On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars

Abstract

The O-rich Asymptotic Giant Branch (AGB) stars experience strong mass loss with efficient dust condensation and they are major sources of dust in the interstellar medium. Alumina dust (Al${}_2$O${}_3$) is an important dust component in O-rich circumstellar shells and it is expected to be fairly abundant in the winds of the more massive and O-rich AGB stars. By coupling AGB stellar nucleosynthesis and dust formation, we present a self-consistent exploration on the Al${}_2$O${}_3$ production in the winds of AGB stars with progenitor masses between $\sim$3 and 7 M${}_{\odot }$ and metallicities in the range 0.0003 $\le$ Z $\le$ 0.018. We find that Al${}_2$O${}_3$ particles form at radial distances from the centre between $\sim 2$ and 4 R${}_{\ast }$ (depending on metallicity), which is in agreement with recent interferometric observations of Galactic O-rich AGB stars. The mass of Al${}_2$O${}_3$ dust is found to scale almost linearly with metallicity, with solar metallicity AGBs producing the highest amount (about 10${}^{-3}$ M${}_{\odot }$) of alumina dust. The Al${}_2$O${}_3$ grain size decreases with decreasing metallicity (and initial stellar mass) and the maximum size of the Al${}_2$O${}_3$ grains is $\sim$0.075 $\mu m$ for the solar metallicity models. Interestingly, the strong depletion of gaseous Al observed in the low-metallicity HBB AGB star HV 2576 seems to be consistent with the formation of Al${}_2$O${}_3$ dust as predicted by our models. We suggest that the content of Al may be used as a mass (and evolutionary stage) indicator in AGB stars experiencing HBB.

13 pages, 8 figures, accepted for publication in MNRAS

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• A study of oxygen-rich post-AGB stars in the Milky Way to understand the production of silicates from evolved stars
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• Exploring circumstellar effects on the lithium and calcium abundances in massive Galactic O-rich AGB stars
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