Publications (3)0 Total impact
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ABSTRACT: 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 1M<M<8M
throughthe 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 3.5M. These fndings 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 ?nd that the cosmic yield at Z=0.008 is a factor 5 larger than at
Z=0.001. In the lower metallicity case carbon dust dominates after about 300
Myr, but at Z=0.008 the dust mass is dominated by silicates at all times,with a
prompt enrichment occurring after about 40 Myr, associated with the evolution
of stars with masses M =7.5 -8M.
05/2012;
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ABSTRACT: We compute the mass and composition of dust produced by stars with masses in
the range 1Msun<M<8 Msun and with a metallicity of Z=0.001 during their AGB and
Super AGB phases. Stellar evolution is followed from the pre-main sequence
phase using the code ATON which provides, at each timestep, the thermodynamics
and the chemical stucture of the wind. We use a simple model to describe the
growth of the dust grains under the hypothesis of a time-independent,
spherically symmetric stellar wind. We find that the total mass of dust
injected by AGB stars in the interstellar medium does not increase
monotonically with stellar mass and ranges between a minimum of 10^{-6}Msun for
the 1.5Msun stellar model, up to 2x10^{-4} Msun, for the 6Msun case. Dust
composition depends on the stellar mass: low-mass stars (M < 3Msun) produce
carbon-rich dust, whereas more massive stars, experiencing Hot Bottom Burning,
never reach the carbon-star stage, and produce silicates and iron. This is in
partial disagreement with previous investigations in the literature, which are
based on synthetic AGB models and predict that, when the initial metallicity is
Z=0.001, C-rich dust is formed at all stellar masses. The differences are due
to the different modelling of turbulent convection in the super-adiabaticity
regime. Also in this case the treatment of super-adiabatic convection shows up
as the most relevant issue affecting the dust-formation process. We also
investigate Super AGB stars with masses 6.5Msun<M<8 Msun that evolve over a ONe
core.Due to a favourable combination of mass loss and Hot Bottom Burning, these
stars are predicted to be the most efficient silicate-dust producers, releasing
[2 - 7]x 10^{-4} Msun masses of dust. We discuss the robustness of these
predictions and their relevance for the nature and evolution of dust at early
cosmic times.
11/2011;
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ABSTRACT: Theoretical models suggest that Type-II SNe are efficient dust
factories, able to produce 0.1-1 Msun of dust within 0.01-0.1
Gyr from the progenitor birth. These sources can easily explain the
observations of dust emission in high redshift (z˜6) QSOs, when
the Universe age was ≤1 Gyr. However, such high masses of dust are
not observed in recent supernovae (SNe) and SN remnants (SNR), thus
questioning the validity of models. We discuss the dependence of
formation models on the assumptions, the evolution and survival of
grains in the SN ejecta, and the extinction/emission properties of the
resulting dust. Finally, we comment on the relative contribution of SNe
and AGB stars to the dust production in the early universe.
11/2009; 414:65.
