Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.410n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 410
Other
10
Helium, Interstellar Dust, Neon, Neon-E, Nucleosynthesis, Silicon Carbide
Scientific paper
We have analyzed 61 individual interstellar SiC grains (Murchison KJH, ~4.6- micrometer nominal diameter) first by ion microprobe to determine C- and N- isotopic compositions and then by laser-gasas-extraction to determine He and Ne compositions. This work complements our initial investigation of the ^4He, ^22Ne, and C compositions in a suite of 92 individual SiC grains from the Murchison KJG SiC fraction (~3 micrometers) [1,2]. Three grains in the present suite carried ^4He and ^22Ne-E(H) above detection limits (Table 1). The results presented here support our initial conclusions [1,2]: (1) only a small fraction (~5%) of the SiC grains is gas-rich and contains essentially all the Ne-E measured in the bulk residues and (2) ^4He accompanies ^22Ne-E(H) in each gas-rich grain and vice versa. The gas-poor grains were either degassed or were not exposed to the stellar source(s) that produced Ne-E(H). One extremely gas-rich grain in the present suite (7611, estimated ^4He and ^22Ne concentrations of 2.0 and 0.02 ccSTP/g respectively) contained sufficient 20Ne to establish a precise value for the (^20Ne/^22Ne)E(H) ratio (0.09 +/- 0.02). This ratio is consistent both with the bulk KJH measurements [3,4] and those expected from AGB (asymptotic giant branch) star He-burning shells [5], leaving little or no room in the grain for an additional complement of envelope-derived gases. The ^4He/^22Ne ratio in grain 7611 is also consistent with an AGB star He-shell composition [5]. The elevated ^4He/^22Ne ratios in the other two gas-rich grains indicate that these grains contain a larger complement of envelope-derived gases. Spallation-derived gases are not detectable in the individual grains; however, the upper limit to the (^21Ne/^22Ne)E(H) ratio (<7 x 10^-4) in grain 7611 suggests that the Ne-E-rich grains alone may not contain most (>90%) of the spallation-derived gases measured in the bulk residues. This would imply that the 3.4-Ga interstellar cosmic ray exposure age of the gas-rich SiC grains, calculated by assuming that the Ne-E-rich grains carry all the spallation- derived gases measured in the KJ-series SiC residues [4], may be too high by as much as a factor of 20. The C- and N-isotopic compositions for these three grains are typical for Murchison KJH SiC grains analyzed to date and are discussed in more detail in [6]. Because accompanying ^20Ne is observed in one SiC grain that is extremely rich both in ^4He and in ^22Ne, scenarios invoking a ^22Na origin for Ne-E(H) [e.g., 7] are eliminated. Table 1, which appears here in the hard copy, shows He, Ne, C, and N compositions for Ne-E(H)-rich SiC grains (Murchison KJH) compared to bulk measurements and model AGB star envelope and He-shell compositions. References: [1] Nichols R. H. Jr. et al. (1992) LPS XXIII, 989-990. [2] Nichols R. H. Jr. (1992) Ph.D. thesis, Washington University, St. Louis, 171 pp. [3] Lewis R. S. et al. (1990) Nature, 348, 293-298. [4] Lewis R. S. et al. (1993) GCA, submitted. [5] Gallino R. et al. (1990) Nature, 348, 298-302. [6] Hoppe P. et al. (1993) Astrophys. J., submitted. [7] Brown L. E. and Clayton D. D. (1992) Science, 258, 970-972.
Amari Sachiko
Hohenberg Charles M.
Hoppe Peter
Lewis Reed S.
Nichols Robert Hill Jr.
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