Computer Science
Scientific paper
Sep 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000m%26ps...35..997a&link_type=abstract
Meteoritics & Planetary Science, vol. 35, no. 5, pp. 997-1014 (2000).
Computer Science
16
Scientific paper
We report measurements of isotopic ratios of C, N, Mg, Si, Ca, Ti, Cr and Fe in bulk samples (aggregates of many grains) of up to seven different fractions of SiC, ranging from 0.38 to 3.0 μm in diameter, from the Murchison CM2 carbonaceous chondrite. 12C/13C ratios range from 37 to 42 and 14N/15N ratios from 370 to 520, within the range of single grain measurements on coarser samples and in agreement with an asymptotic giant branch (AGB) star origin of most of the grains. Variations among size fractions do not show any simple trend and can be explained by varying contamination with isotopically normal material. Silicon isotopic ratios vary only little and, with one exception, lie to the right of the single-grain mainstream correlation line. This might indicate a higher percentage of the minor populations Y and Z among finer grain size fractions. All bulk samples have large 26Mg excesses attributed to the presence of short-lived 26Al at the time of grain formation. Inferred 26Al/27Al ratios are much larger than those measured in single larger mainstream grains. This is probably because of the presence of SiC grains of type X; we obtain an estimate of 0.4 for their 26Al/27Al ratio. Our Ca isotopic measurements, the first made on presolar SiC grains, show excesses in 42Ca and 43Ca, in general agreement with theoretical expectations for AGB stars. 44Ca excesses are much larger than expected and are probably due to X grains, which have high 44Ca excesses due to the decay of short-lived 44Ti produced in supernova explosions. We arrive at an estimate of 0.014 for the initial 44Ti/48Ti ratio of the X grains, within the range obtained from previous single X grain measurements. The Ti isotopic ratios of the bulk samples show a V-shaped pattern with excesses of all isotopes relative to 48Ti. 46Ti, 47Ti and 50Ti show excesses relative to the correlation between Ti and Si ratios for single grains and are in general agreement with theoretical models of s-process nucleosynthesis in AGB stars. In contrast, 49Ti does not show any excess relative to the single grain data; it also fails to agree with theory, which predicts much larger excesses than observed. Measured 53Cr/52Cr and 57Fe/56Fe ratios are normal within errors. The first result is expected even for Cr in AGB star envelopes, but the second result suggests that most of the Fe analyzed originates from contamination. We have found no simple trends in isotopic composition with respect to grain size that can be interpreted in terms of nucleosynthetic origin, unlike the results for Kr, Xe, Ba, and Sr.
Amari Sachiko
Lewis Roy S.
Zinner Ernst
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