Presolar grains

Presolar grains are isotopically-distinct clusters of material found in the fine-grained matrix of primitive meteorites, whose differences from the surrounding meteorite suggest that they are older than the solar system [Maria Lugaro (2005) Stardust from meteorites: An introduction to presolar grains (World Scientific, NY) ISBN 9812560998] . Crystallinity in these clusters ranges from that of micrometre-sized silicon carbide crystals, down to that of diamond and unlayered graphene crystals with fewer than 100 atoms. These grains were probably formed in supernovae or the stellar outflows of red giant stars, and then incorporated into the molecular cloud from which the solar nebula separated to form our solar system. Presolar grains identified so far consist of refractory minerals which survived the collapse of the solar nebula, and the subsequent formation of planetesimals.

In the 1960s, neon [D. C. Black and R. O. Pepin (1969) Trapped neon in meteorites. II., "Earth Planet. Sci. Lett." 36, 377-394] and xenon [J. H. Reynolds and G. Turner (1964) Rare gases in the chondrite Renazzo, "J. Geo. Phys. Res." 69, 3263-3281] components with unusual isotopic ratios were discovered in primitive meteorites. One possible explanation: The existence of intact presolar grains containing anomalous noble gas within these meteorites [Ahnert-Rohlfs E. (1954) "Vorläufige Mitteilung über Versuche zum Nachweis von Meteoritischem Staub", Mitteilung der Sternwarte Sonneberg 45] .

In 1987 diamond [Lewis R.S., Tang M., Wacker J.F., Anders E. and Steel E. (1987) "Interstellar diamonds in meteorites", Nature 326, 160-162] and silicon carbide [Bernatowicz, T., Fraundorf, G., Ming, T., Anders, E., Wopenka, B., Zinner, E., and Fraundorf, P. (1987) "Evidence for interstellar SiC in the Murray carbonaceous meteorite", Nature 330, 728.] grains were found to be carriers of these noble gases. Significant major-element isotopic anomalies were in turn found within these grains [Ernst Zinner (1996) Stardust in the laboratory, "Science" 271:5245, 41-42] .

Because identified presolar stardust grains formed in the neighborhood of specific stellar nucleosynthesis sources, the isotopic composition of their elements is usually different from the isotopic composition of solar-system matter as well as from the galactic average. These isotopic signatures often fingerprint very specific astrophysical nuclear processes [Ernst Zinner (1998) Stellar nucleosynthesis and the isotopic composition of presolar grains from primitive meteorites, "Annual Review of Earth and Planetary Sciences" 26:147-188.] , and thus they go beyond proving interstellar origin to providing independent insight into the way stars work [T. J. Bernatowicz and R. M Walker (1997) Ancient stardust in the laboratory, "Physics Today" 50:1212, 26-32] .

Types of presolar material

There are different types of presolar grains. Presolar grains consisting of the following minerals have so far been identified:
*diamond (C) nanometer-sized grains (~ 2.6 nm diameter) [P. Fraundorf, G. Fraundorf, T. Bernatowicz, R. Lewis, and M. Tang (1989) "Ultramicroscopy" 27:401–412.] possibly formed by vapor deposition [T. L. Daulton, D. D. Eisenhour, T. J. Bernatowicz, R. S. Lewis and P. R. Buseck (1996) Genesis of presolar diamonds: Comparative high-resolution transmission electron microscopy study of meteoritic and terrestrial nano-diamonds, "Geochimica et Cosmochimica Acta" 60:23, 4853-4872]
*graphite (C) particles and onions [T. Bernatowicz, R. Cowsik, P. C. Gibbons, K. Lodders, B. Fegley Jr., S. Amari and R. S. Lewis (1996) Constraints on stellar grain formation from presolar graphite in the Murchison meteorite, "Ap. J." 472:760-782] , some with unlayered graphene cores [P. Fraundorf and M. Wackenhut (2002) The core structure of pre-solar graphite onions, "Ap. J. Lett." 578:2, L153-156]
*silicon carbide (SiC) submicrometre to micrometre sized grains. Presolar SiC occurs as single-polytype grains or polytype intergrowths. The atomic structures observed contain the two lowest order polytypes: hexagonal 2H and cubic 3C (with varying degrees of stacking fault disorder) as well as 1-dimensionally disordered SiC grains. [T. Daulton, T. J. Bernatowicz, R. S. Lewis, S. Messenger, F. J. Stadermann and S. Amari (2002) Polytype distribution in circumstellar silicon carbide, "Science" 296:5574, 1852-1855] In comparison, terrestrial laboratory synthesized SiC is known to form over a hundred different polytypes.
*titanium carbide (TiC) and other carbides within C and SiC grains [T. Bernatowicz, S. Amari, E. Zinner, & R. Lewis (1991) Presolar grains within presolar grains, "Ap J Lett", 373:L73]
*silicon nitride (Si₃N₄)
*corundum (Al₂O₃) [I. D. Hutcheon, G. R. Huss, A. J. Fahey and G. J. Wasserberg (1994) Extreme Mg-26 and O-17 enrichments in an Orgueil corundum: Identification of a presolar oxide grain, "Ap. J. Lett." 425:2, L97-L100]
*spinel (MgAl₂O₄) [E. Zinner, S. Amari, R. Guiness, A. Nguyen, F. J. Stadermann, R. M. Walker and R. S. Lewis (2003) Presolar spinel grains from the Murray and Murchison carbonaceous chondrites, "Geochimica et Cosmochimica Acta" 67:24, 5083-5095]
*hibonite ((Ca,Ce)(Al,Ti,Mg)₁₂O₁₉) [T. R. Ireland (1990) Presolar isotopic and chemical signatures in hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite, "Geochmica et Cosmochimica Acta" 54:3219-3237]
*titanium oxide (TiO₂)
*silicate minerals (olivine and pyroxene)

Characterization of presolar materials

Presolar grains are investigated using scanning or transmission electron microscopes (SEM/TEM), and mass spectrometric methods (noble gas mass spectrometry, resonance ionization mass spectrometry (RIMS), secondary ion mass spectrometry (SIMS, NanoSIMS)). Presolar grains that consist of diamonds are only a few nanometers in size, and are therefore also called nanodiamonds. Because of their small size, nanodiamonds are hard to investigate and, although they are among the first presolar grains discovered, relatively little is known about them. The typical sizes of other presolar grains are in the range of micrometres. Presolar grains represent material from outside our solar system.

Information carried by presolar grains

The study of presolar grains provides information about nucleosynthesis and stellar evolution [Donald D. Clayton and Larry R. Nittler (2004) Astrophysics with presolar stardust, "Annual Review of Astronomy and Astrophysics" 42:39-78] . Grains bearing the isotopic signature of rapid-process nucleosynthesis are useful in testing models of supernovae explosions. Other grains provide isotopic and physical information on asymptotic giant branch stars, which have manufactured the lion's share of the refractory elements lighter than iron in the galaxy. Because the elements in these particles were made at different times (and places) in the early Milky Way, the set of collected particles further provides insight into galactic evolution prior to formation of our solar system.

Along with providing information on nucleosynthesis, solid grains provide information on the physico-chemical conditions under which they formed, and on events subsequent to their formation. For example red giants produce much of the carbon in our galaxy, and their atmospheres are cool enough (unlike that of our sun) to precipitate solid particles for injection into the interstellar medium by radiation pressure. Hence particles bearing the signature of stellar nucleosynthesis are providing us with information on: (i) condensation processes in red giant atmospheres, (ii) radiation and heating processes in the interstellar medium, and (iii) the types of particles that carried the elements of which we are made across the galaxy to Sol.

References

ee also

*cosmic dust
*interplanetary dust cloud
*circumstellar dust
*extraterrestrial materials
*cosmochemistry

External links

*http://presolar.wustl.edu/work/grains.html
*http://www.astro.uiuc.edu/~akspeck/witch-stuff/Research/chapter3/node3.html
*http://www.ciw.edu/lrn/psg_main.html
*http://www.psrd.hawaii.edu/July97/Stardust.html
*http://www.stecf.org/~ralbrech/amico/intabs/ottu.html
*http://www.mpch-mainz.mpg.de/~kosmo/sims/sternenstaub/sternenstaub.htm (images of presolar grains, text in German)
*http://presolar.wustl.edu/~pgd (large database of isotopic data on presolar grains)
* [http://www.psrd.hawaii.edu/ Planetary Science Research Discoveries] Educational journal with articles about presolar grains and cosmochemistry.