Presolar grains environments

Presolar grains as tracers of circumstellar and interstellar environments... 

What can presolar grains tell us about the environment of their formation ... 

Bematowicz, T. J., Croat, T. K. and Daulton, T. L. (2006) Origin and evolution of carbonaceous presolar grains in stellar environments. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H.Y., Jr. Tucson University of Arizona Press, pp. 109-126. 

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

Bulk isotopic compositions of chondrites. Isotopic compositions of bulk chondrites are essentially uniform within variations of 0.1-0.01% except for light elements such as H, C, N, and O and for presolar grains (see e.g. Lodders 2003 Palme Jones 2003). Presolar grains have isotopic compositions significantly different from those of Solar System materials, suggesting that they were dust particles formed in circumstellar environments and incorporated into the proto-solar molecular cloud (Chapter 2 and see e.g. Nittler 2003 Zinner 2005). Presolar grains are thus considered to be the first dust components that formed in the proto-solar disk. The rarity of presolar grains in chondrites (several ppb for silicon nitride to 200 ppm... 

Many different processes were involved in making each chondritic component. Unaltered chondrite matrices may contain at least six different types of micrometer-to-nanometer-sized components, which formed in diverse environments amorphous FeO-rich silicate, forsterite and enstatite grains, refractory grains, presolar grains, carbonaceous material, and iron-rich olivine. Chondrules formed by several nebular processes (closed-system melting, condensation, and possibly evaporation) and at least one asteroidal process (impact melting in regoliths). CAIs may be condensates, residues or processed versions of both. An exception to this preference for complexity is provided by the amoeboid olivine inclusions all AO As could have formed by the same basic process nebular condensation. Aluminum-rich chondrules may provide a second exception, at least within carbonaceous chondrites. 

Perhaps the best current way to view the disk of debris from which the Earth accreted was as an environment of vigorous mixing. Volatile-depleted material that had witnessed very high temperatures at an early stage (CAIs) mixed with material that had been flash melted (chondrules) a few milhon years later. Then presolar grains that had escaped these processes rained into the... 

In this chapter, we reviewed the methods and results of chemical equilibrium calculations applied to solar composition material. These types of calculations are applicable to chemistry in a variety of astronomical environments including the atmospheres and circumstellar envelopes of cool stars, the solar nebula and protoplanetary accretion disks around other stars, planetary atmospheres, and the atmospheres of brown dwarfs. The results of chemical equilibrium calculations have guided studies of elemental abundances in meteorites and presolar grains and as a result have helped to refine nucleosynthetic models of element formation in stars. 

Genuine star dust is preserved in meteorites. Most of the presolar grains comes from RG and from 0-rich and C-rich AGB stars. Dust from supemovae and novae has also been found. Elemental, isotopic, and structural analyses of this star dust gives details on stellar nucleosynthesis and dust formation conditions in the circumstellar environments. 

In addition to the above tools, we are developing other tools related to Galactic chemical evolution. These include a Nuclear Reactions Tool, a Nuclear Network Tool, and a Stellar Ejecta Tool. The Nuclear Reactions Tool will help users calculate nuclear reaction rates and help organize, view, and sort many of the common parameters need for these calculations. The Nuclear Network Tool will provide an easy way to evolve a system of species through time for a given environment s temperature and pressure. The features of the Stellar Ejecta Tool are designed to help a user understand the isotopic anomalies found in primitive meteorites or presolar grains. The Stellar Ejecta Tool will provide an easy way to view the isotopic abundance of a star s ejecta, run a nuclear decay network on this material, and then mix it with a second distribution of isotopic abundances. In this way it can simulate systems such as a late injection of material into the early solar nebula. When these tools are released, we will announce them over the Webnucleo mail list (see below). 

The most extreme example of isotopic anomalies is provided by the laboratory analyses of individual preserved presolar dust grains extracted from primitive meteorites (Anders and Zinner 1993). These micron-size or smaller grains of SiC, graphite, and (less commonly analyzed) refractory oxides formed in the outflows of evolved stars and their isotopic compositions of C, N, O and other major and even trace elements quantitatively reflect the unique nucleosynthetic environment of that particular star, which may differ from average solar system compositions by one or more orders of magnitude. That such... 

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