When a massive star runs out of helium to burn in its core, it joins what astronomers call the asymptotic giant branch (AGB). The star puffs up to the size of Earth’s orbit around the Sun, and burns carbon and other elements in its core to produce aluminum, magnesium, and silicon. Atoms of Al, Mg, and Si make their way to the star’s few-thousand-kelvin outer envelope, where they condense to form grains of cosmic dust. Astronomers infer the presence of the dust from broad emission features in the mid-IR.
More than one type of grain can plausibly account for a broad feature. Nailing a feature’s origin entails proving two things: first, that a hypothesized type of grain can form in the atmosphere of an AGB star; second, that the grains can radiate at the right range of wavelengths. Kyoto University’s Aki Takigawa and her collaborators in Japan and Korea have made that two-pronged case for one type of dust rich in alumina (Al2O3). They found that an 11–12 μm emission feature observed in certain AGB stars could come from dust that is crystalline, rather than amorphous as had previously been assumed.
Takigawa and her collaborators did not start off with a particular type of alumina-rich grain in mind. Rather, they sent various mixes of powdered Al, Mg, and Si through an induction thermal plasma system and characterized whatever grains came out. The process began when the powder mixtures were streamed through a set of radio-frequency coils. The coils’ power, 6 and 30 kW, and frequency, 4 and 13.6 MHz, were so high that the induced currents turned the metal powders into plasma. The plasma, along with argon carrier gas and oxygen, then expanded inside a vacuum chamber. There, as in an AGB star’s atmosphere, the atoms reacted, cooled, and condensed into grains.
The researchers used electron microscopy to determine the grains’ sizes and shapes, x-ray and electron diffraction to determine their structures, and Fourier transform IR spectroscopy to determine their spectra. Several mixes of powders yielded grains that emitted a broad peak at 11–14 μm, but only one—90% Al and 10% Si (labeled Al90Si10)—did so without also emitting sharper features at nearby wavelengths, which, if present in the spectra of AGB stars, would have been seen by astronomers.
Although the Al90Si10 grains included an amorphous phase, the largest contributor to their diffraction patterns (see figure for an example) was a metastable crystalline phase known as γ-alumina. Previous work had demonstrated that amorphous alumina grains produced in a wet-chemistry process, sol–gel, could reproduce the 11–12 μm emission feature. Forming amorphous alumina grains in a gas phase requires rapid cooling. In the vacuum chamber that Takigawa and colleagues used, the temperature of the expanding plasma fell as quickly as 105 K/s, yet none of the alumina grains they made were mostly amorphous. Whatever the cooling rates are in an AGB star’s atmosphere, it seems they can be high and still sustain mostly crystalline grains.
T Cephei is one of the AGB stars whose mid-IR spectrum includes a peak at 11–12 μm. To derive the rest of the star’s mid-IR dust spectrum, Takigawa and her colleagues took an archival spectrum of T Cephei obtained in the 1990s by the Infrared Space Observatory (ISO). Then they subtracted the spectrum of a 2683 K blackbody to represent the star’s continuum emission. It turned out that the measured spectrum of the Al90Si10 grains could account for most of the dust emission observed by ISO, but not all of it. Peaks at 10.5, 13, and 20 μm remained. Suggestively, the residual peaks resemble spectra obtained in the lab from a variety of grains, including ones made from magnesium-iron silicates, iron(II)oxide, and a hard crystalline form of Al2O3 called corundum.
As they age, T Cephei and other AGB stars shed up to 70% of their envelopes in the form of a wind. The wind seeds the interstellar medium with raw material for subsequent generations of stars. Remarkably, grains from previous generations survive the formation of new solar systems and wind up in planets, asteroids, and meteorites. Grains of alumina stardust have been found in terrestrial meteorites, but none with a 10%, or any, admixture of Si. Takigawa hypothesizes that the absence might have to do with the acids used to separate alumina from meteoritic material. Whereas corundum survives the acid wash, γ-alumina dissolves. Chondrites, the type of meteorites that harbor stardust, are rich in silicate minerals. Isolating and identifying Si-doped alumina grains is challenging. Takigawa will present the results of her search for the grains at next week’s annual meeting of the Meteoritical Society in Sapporo, Japan. (A. Takigawa et al., Astrophys. J. Lett. 878, L7, 2019.)