We present strontium, barium, carbon, and silicon isotopic compositions of 61 acidcleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing both strontium and barium contamination. For the first time, by using correlated 88 Sr/ 86 Sr and 138 Ba/ 136 Ba ratios in mainstream SiC grains, we are able to resolve the effect of 13 C concentration from that of 13 C-pocket mass on s-process nucleosynthesis, which points towards the existence of large 13 C-pockets with low 13 C concentrations in AGB stars. The presence of such large 13 C-pockets with a variety of relatively low 13 C concentrations seems to require multiple mixing processes in parent AGB stars of mainstream SiC grains.

Short title: Barium isotopic composition of mainstream SiCs 10 NuGrid collaboration, http://www.nugridstars.org. ABSTRACTWe 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 δ( 138 Ba/ 136 Ba) values are found, down to −400 ‰, which can only be modeled with a flatter 13 C profile within the 13 C pocket than is normally used. The dependence of δ( 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(α,n) 25 Mg rate in the stellar temperature regime relevant to AGB stars, based on δ( 134 Ba/ 136 Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ( 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 ½ = 2.3 Ma) to 135 Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before 135 Cs decayed.

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Graphi cal Abstract 3 Highlights• CHILI is a new resonance ionization mass spectrometer at the University of Chicago • CHILI has been developed for isotopic analysis of small samples in cosmochemistry • CHILI combines high spatial resolution and high sensitivity • Sr, Ba, Fe, and Ni isotopes have been measured in presolar grains • A new timing scheme allows analysis of all Fe and Ni isotopes without interferences 4 ABSTRACT We describe CHILI, the Chicago Instrument for Laser Ionization, a new resonance ionization mass spectrometer developed for isotopic analysis at high spatial resolution and high sensitivity of small samples like contemporary interstellar dust grains returned by the Stardust spacecraft. We explain how CHILI addresses the technical challenges associated with such analyses by pushing most technical specifications towards their physical limits. As an initial demonstration, after many years of designing and developing CHILI, we have analyzed presolar silicon carbide grains for their isotopic compositions of strontium, zirconium, and barium. Subsequently, after further technical improvements, we have used CHILI to analyze, for the first time without interference, all stable isotopes of iron and nickel simultaneously in presolar silicon carbide grains. With a special timing scheme for the ionization lasers, we separated iron and nickel isotopes in the time-of-flight spectrum such that the isobaric interference between 58 Fe and 58 Ni was resolved. In-depth discussion of the astrophysical implications of the presolar grain results is deferred to dedicated later publications. Here we focus on the technical aspects of CHILI, its status quo, and further developments necessary to achieve CHILI's ultimate goals, ~10 nm lateral resolution and 30-40 % useful yield.

Abstract-Atom-probe tomography (APT) is currently the only analytical technique that, due to its spatial resolution and detection efficiency, has the potential to measure the carbon isotope ratios of individual nanodiamonds. We describe three different sample preparation protocols that we developed for the APT analysis of meteoritic nanodiamonds at sub-nm resolution and present carbon isotope peak ratios of meteoritic and synthetic nanodiamonds. The results demonstrate an instrumental bias associated with APT that needs to be quantified and corrected to obtain accurate isotope ratios. After this correction is applied, this technique should allow determination of the distribution of 12 C/ 13 C ratios in individual diamond grains, solving the decades-old question of the origin of meteoritic nanodiamonds: what fraction, if any, formed in the solar system and in presolar environments? Furthermore, APT could help us identify the stellar sources of any presolar nanodiamonds that are detected.

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The isotopic composition of ruthenium (Ru) in individual presolar silicon carbide (SiC) stardust grains bears the signature of s-process nucleosynthesis in asymptotic giant branch stars, plus an anomaly in 99Ru that is explained by the in situ decay of technetium isotope 99Tc in the grains. This finding, coupled with the observation of Tc spectral lines in certain stars, shows that the majority of presolar SiC grains come from low-mass asymptotic giant branch stars, and that the amount of 99Tc produced in such stars is insufficient to have left a detectable 99Ru anomaly in early solar system materials.