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تسجيل مستخدم جديدDust from AGB stars
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Anja C. Andersen
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Dust is formed in the expanding atmosphere during late stages of stellar evolution. Dust influences the dynamics and thermodynamics of the stellar atmosphere by its opacity. The dust opacity depends both on the optical properties of the grain material as well as on the amount of dust present. A rich source of information on some mineral phases of dust in AGB stars comes from the study of presolar grains from meteorites. This paper presents a short overview of presolar grains studies and describes how the optical properties of dust grains are obtained in the laboratory.
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We study the evolution of extremely metal-poor AGB stars, with metallicities down to [Fe/H]=-5, to understand the main evolutionary properties, the efficiency of the processes able to alter their surface chemical composition and to determine the gas
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To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies with the 10-$mu$
m camera VISIR at the VLT. We derive mass-loss rates for 29 AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated from the $K-[9]$ and $K-[11]$ colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between $K-[12]$ colour and dust mass-loss rates is observed for AGB stars from galaxies with different metallicities, i.e. the Galaxy, the LMC and the SMC. The derived dust mass-loss rates are in the range 5$times10^{-10}$ to 3$times10^{-8}$ M$_{odot}$yr$^{-1}$ for the observed AGB stars in Sgr dSph and around 5$times10^{-9}$ M$_{odot}$yr$^{-1}$ for those in Fornax; while values obtained with the two different methods are of the same order of magnitude. The mass-loss rates for these stars are higher than the nuclear burning rates, so they will terminate their AGB phase by the depletion of their stellar mantles before their core can grow significantly. Some observed stars have lower mass-loss rates than the minimum value predicted by theoretical models.
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 w
ind 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 find 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.
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pheres. We used Al2O3 and warm silicate grains. Our S Ori and R Cnc data could be well described by an Al2O3 dust shell alone, and our GX Mon data by a mix of an Al2O3 and a silicate shell. The best-fit parameters for S Ori and R Cnc included photospheric angular diameters Theta(Phot) of 9.7+/-1.0mas and 12.3+/-1.0mas, optical depths tau(V)(Al2O3) of 1.5+/-0.5 and 1.35+/-0.2, and inner radii R(in) of 1.9+/-0.3R(Phot) and 2.2+/-0.3R(Phot), respectively. Best-fit parameters for GX Mon were Theta(Phot)=8.7+/-1.3mas, tau(V)(Al2O3)=1.9+/-0.6, R(in)(Al2O3)=2.1+/-0.3R(Phot), tau(V)(silicate)=3.2+/-0.5, and R(in)(silicate)=4.6+/-0.2R(Phot). Our model fits constrain the chemical composition and the inner boundary radii of the dust shells, as well as the photospheric angular diameters. Our interferometric results are consistent with Al2O3 grains condensing close to the stellar surface at about 2 stellar radii, co-located with the extended atmosphere and SiO maser emission, and warm silicate grains at larger distances of about 4--5 stellar radii. We verified that the number densities of aluminum can match that of the best-fit Al2O3 dust shell near the inner dust radius in sufficiently extended atmospheres, confirming that Al2O3 grains can be seed particles for the further dust condensation. Together with literature data of the mass-loss rates, our sample is consistent with a hypothesis that stars with low mass-loss rates form primarily dust that preserves the spectral properties of Al2O3, and stars with higher mass-loss rate form dust with properties of warm silicates.
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and Small Magellanic Clouds (LMC, SMC). The measured DPR of C-AGB in the LMC are reproduced only if the mass loss from AGB stars is very efficient during the carbon-star stage. The same yields over-predict the observed DPR in the SMC, suggesting a stronger metallicity dependence of the mass-loss rates during the carbon- star stage. DPR of O-AGB stars suggest that rapid silicate dust enrichment occurs due to efficient hot-bottom-burning if mstar > 3 Msun and Z > 0.001. When compared to the most recent observations, our models support a stellar origin for the existing dust mass, if no significant destruction in the ISM occurs, with a contribution from AGB stars of 70% in the LMC and 15% in the SMC.
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