High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum

Corundum ($\alpha$-Al$_{2}$O$_{3}$) and amorphous or metastable Al$_{2}$O$_{3}$ are common components of circumstellar dust observed around O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites. We report a detailed isotopic and microstructural investigation of a unique presolar corundum grain, QUE060, identified in an acid residue of the Queen Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic compositions, this 1.4 $\mu$m diameter grain formed in a low- or intermediate-mass AGB star. It has four developed rhombohedral $\{$011$\}$ faces of corundum and a rough, rounded face with cavities. High Mg contents (Mg/Al $>$ 0.004) are due to the decay of radioactive $^{26}$Al. No spinel (MgAl$_{2}$O$_{4}$) inclusions that might have exsolved from the corundum are observed, but there are several high-Mg domains with modulated structures. The subhedral shape of grain QUE060 is the first clear evidence that corundum condenses and grows to micrometer sizes in the extended atmospheres around AGB stars. The flat faces indicate that grain QUE060 experienced little modification by gas-grain and grain-grain collisions in the interstellar medium (ISM) and solar nebula. The Mg distribution in its structure indicates that grain QUE060 has not experienced any severe heating events since the exhaustion of $^{26}$Al. However, it underwent at least one very transient heating event to form the high-Mg domains. A possible mechanism for producing this transient event, as well as the one rough surface and cavity, is a single grain-grain collision in the ISM. These results indicate that grain QUE060 is the most pristine circumstellar corundum studied to date.


Introduction
Corundum, the only thermodynamically stable phase of alumina (Al 2 O 3 ), is one of the most refractory dust species expected to condense from a gas of the solar composition (e.g., Ebel 2006). Mid-infrared (MIR) spectroscopic observations have revealed that more than 90% of semi-regular variables and 20% of Mira variables, which are on the asymptotic giant branch (AGB), show single peaks at 13 µm (e.g., Speck et al. 2000;Sloan et al. 2003). Corundum is the most plausible dust species capable of producing this 13 µm feature (Zeidler et al. 2013;Takigawa et al. 2015).
Amorphous or metastable alumina, which shows a broad MIR spectral feature peaking at 11-12 µm, is roughly as abundant as silicate dust around many O-rich AGB stars (e.g., Little-Marenin and Price 1986;Speck et al. 2000;Sloan et al. 2003). Recent radio observation of such an alumina-rich star, W Hya, shows that AlO gas molecules efficiently condense to dust while most SiO molecules remain in gas (Takigawa et al. 2017). These authors also suggested that alumina dust grows and accumulates near W Hya from the observed gas distribution and dust mass. However, dust formation in the extended atmospheres of alumina-rich AGB stars is poorly understood because of the limited mineralogical and crystallographic information that is available. It is not even clear if circumstellar corundum is a direct condensate or if it forms by post-condensation transformation of amorphous or metastable alumina that formed in very rapid cooling gases. Presolar dust grains that must have formed prior to the birth of the Solar System are found in primitive chondritic meteorites and cometary dust, and have isotopic compositions indicating origins around evolved stars, such as AGB stars and supernovae. They may not be fully representative of all circumstellar dust grains, but because they can be studied in the laboratory with state-of-the-art microanalytical instrumentation, they can provide detailed quantitative information about the histories of individual dust grains that cannot be obtained by astronomical observations. Presolar Al 2 O 3 grains with isotopic compositions indicating AGB star origins have been identified in primitive chondrites (e.g., Hutcheon et al 1994;Nittler et al. 1997;Takigawa et al. 2014). The morphologies and crystal structures of presolar grains from AGB stars will reflect the condensation conditions in the circumstellar envelopes of their parent stars (e.g., pressure, temperature, cooling rate and chemical composition of the envelope), but may also record alteration in the interstellar medium (ISM) and protosolar disk. Refractory presolar grains like SiC, graphite, Al 2 O 3 and MgAl 2 O 4 (spinel) can be isolated from their host meteorites via physical and chemical processes (Amari et al. 1994;Bernatowicz et al. 2003;Cody et al. 2002). However, most of the Al 2 O 3 grains in meteorite residues have solar origins (Nittler et al. 1997(Nittler et al. , 2008Choi et al. 1998;Takigawa et al. 2014), and presolar Al 2 O 3 grains thus must be identified by secondary ion mass spectroscopic (SIMS) isotopic measurements. Since SIMS sputters and alters grain surfaces, determining grain morphologies requires detailed scanning electron microscope (SEM) examination of each grain before the SIMS measurements.
Morphological studies of pristine presolar Al 2 O 3 grains are thus severely limited in number (Choi et al. 1998;Takigawa et al. 2014). Transmission electron microscopy (TEM) studies of presolar oxide grains were made possible with the development of focused ion beam (FIB) methods (Stroud et al. 2004), but the number of such studies is also small (Stroud et al. 2004(Stroud et al. , 2007Zega et al. 2011;Zega et al. 2014). The microstructures of presolar Al 2 O 3 grains reported to date include an amorphous grain, several corundum grains, and a crystalline Al 2 O 3 tentatively identified as having a hexagonal crystal structure (Stroud et al. 2004(Stroud et al. , 2007. Here we report on a unique subhedral presolar Al 2 O 3 grain found in a residue of the Queen Alexandra Range (QUE) 97008 ordinary chondrite. From a systematic study of Al 2 O 3 grains in acid residues that combined SEM, SIMS, FIB, and TEM, in that order, we discuss the origins of the morphology and interior microstructures, and thermal history of the grain.

Experimental
A residue of the primitive (LL3.05) QUE 97008 meteorite was prepared using the CsF procedure described in Cody et al. (2002) and Cody and Alexander (2005) to remove the bulk of the meteoritic material, follow by treatment with boiling perchloric acid, to destroy carbonaceous material and chromite (FeCr 2 O 4 ). Droplets of the residue suspended in isopropanol and MilliQ water were deposited onto a high purity Au substrate so that on drying the grains were sparsely dispersed over its surface. Micron-sized Burma spinel grains were used as a standard to correct for the instrumental mass fractionation and to determine the relative sensitivity factor of secondary Mg and Al ions. The standard 26 Mg/ 24 Mg of 0.13932 (Catanzaro et al. 1966) and a sensitivity factor of 1.16 were used to calculate the initial 26 Al/ 27 Al ratio.
Ultra-thin sections of a few identified presolar grains were prepared with the FIB-SEM at NRL. TEM studies were carried out with field-emission scanning 6 transmission electron microscopes (STEM) at NRL (JEOL JEM-2200FS) and Kyoto University (JEOL JEM-2100F equipped with JED 2300D). Chemical compositions were measured by STEM-EDS. The K-factor method (Cliff and Lorimer 1975) was used for quantification of the EDS data. We focus here on the TEM data from one particularly interesting grain, QUE060; results for the other grains will be reported elsewhere.

Isotopic compositions
The O isotopic measurements revealed six presolar grains from the acid residue of QUE 97008, and one from that of RC075 ( Fig. 1 and Table 1). No new presolar grains were identified from the residues of Semarkona and Bishunpur. Combined with the grains reported by Takigawa et al. (2014), the presolar grain fraction in Al 2 O 3 grains of residues from Semarkona, Bishunpur, RC 075, and QUE 97008 are 0/25, 2/72, 8/87, and 6/72, respectively.
Grain QUE060 is enriched in 17 O compared to terrestrial by a factor of ~2.8, but depleted in 18 O by a factor of 25 (Table 1), which identifies it as a Group 2 grain according to the classification scheme for presolar oxide grains (Nittler et al. 1997 (Lugaro et al. 2017) or a lower-mass (< 2 M Sun ) AGB star undergoing cool-bottom-processing (Nittler et al. 2008) Morphology and surface structure Figure 2 shows secondary electron images of grain QUE060 on an Au substrate that were acquired prior to modification of its surface morphology by the SIMS isotopic measurements. The mean of the longest and shortest dimensions of grain QUE060 is about 1.4 µm. Four flat faces are clearly observed on the grain (faces a-d in Fig. 2).
Such flat and smooth faces were not observed for any other presolar Al 2 O 3 grains examined here, nor have they been reported in previous studies (Choi et al. 1998;Takigawa et al. 2014). The area e shows a rough and rounded surface with no clear edge and seems to have cavities (Figs. 2B-D). The rough surface structure of area e is similar to the characteristic surface structures observed on most previously studied presolar Al 2 O 3 grains (Choi et al. 1998;Takigawa et al. 2014).
The EBSD patterns taken prior to the SIMS measurements on multiple locations on face b (Fig. 2) were indexed to that of corundum. The EBSD analysis shows that at least the very surface layer (<30 nm in depth) sampled by the measurements, which was subsequently lost during the SIMS measurements, originally had the corundum crystal structure, which is consistent with that of grain interior as shown in the following section.

Interior microstructures
A FIB lift-out section of grain QUE060, cut perpendicular to the Au substrate along the dashed lines in Fig. 2F, was prepared for TEM analysis (Fig. 3). The SEM image in  Fig. 2. Comparing the initial shape and size ( Fig. 2D) with those in the FIB-section (Fig. 3A), most of the grain volume was preserved after the SIMS measurements. A cavity seen on area e of Fig. 2 was also observed on the left part of the grain facing the Au substrate (Fig. 3A). The ion beam damage caused by the SIMS measurements and FIB lift-out cannot be responsible for the cavity formation because the cavity was not exposed to the electron and ion beams.
Very similar electron diffraction patterns of corundum (Fig. 3B) were obtained from most locations across the whole grain (the few exceptions will be described later), which indicates that this grain is a single crystal of corundum. From the EBSD pattern and the electron diffraction patterns along the three different zone axes, the faceted faces of a, b, c, and d in Fig. 2 are indexed to rhombohedral 011 , 110 , 101 , and 101 faces, which are crystallographically identical 011 planes and generally called r-planes (Hartman 1980). Note that we took the smallest rhombohedral unit cell to describe the crystal faces and directions of corundum in this paper.
The bright-field (BF) TEM image in Fig. 3A can be seen to be darkening from right to left. Electron diffraction patterns taken at the center of the grain and the edge of the cavity showed a rotational difference of ~0.3 degrees. The gradual change of the contrast in the BF-TEM image from left to right (large scale contrast) is thus due to rotational differences of the crystallographic orientation. Both the BF and the dark-field (DF) TEM images ( Fig. 3A and D) show small scale (<30 nm) brightness contrast throughout the grain, which indicates that distortions are distributed everywhere in the grain. The selected-area electron diffraction (SAED) patterns from the area b in Fig. 3A showed satellite spots (Fig. 3C), indicating modulated structures in the corundum crystal. These satellite spots were observed from several domains corresponding to the bright areas in the DF-TEM image (Fig. 3E). These domains are <130 nm in diameter.
The average Mg/Al ratio in the grain measured with STEM-EDS is 0.014(1) The detected Mg is essentially pure radiogenic 26 Mg based on the measured Mg isotopic composition (Table 1) and the ratio obtained with EDS is consistent with the 26 Mg/ 27 Al of 0.0126(15) determined by NanoSIMS. The Mg/Al ratios are not uniform within the grain, but are always greater than Mg/Al ~0.004(1) (2000 ppm Mg by weight), which is significantly higher than the Mg detection limit of 0.001 in our measurements. The domains showing satellite spots contain a higher amount of Mg (Fig. 3F). The Mg/Al ratios in such domains are up to 0.038(3), which is significantly higher than those of the surrounding regions. The DF image of satellite spots shows that the modulated structure is not present in the lower Mg-content regions (Fig. 3E).

Formation of grain QUE060
The existence of presolar corundum grains does not rule out the possibility that the grains originally condensed as amorphous or metastable alumina and later transformed to corundum at >1000°C (Levin and Brandon 1998). Had grain QUE060 initially condensed as amorphous alumina around its parent AGB star, the faceted surfaces would have to have formed later by surface diffusion of molecules following crystallization. Because amorphous Al 2 O 3 formation needs very rapid cooling of the gas (e.g., Dragoo and Diamond 1967), reheating is required for crystallization and surface diffusion. Heating events in the outflows of AGB stars beyond the dust formation regions are mainly due to shock wave propagation. These are transient events, whereas sustained high temperatures are needed for faceted face formation by surface diffusion.
Melting or surface diffusion in the protosolar disk can be ruled out because isotopic exchange with the surrounding gas is very efficient at high temperatures and would have erased the isotopic anomalies. Thermal alteration in the parent body of QUE 97008 is also unlikely because QUE 97008 (LL3.05) shows very little evidence of aqueous alteration or thermal metamorphism (Grossman and Brearley, 2005). Hence, the faceted faces on grain QUE060 suggest that this grain originally condensed as crystalline corundum.
Corundum grains found naturally in the Earth or synthesized with flux methods in laboratories generally show well developed 111 faces (c-planes). However, the most developed faces theoretically predicted using attachment energies and from ab-initio calculations are 110 (Hartman 1980(Hartman , 1989Rhol and Gay 1995) as observed for grain QUE060. Hartman (1989) argued that the development of 111 faces is due to adsorption of atoms on the 111 faces, where the density of unsaturated bonds is highest, and that 110 faces should develop at high temperature and low supersaturation conditions. It is, therefore, most likely that the faceted faces observed on grain QUE060 developed during condensation and growth of crystalline Al 2 O 3 around its parent AGB star. There is no evidence of reaction between grain QUE060 and the surrounding gas, which indicates that the grain remained and grew within the high temperature region near the central star where no other mineral was thermodynamically stable. The grain was then transported rapidly to the low temperature and gas density regions along with the accelerated outflow so that other refractory grains could not grow on it or form by gas-grain reaction.

Formation of the rough surface and cavities
The uniqueness of the very flat 110 faces on grain QUE060 indicates that this grain did not spend a very long time in the diffuse ISM. The rough surface area on grain QUE060 (area e in Fig. 2) may be a secondary feature possibly formed by dust-gas or dust-dust collisions in the ISM or protosolar disk. Corundum is not dissolved even by more harsh acid treatments using HF, but amorphous and metastable alumina phases do dissolve into HF-HCl or HClO 4 (Takigawa et al. 2014). If partial amorphization or deformation of the crystal structure occurred as the result of such a collision, the damaged areas could have been dissolved during the isolation procedures.
A grain-grain collision (Jones et al. 1996) is also a possible mechanism for forming the cavity observed in the FIB section. In addition to forming a crater, a high velocity collision would have generated a shock wave that propagated through the grain (Jones et al. 1996). The rotational misorientation from left to right in the FIB section may be explained by such a shock wave propagating from the cavity.

Thermal history after 26 Al decay
The inferred initial 26 Al/ 27 Al of grain QUE060 is about 0.01. Thus, the radiogenic 26 Mg produced by 26 Al decay should have initially occupied roughly one percent of the octahedral Al-sites in the corundum structure. The solubility limit of MgO in corundum is 130 ppm at 1600 °C (Miller et al. 2006)  formation around an AGB star is much shorter than the 720,000 yr half-life of 26 Al. A collisional event in the ISM that could also have been responsible for the crater formation discussed above is an attractive explanation for the heat source that enabled the Mg-diffusion.

Implication for astromineralogy
The faceted faces of the presolar corundum grain QUE060, indexed to {011} faces, provide strong evidence for the direct condensation of corundum around an AGB star.
Because of its very refractory nature, corundum condenses from a hot gas within a few stellar radii from the central star, where acceleration of the stellar wind does not occur.
The micron size of grain QUE060 indicates that the grain had to survive several pulsation periods to grow (Gobrecht et al. 2016). Small alumina grains (<0.1 µm) are transparent to the stellar radiation, but micron-sized grains efficiently scatter the stellar light and contribute more to the acceleration of the stellar wind (Höfner 2008