Tungsten isotopic compositions in stardust SiC grains from the Murchison meteorite: Constraints on the s-process in the Hf-Ta-W-Re-Os region
Date
2012
Authors
Avila, Janaina
Lugaro, Maria
Gyngard, Frank
Zinner, Ernst
Cristallo, Sergio
Holden, Peter
Buntain, Joelene
Amari, Sachiko
Karakas, Amanda
Ireland, Trevor
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IOP Publishing
Abstract
We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes182,183,184,186W and179,180Hf were measured on both an aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction) believed to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate shows small deviations from terrestrial (= solar) composition in the182W/184W and183W/184W ratios, with deficits in182W and183W with respect to184W. The186W/184W ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar182W/184W,183W/184W, and186W/184W ratios. We have compared the SiC data with theoretical predictions of the evolution of W isotopic ratios in the envelopes of AGB stars. These ratios are affected by the slow neutron-capture process and match the SiC data regarding their182W/184W,183W/184W, and179Hf/180Hf isotopic compositions, although a small adjustment in the s-process production of183W is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the186W/184W ratios observed in the SiC grains, even when the current185W neutron-capture cross section is increased by a factor of two. Further study is required to better assess how model uncertainties (e.g., the formation of the13C neutron source, the mass-loss law, the modeling of the third dredge-up, and the efficiency of the22Ne neutron source) may affect current s-process predictions.
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Keywords: dust, extinction; nuclear reactions, nucleosynthesis, abundances; stars: AGB and post-AGB; stars: carbon
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Astrophysical Journal, The
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Journal article
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Open Access
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