Dust from AGBs: from carbon to silicates, the role of the Hot Bottom Burning

To better understand the role of Asymptotic Giant Branch stars as dust polluters in the Universe, we calculate the dust formed around AGBs in the metallicity range 0.05 ≤ Z/Z ≤ 0.4, following the complete evolution of models with masses in the range 1 M ≤ M ≤ 8 M . We assume that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface. The dust formed is extremely sensitive to the initial mass of the star: carbonaceous grains are prevalent around lower mass objects, whereas high mass models produce silicates and alumina dust, because of the strong Hot Bottom Burning experienced. The transition between the two regimes occurs around 3M , partly depending on the metallicity assumed. Dust production in low-mass stars is almost independent of Z, whereas in high-mass objects the rate of silicate production increases with metallicity, because of the higher abundance of silicon in the envelope.