We calculate the dust formed around AGB and SAGB stars of metallicity Z=0.008 by following the evolution of models with masses in the range 1 M _ { \odot } \leq M \leq 8 M _ { \odot } through the thermal pulses phase , and assuming that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface .
We find that , because of the strong Hot Bottom Burning ( HBB ) experienced , high mass models produce silicates , whereas lower mass objects are predicted to be surrounded by carbonaceous grains ; the transition between the two regimes occurs at a threshold mass of 3.5 M _ { \odot } .
These findings are consistent with the results presented in a previous investigation , for Z=0.001 .
However , in the present higher metallicity case , the production of silicates in the more massive stars continues for the whole AGB phase , because the HBB experienced is softer at Z=0.008 than at Z=0.001 , thus the oxygen in the envelope , essential for the formation of water molecules , is never consumed completely .
The total amount of dust formed for a given mass experiencing HBB increases with metallicity , because of the higher abundance of silicon , and the softer HBB , both factors favouring a higher rate of silicates production .
This behaviour is not found in low mass stars , because the carbon enrichment of the stellar surface layers , due to repeated Third Drege Up episodes , is almost independent of the metallicity .
Regarding cosmic dust enrichment by intermediate mass stars , we find that the cosmic yield at Z=0.008 is a factor \sim 5 larger than at Z=0.001 .
In the lower metallicity case carbon dust dominates after \sim 300 Myr , but at Z=0.008 the dust mass is dominated by silicates at all times , with a prompt enrichment occurring after \sim 40 Myr , associated with the evolution of stars with masses M \sim 7.5 - 8 M _ { \odot } .
These conslusions are partly dependent on the assumptions concerning the two important macro–physics inputs needed to describe the AGB phase , and still unknowm from first principles : the treatment of convection , which determines the extent of the HBB experienced and of the Third Dredge–up following each thermal pulse , and mass loss , essential in fixing the time scale on which the stellar envelope is lost from the star .