Sounding-rocket microgravity experiments on alumina dust

Shinnosuke Ishizuka,Yuko Inatomi,Shinsuke Takeuchi,Hiroshi Kimura,Yuki Kimura,Itsuki Sakon,Tomoya Yamazaki

doi:10.1038/s41467-018-06359-y

Abstract

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.

Highlights

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 is a potential origin of a 13 μm feature that appears close to stars
Large Millimeter Array (ALMA) visualized the spatial distribution of molecular rotational bands around the alumina (Al2O3)rich asymptotic giant branch (AGB) star W Hya demonstrating that AlO gases are consumed within the dust formation region while SiO molecules remain gaseous even at the outside of the dust formation region[2]
The presence of titania (TiO2) in the gas phase far from the dust shell is reported for a red supergiant environment[3]. Several minerals, such as Al2O3, TiO2, and spinel (MgAl2O4), have been proposed as carriers of the 13 μm feature[4], Al2O3 is the most likely mineral to be capable of surviving sublimation and, owing to its transparency, of withstanding the radiation pressure from the central star; it is considered to condense earlier than other abundant minerals, such as silicates

Summary

Introduction

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. The presence of titania (TiO2) in the gas phase far from the dust shell is reported for a red supergiant environment[3] Several minerals, such as Al2O3, TiO2, and spinel (MgAl2O4), have been proposed as carriers of the 13 μm feature[4], Al2O3 is the most likely mineral to be capable of surviving sublimation and, owing to its transparency, of withstanding the radiation pressure from the central star; it is considered to condense earlier than other abundant minerals, such as silicates. The discovery of micron-sized presolar Al2O3 dust with an unusual oxygen isotopic composition in meteorites strongly suggests that some of the Al2O3 dust has been formed around AGB stars. e.g. ref. 5

Methods

Results

Conclusion