Dust Chemical Reaction around AGB Stars: Identification of Presolar Metastable Aluminum-Oxide Grain

Abstract

We conducted surface microstructual and chemical analyses of the matrix region in carbonaceous chondrite DOM 08006, followed by isotopic measurements to identify presolar grains. As a result, the abundance of presolar grains in the analyzed area was approximately 1.4 times higher than that reported in previous studies. One of the identified presolar oxide grains consists of two Al-rich grains with a defect spinel structure: a large grain containing Ca and Fe, and a smaller one without Ca or Fe, identified as delta alumina. This grain represents the first in-situ identification of a presolar metastable alumina. The formation sequence is inferred as initial condensation of delta alumina under high-temperature conditions in the circumstellar region of AGB star, followed by formation of the Ca-bearing defect spinel phase and subsequent reaction with Fe gas. Since delta alumina with a defective spinel structure can incorporate various metal cations into the vacancies, it might react with surrounding gases to form a variety of oxides, leading to the diversity in the spectrum of Al-rich oxide dust.