Dust production – AGB stars Anja C. Andersen Dark Cosmology Centre Niels Bohr Institute University of Copenhagen

The reason galaxies should care about AGB stars are because they are the major dust producers !

Why doesn’t metallicity play a role in the type of dust observed? !

Dust is “simple”! Two kind: carbon dust and silicates.

Magnesium-iron silicates The most stable condensate formed from the abundant elements O, Si, Mg and Fe. Mg 2x Fe 2(1-x) SiO 4 with x  [0,1] x=1  fosterite, x=0  fayalite, 0<x<1  olivine Pure fosterite is stable up to much higher temperatures, than pure fayalite. Mg x Fe (1-x) SiO 3 with x  [0,1] x=1  enstatite, x=0  ferrosilite, 0<x<1  pyroxene

Example of oxide grain growth Olivine: 2xMg + 2(1-x)Fe + SiO + 3H 2 O  Mg 2x Fe 2(1-x) SiO 4 (s) + 3H 2 Pyroxene: xMg + (1-x)Fe + SiO + 2H 2 O  Mg x Fe (1-x) SiO 3 (s) + 2H 2 It is even more complicated since: Fe 2 SiO 4 (s) + 2Mg(g)  Mg 2 SiO 4 (s) + 2Fe(g)

Carbon dust Graphite, amorphous carbon, diamond, SiC.

Amorphous Carbon Calculated extinction efficiency of amorphous carbon from optical constants (n & k) published by Maron (1990), Rouleau & Martin (1991), Preibisch et al. (1993), Zubko et al. (1996) and Jäger et al. (1998). In contrast to the gray models, for the frequency- dependent models both the absolute value and the slope of the dust opacity data as a function of become relevant. Andersen et al. 1999

Comparison with observations Observations by Whitelock et al Models vs. three observed carbon Miras with moderately thick dust shells and comparable outflow velocities. Andersen et al. 2003

Why do carbon always get all the attention? Easier to identify presolar carbon grains. More laboratory data - thanks to the combustion industry. Relatively simple grain growth, graphite, diamond and amorphous carbon are made only of carbon!

Dust formation Dust nucleation = the step from molecule, over macromolecule to tiny solid. Need high gas super saturation pressure. Grains seeds cannot form in the ISM. Grain growth in the ISM. Dust seeds is formed in AGB stars and SN. (e.g. Sedlmayr 1994)

Chemistry: The Simple Picture High bond energy of CO leads to 3 cases: C/O < 1: all C bound in CO, O-bearing molecules dominate gas. Grains: silicates. C/O = 1: all C and O in CO, less abundant elements dominate molecules. Grains: ?!?. C/O > 1: all O bound in CO, hydrocarbon molecules dominate gas. Grains: carbon. Spectral types: M (C/O 1)

Bond energies of abundant elements

Flavor of high mass AGB stars Depends on the efficiency of the third dredge-up Hot bottom burning. H burning via the CN cycles. This turn C12 into C13 and N14. When HBB is active, the added C12 can be burned into N14 and the star will not become a carbon star. Flavor will then relay on whether PopIII produce more O or C! Mass loss efficiency

T ~ 20 mill. K

Dust driven mass loss of AGB stars Observations S-stars (solid line), M-stars (dotted line), C-stars (dashed line) Ramstedt et al. (2006;2009) Mass loss rates Wind velocity distribution S-stars M-stars C-stars

AGB star: not to scale! Habing & Olofsson, 2003, Asymptotic Giant Branch Stars, Springer

Dust driven mass loss of AGB stars Theoretical modeling C-stars have carbon dust driven winds. M-stars ?? Silicate dust driven winds doesn’t seem to work (Woitke 2006, Höfner 2006). However, see poster by Susanne Höfner (M6). S-stars ?? Possibly carbon dust (Höfner & Andersen 2007).

Schematic radial course of temperature and density of an expanding stellar wind. Shown is the evolution of a complex chemistry and dust nucleation and growth. Nucleation can be homogeneous or heterogeneous and so can grain growth. Nucleation & growth Sedlmayr (1994)

The Moment method describes the time evolution of an ensemble of macroscopic dust grains of various sizes. (Gail et al. 1984, Gail & Sedlmayr 1988, Gauger et al. 1990). Input: nucleation rate, dust opacity, intrinsic dust density, sticking coefficient. Nucleation, growth and destruction of dust grains are supposed to proceed by reactions involving C, C 2, C 2 H and C 2 H 2.

Andersen et al. 2003

CO after Susanne Höfner 2008

Non-gray dust effects of small grains Extinction (Q/a) = absorption + scattering Iron-rich silicates can only form at large stellar radii where the density is low, because their grain temperature > gas temperature. Höfner & Andersen 2007 Difference between T d and T r increases with increasing slope

AGB star dust - summery C-rich AGB stars can account for the carbon dust in space. M-rich AGB stars can account for the iron-free silicates in space. Small carbon grains < 0.1 μm, while the silicates are larger. The STARDUST mission returned inter-planetary dust samples and samples from comet Wild2 in 2005.

Summa summarum In a Schneider, Bianchi, Ferrara, Gallerani, Valiante (Italian!) universe: high-mass AGB stars will produce carbon dust and end their life as WDs. In a Nozawa, Kozasa, Nomoto (Japanese!) universe: AGB stars will not be able to loss mass very efficiently and therefore explode as 1.5 SNs!