Crystallization of stardust analogs under an electron flux atmosphere

The atomic microstructural evolution of circumstellar dust grains, which seed the interstellar medium, remains poorly understood. Amorphous alumina and its crystalline polymorphs, including corundum, have been found in the circumstellar shell of evolved stars. Evidence includes both astronomical observations of mid-infrared spectroscopic features and laboratory analyses of presolar grains. In this work, we show that electron fluxes can stimulate crystallization of amorphous alumina stardust analog materials using transmission electron microscopy. Crystallization experiments conducted at varying electron energies and flux conditions demonstrate a critical threshold cumulative electron dose of ∼10²⁴ e^-/m² for crystallization, suggesting that the crystallization process can occur through atomic rearrangement due to electron interaction with the amorphous matrix. Throughout the crystallization process, time-resolved diffraction reveals the transition from amorphous to a transitional η-Al₂O₃ phase. The same transitional phase was confirmed to occur via thermal annealing at 800 °C, while annealing at 1,300 °C produced the stable crystalline phase α-Al₂O₃ (corundum). In both processes, the structural evolution through atomic rearrangement was characterized by quantifying the average interatomic distance between neighboring atoms using the electron pair distribution function analysis. Extrapolating to astronomical timescales, our findings suggest that electron bombardment may play a significant role in the crystallization of stardust grains, highlighting its potential importance in astrophysical environments, such as the circumstellar envelopes of planetary nebulae.