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 \leq Z \leq 0.018 .
We find that Al _ { 2 } O _ { 3 } particles form at radial distances from the centre between \sim 2 and 4 R _ { * } ( 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 .