feature for the following three reasons, even if tantalum oxide formed separately from alumina. The first reason is that tantalum oxide has a wider band widths. Its FWHM are 1.5 m and 2.4 m, respectively, which are three to five times more than that of the obtained 13.55 m feature. The second reason is that the mass absorption coefficient is smaller. The mass absorption coefficient of alumina is more than six times larger than that of tantalum oxides ( Supplementary Fig. 10).
Instead, the total amount of tantalum in the experimental chamber is only four times more than aluminum. The vapor pressure of aluminum (~10 4 Pa at 2350 K) is much higher than that of tantalum (10 -5 Pa at 2350 K) and only a small amount of tantalum evaporates before melting due to the very low vapor pressure (less than 1 Pa), even at the melting point of Ta (3290 K).
Breaking of the Ta wire can be confirmed from the video image shown in Supplementary Fig.   4. Consequently, tantalum oxide cannot be a major carrier of the 13.55 m feature. The third reason is that the probability of the oxidation of Ta is lower. The evaporated aluminum vapor consumed oxygen; therefore, no more oxygen remains for the oxidation of tantalum. For these reasons, we selected tantalum as an evaporation source and as a contaminant of the alumina surface rather than oxide, based on the calculation of the IR spectra given in Figure 3.

Supplementary Note 2
We demonstrated the dependences of facet ( Supplementary Fig. 5 method. 35 The optical constants of -Al2O3, Ta, Ta2O5, and amorphous alumina can be found in references 12, 36, 37, and 38, respectively. Faceting, higher temperatures and surface contamination induce a shift of the 13 m feature toward longer wavelengths.

Supplementary Note 3
High temperatures affect the peak-wavelength position of the 13 m band of -Al2O3, shifting it toward a longer wavelength (~0.4 m at 928 K). 12 Although the temperature of the particles can be elevated by the latent heat of crystallization, they are cooled down immediately by the buffer Ar gas, which can warm up by 500 K through short-term heating.
In this study, the temperature of the electrode measured by the thermocouple was 373 K at The spectra were not offset, and the absorption appeared as a reduction in the signal. After the subtraction of the spectrum measured when the IR light was blinded (Fbl), the absorbance spectra were obtained from the sample spectra (F) and background spectra (FBG); The corresponding absorbance spectra when BG was used as a background spectrum for a-c are shown as a-c in Figure 2.
Supplementary Fig. 6. Temperature dependences of calculated IR spectra of -Al2O3 particles: (a) sphere calculated with Mie theory; (b) faceted particle calculated using DDA method. Spectra i-iv were recorded at a temperatures of 300, 551, 738, and 928 K, respectively.