s‐Process Sr Isotopic Compositions in Presolar SiC from the Murchison Meteorite

Abstract

We have used thermal ionization mass spectrometry to measure the isotopic compositions of Sr in size-sorted samples (the KJ series) of interstellar SiC separated from the Murchison meteorite. The Sr compositions are radically anomalous in comparison to those in normal solar system materials. As for other heavy elements in presolar SiC, the anomalies can be understood to represent enrichment of s-process (slow neutron capture) nucleosynthetic products, plausibly occurring in asymptotic giant branch stars. The inferred s-process 88 Sr/ 86 Sr ratios are variable and correlate with grain size: this ratio decreases (suggesting lower effective neutron exposure) with increasing grain size. The sense of this trend is the same as that previously observed in Ba, but opposite to that previously observed in Kr, in the same samples. The Sr and Ba trends indicate covariation of the factors governing neutron exposure and grain growth kinetics in a manner not yet understood. The contrasting trend for Kr suggests that the mechanisms for the introduction of volatile (Kr) and refractory (Sr and Ba) elements into SiC produce a biased sampling of the population of different nucleosynthetic products. The relative abundances of s-process 86 Sr, 87 Sr, and 88 Sr in Murchison SiC are very close to those for early solar system Sr, but in view of the different s-process superpositions and other contributions to all three of these isotopes, the close agreement must be regarded as coincidental.

Nucleosynthetic Signatures in Presolar SiC and Graphite Grains

“…Silicon isotopic ratios in mainstream, Y and Z grains show a systematic trend. First, average 29 Si/ 28 Si ratios systematically decrease from mainstream Bulk analyses of SiC-rich fractions show that they are enriched in s-process Kr [26][27][28], Xe [27,28], Sr [29], Ba [30][31][32], Nd and Sm [31]. Since the majority of SiC grains are mainstream grains, the s-process enrichments were most likely carried by mainstream grains.…”

“…Low-density graphite grains show 18 O enrichment in many of the grains that are taken as proof that they formed in core-collapse supernovae: 18 O is produced via 14 N(α,γ) 18 F(e + ν) 18 O in the partial He burning zone, resulting 18 O/ 16 O ratio of 1.68 in a 15M sun model by Rauscher et al [40]. Many low-density graphite grains show 28 Si excesses, similar to SiC X grains from supernovae, while a few show 29 Si and 30 Si excesses. 28 Si is produced during O burning and excesses in neutron-rich isotopes are indicative of neutron captures in various nucleosynthetic zones.…”

Presolar grains

“…The first approach was used in early measurements of Ba, Nd and Sm in aggregates of presolar SiC by secondary ion mass spectrometry [5] and in recent measurements of SiC-bearing presolar diamond aggregates by inductively coupled plasma (ICP) mass spectrometry [6]. The second approach has been used to measure the isotopic compositions of Sr [7], Ba [8,9], Nd [10], Sm [10] and Dy [11] in SiC aggregates by thermal ionization mass spectrometry. When elements are only partially separated from one another, the properties of elements can be used to reduce interferences.…”

“…When elements are only partially separated from one another, the properties of elements can be used to reduce interferences. For example, before measuring Sr isotopes, the filament is heated to a temperature where Rb evaporates and Sr remains on the filament, reducing the isobaric interference between 87 Rb and 87 Sr [7]. The third approach has been applied to measurements of Sr [12], Zr [13], Mo [14], Ru [15] and Ba [16] in individual presolar SiC grains by laser desorption and laser resonant ionization mass spectrometry [17].…”

“…The abundances of heavy trace elements in presolar grains are low, even when overproduction due to the s-process in mainstream SiC grains is taken into account [18]. For this reason, all early measurements were made on aggregates of thousands of presolar grains [5][6][7][8][9][10][11]. With one exception, all measurements of…”