Cool bottom processes on the thermally-pulsing AGB and the isotopic composition of circumstellar dust grains

(Abridged) We examine the effects of cool bottom processing (CBP) on several isotopic ratios in the convective envelope during the TP-AGB phase of evolution in a 1.5 M_sun initial-mass star of solar initial composition. We use a parametric model which treats extra mixing by introducing mass flow between the convective envelope and the underlying radiative zone. The parameters of this model are the mass circulation rate (Mdot) and the maximum temperature (T_P) experienced by the circulating material. The effects of nuclear reactions in the flowing matter were calculated using a set of structures of the radiative zone selected from a complete stellar evolution calculation. The compositions of the flowing material were obtained and the resulting changes in the envelope determined. Abundant ^26Al was produced by CBP for log T_P > 7.65. While ^26Al/^27Al depends on T_P, the isotopic ratios in CNO elements depend dominantly on the circulation rate. The correspondence is shown between models of CBP as parameterized by a diffusion formalism within the stellar evolution model and those using the mass-flow formalism employed here. The isotopic ratios are compared with the data on circumstellar dust grains. It is found that the ratios ^{18}O/^{16}O, ^{17}O/^{16}O, and ^26Al/^27Al observed for oxide grains formed at C/O < 1 are reasonably well-understood. However, the ^15N/^14N, ^12C/^13C, and ^26Al/^27Al in carbide grains (C/O > 1) require many stellar sources with ^14N/^15N at least a factor of 4 below solar. The rare grains with ^12C/^13C < 10 cannot be produced by any red-giant or AGB source.Comment: 35 pages, plus 18 included figures. Scheduled for January 10, 2003 issue of Ap

Cosmic-ray strangelets in the Earth's atmosphere

If strange quark matter is stable in small lumps, we expect to find such lumps, called ``strangelets'', on Earth due to a steady flux in cosmic rays. Following recent astrophysical models, we predict the strangelet flux at the top of the atmosphere, and trace the strangelets' behavior in atmospheric chemistry and circulation. We show that several strangelet species may have large abundances in the atmosphere; that they should respond favorably to laboratory-scale preconcentration techniques; and that they present promising targets for mass spectroscopy experiments.Comment: 28 pages, 4 figures, revtex