107 research outputs found
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
Atomic layer deposition-based functionalization of materials for medical and environmental health applications
Nanoporous alumina membranes exhibit high pore densities, well-controlled and uniform pore sizes, as well as straight pores. Owing to these unusual properties, nanoporous alumina membranes are currently being considered for use in implantable sensor membranes and water purification membranes. Atomic layer deposition is a thin-film growth process that may be used to modify the pore size in a nanoporous alumina membrane while retaining a narrow pore distribution. In addition, films deposited by means of atomic layer deposition may impart improved biological functionality to nanoporous alumina membranes. In this study, zinc oxide coatings and platinum coatings were deposited on nanoporous alumina membranes by means of atomic layer deposition. PEGylated nanoporous alumina membranes were prepared by self-assembly of 1-mercaptoundec-11-yl hexa(ethylene glycol) on platinum-coated nanoporous alumina membranes. The pores of the PEGylated nanoporous alumina membranes remained free of fouling after exposure to human platelet-rich plasma; protein adsorption, fibrin networks and platelet aggregation were not observed on the coated membrane surface. Zinc oxide-coated nanoporous alumina membranes demonstrated activity against two waterborne pathogens, Escherichia coli and Staphylococcus aureus. The results of this work indicate that nanoporous alumina membranes may be modified using atomic layer deposition for use in a variety of medical and environmental health applications
- …