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- W3178164132 abstract "We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, with and without previously formed internal crystals of titanium carbide (TiC). A lower mass limit of 1.1 Mfor stars capable of contributing grains to the solar nebula isderived. Thismass limit, in conjunction with a mass-luminosity relation for carbon stars, identifies the region of the H-R diagram relevant to the production of presolar graphite. Detailed dynamical models of AGB outflows, along with constraints provided by kinetics and equilibrium ther- modynamics, indicate that grain formation occurs at radiifrom2.3 to 3.7 AU for AGB carbon stars in the 1.1-5M� range. This analysis also yields time intervals available for graphite growth that are on the order of a few years. By considering the luminosity variations of carbon stars, we show that grains formed during minima in the luminosity are likely to be evaporated subsequently, while those formed at luminosity maxima will survive. We calculate strict upper limits on grain sizes for graphite and TiC in spherically symmetric AGB outflows. Graphite grains can reach diameters in the observed micron size range (1-2 � m) only under ideal growth conditions (perfect sticking efficiency,noevaporation,nodepletionofgasspeciescontributingtograingrowth),andthenonlyinoutflowsfrom carbon stars with masses P2.5 M� . The same is true for TiC grains that are found within presolar graphite, which have mean diameters of 24 � 14 nm. In general, the mass-loss rates that would be required to produce the observed grain sizes in spherically symmetric outflows are at least an order of magnitude larger than the maximum observed AGB carbon star mass-loss rates. These results, as well as pressure constraints derived from equilibrium ther- modynamics,forceustoconcludethatpresolargraphiteandTiCmustforminregionsofenhanceddensity(clumps, jets)inAGBoutflowshavingsmallangularscales.AsshowninthecompanionpaperbyCroatetal.,theenrichment of 12 C in many AGB graphites, and the overabundances of the s-process elements Mo, Zr, and Ru in the carbides found within them, often greatly exceed the values observed astronomically in AGB outflows. These observations notonlylendfurthersupporttotheideathattheoutflowsareclumpy,butalsoimplythattheoutflowingmatterisnot well mixed in the circumstellar envelope out to the radii where grain condensation takes place. Subject headingg dust, extinction — stars: AGB and post-AGB — stars: carbon" @default.
- W3178164132 created "2021-07-19" @default.
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- W3178164132 date "2005-01-01" @default.
- W3178164132 modified "2023-09-23" @default.
- W3178164132 title "Constraints on Grain Formation Around Carbon Stars from Laboratory Studies of Presolar Graphite" @default.
- W3178164132 hasPublicationYear "2005" @default.
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