Dust formation around AGB and SAGB stars: a trend with metallicity?
Article first published online: 5 JUL 2012
© 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS
Monthly Notices of the Royal Astronomical Society
Volume 424, Issue 3, pages 2345–2357, 11 August 2012
How to Cite
Ventura, P., Criscienzo, M. D., Schneider, R., Carini, R., Valiante, R., D'Antona, F., Gallerani, S., Maiolino, R. and Tornambé, A. (2012), Dust formation around AGB and SAGB stars: a trend with metallicity?. Monthly Notices of the Royal Astronomical Society, 424: 2345–2357. doi: 10.1111/j.1365-2966.2012.21403.x
- Issue published online: 1 AUG 2012
- Article first published online: 5 JUL 2012
- Manuscript Accepted: 28 MAY 2012
- Manuscript Received: 28 MAY 2012
- Observatory of Rome
- stars: abundances;
- stars: AGB and post-AGB. ISM: abundances;
- dust, extinction
We calculate the dust formed around asymptotic giant branch (AGB) and super-AGB stars of metallicity Z = 0.008 by following the evolution of models with masses in the range 1 M⊙ ≤ M ≤ 8 M⊙ through the thermal pulses phase, 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⊙. 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 dredge-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 of ∼5 larger than at Z = 0.001. In the lower metallicity case carbon dust dominates after ∼300 Myr, but at Z = 0.008 the dust mass is dominated by silicates at all times, with a prompt enrichment occurring after ∼40 Myr, associated with the evolution of stars with masses M ∼ 7.5–8 M⊙. These conclusions are partly dependent on the assumptions concerning the two important macrophysics inputs needed to describe the AGB phase, and still unknown 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.