BARIUM ISOTOPIC COMPOSITION OF MAINSTREAM SILICON CARBIDES FROM MURCHISON: CONSTRAINTS FORs-PROCESS NUCLEOSYNTHESIS IN ASYMPTOTIC GIANT BRANCH STARS

Nan Liu,Sergio Cristallo,Oscar Straniero,Michael R Savina,Frank Gyngard,Roberto Gallino,Sara Bisterzo,Andrew M Davis,Michael J Pellin,Nicolas Dauphas,Marco Pignatari,David G Willingham,Falk Herwig

doi:10.1088/0004-637x/786/1/66

Abstract

We present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in $\delta$($^{138}$Ba/$^{136}$Ba) values are found, down to $-$400 permil, which can only be modeled with a flatter $^{13}$C profile within the $^{13}$C pocket than is normally used. The dependence of $\delta$($^{138}$Ba/$^{136}$Ba) predictions on the distribution of $^{13}$C within the pocket in AGB models allows us to probe the $^{13}$C profile within the $^{13}$C pocket and the pocket mass in asymptotic giant branch (AGB) stars. In addition, we provide constraints on the $^{22}$Ne$(\alpha,n)^{25}$Mg rate in the stellar temperature regime relevant to AGB stars, based on $\delta$($^{134}$Ba/$^{136}$Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative $\delta$($^{134}$Ba/$^{136}$Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process ($i$-process), which is activated by the Very Late Thermal Pulse (VLTP) during the post-AGB phase and characterized by a neutron density much higher than the $s$-process. These two grains may have condensed around post-AGB stars. Finally, we report abundances of two $p$-process isotopes, $^{130}$Ba and $^{132}$Ba, in single SiC grains. These isotopes are destroyed in the $s$-process in AGB stars. By comparing their abundances with respect to that of $^{135}$Ba, we conclude that there is no measurable decay of $^{135}$Cs ($t_{1/2}$= 2.3 Ma) to $^{135}$Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before $^{135}$Cs decayed.

Highlights

Presolar silicon carbides (SiC grains) are pristine microcrystals that condensed in carbon-rich stellar winds and/or explosions (Lodders & Fegley 1995), were ejected into the interstellar medium preserving their nucleosynthetic origin, transported to the protosolar nebula, incorporated in meteorite parent bodies, and delivered to Earth in meteorites, where they were discovered over 25 years ago via their exotic isotopic signatures (Bernatowicz et al 1987; Zinner et al 1987; Lewis et al 1990)
134Ba and 136Ba both are pure s-process isotopes shielded by their stable xenon isobars, their relative abundances produced during asymptotic giant branch (AGB) nucleosynthesis may deviate from the solar ratio because of the branch point at 134Cs
Once 135Cs is produced, it is stable (t1/2 = 2.3 Ma; Ma stands for millions of years) on the timescale of the s process in AGB stars (t ∼ 20 ka; Gallino et al 1998) and continues to undergo neutron capture to form unstable 136Cs (t1/2 = 13 days), almost all of which decays to 136Ba

Summary

Introduction

Presolar silicon carbides (SiC grains) are pristine microcrystals that condensed in carbon-rich stellar winds and/or explosions (Lodders & Fegley 1995), were ejected into the interstellar medium preserving their nucleosynthetic origin, transported to the protosolar nebula, incorporated in meteorite parent bodies, and delivered to Earth in meteorites, where they were discovered over 25 years ago via their exotic isotopic signatures (Bernatowicz et al 1987; Zinner et al 1987; Lewis et al 1990). Extensive studies of isotopic anomalies of light elements (A < 56) in presolar SiC grains by Secondary Ion Mass Spectrometry (SIMS) confirmed that different types of SiC grains have different types of parent stars, with the majority of them (mainstream grains) originating from low-mass asymptotic giant branch (AGB) stars (Hoppe et al 1994; Zinner 2004; Clayton & Nittler 2004; Davis 2011). Single mainstream grains by Resonance Ionization Mass Spectrometry (RIMS) showed clear s-process signatures (Nicolussi et al 1997, 1998; Savina et al 2003a, 2004; Barzyk et al 2007), providing constraints on free parameters in AGB nucleosynthesis calculations. The measurement of abundance anomalies in their isotopic compositions allows the study of s-process nucleosynthesis in individual stars at a level of precision unavailable to spectroscopic observations

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