Chicago Center for Cosmochemistry

Chicago, IL, United States

Chicago Center for Cosmochemistry

Chicago, IL, United States
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Liu N.,Carnegie Institution for Science | Stephan T.,University of Chicago | Stephan T.,Chicago Center for Cosmochemistry | Boehnke P.,University of Chicago | And 12 more authors.
Astrophysical Journal Letters | Year: 2017

We report Mo isotopic data of 27 new presolar SiC grains, including 12 14N-rich AB (14N/15N > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takes place, as their stellar source. On the other hand, low-mass CO novae and early R- and J-type carbon stars show 13C and 14N excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. Because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%-15% of all carbon stars) are thus likely to be a dominant source of AB2 grains. © 2017. The American Astronomical Society. All rights reserved..


Liu N.,University of Chicago | Liu N.,Chicago Center for Cosmochemistry | Liu N.,Argonne National Laboratory | Savina M.R.,Chicago Center for Cosmochemistry | And 17 more authors.
Astrophysical Journal | Year: 2014

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 δ(138Ba/136Ba) values are found, down to -400‰, which can only be modeled with a flatter 13C profile within the 13C pocket than is normally used. The dependence of δ(138Ba/136Ba) predictions on the distribution of 13C within the pocket in asymptotic giant branch (AGB) models allows us to probe the 13C profile within the 13C pocket and the pocket mass in AGB stars. In addition, we provide constraints on the 22Ne(α, n)25Mg rate in the stellar temperature regime relevant to AGB stars, based on δ( 134Ba136Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ(134Ba 136Ba) 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 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, 130Ba and 132Ba, in single SiC grains. These isotopes are destroyed in the s process in AGB stars. By comparing their abundances with respect to that of 135Ba, we conclude that there is no measurable decay of 135Cs (t 1/2 = 2.3 Ma) to 135Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before 135Cs decayed. © 2014. The American Astronomical Society. All rights reserved..


Pignatari M.,University of Hull | Pignatari M.,Hungarian Academy of Sciences | Herwig F.,University of Victoria | Herwig F.,Joint Institute for Nuclear Astrophysics | And 21 more authors.
Astrophysical Journal, Supplement Series | Year: 2016

We provide a set of stellar evolution and nucleosynthesis calculations that applies established physics assumptions simultaneously to low- and intermediate-mass and massive star models. Our goal is to provide an internally consistent and comprehensive nuclear production and yield database for applications in areas such as presolar grain studies. Our non-rotating models assume convective boundary mixing (CBM) where it has been adopted before. We include 8 (12) initial masses for Z = 0.01 (0.02). Models are followed either until the end of the asymptotic giant branch phase or the end of Si burning, complemented by simple analytic core-collapse supernova (SN) models with two options for fallback and shock velocities. The explosions show which pre-SN yields will most strongly be effected by the explosive nucleosynthesis. We discuss how these two explosion parameters impact the light elements and the s and p process. For low- and intermediate-mass models, our stellar yields from H to Bi include the effect of CBM at the He-intershell boundaries and the stellar evolution feedback of the mixing process that produces the pocket. All post-processing nucleosynthesis calculations use the same nuclear reaction rate network and nuclear physics input. We provide a discussion of the nuclear production across the entire mass range organized by element group. The entirety of our stellar nucleosynthesis profile and time evolution output are available electronically, and tools to explore the data on the NuGrid VOspace hosted by the Canadian Astronomical Data Centre are introduced. © 2016. The American Astronomical Society. All rights reserved.


Battino U.,University of Basel | Pignatari M.,University of Hull | Pignatari M.,Hungarian Academy of Sciences | Ritter C.,University of Victoria | And 13 more authors.
Astrophysical Journal | Year: 2016

The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up (TDU), where the 13C pocket for the s process in AGB stars forms. In this work, we apply a CBM model motivated by simulations and theory to models with initial mass M = 2 and M = 3 M⊙, and with initial metal content Z = 0.01 and Z = 0.02. As reported previously, the He-intershell abundances of 12C and 16O are increased by CBM at the bottom of the pulse-driven convection zone. This mixing is affecting the 22Ne (α, n) 25Mg activation and the s-process efficiency in the 13C-pocket. In our model, CBM at the bottom of the convective envelope during the TDU represents gravity wave mixing. Furthermore, we take into account the fact that hydrodynamic simulations indicate a declining mixing efficiency that is already about a pressure scale height from the convective boundaries, compared to mixing-length theory. We obtain the formation of the 13C-pocket with a mass of ≈10-4 M⊙. The final s-process abundances are characterized by 0.36 < [s/Fe] < 0.78 and the heavy-to-light s-process ratio is -0.23 < [hs/ls] < 0.45. Finally, we compare our results with stellar observations, presolar grain measurements and previous work. © 2016. The American Astronomical Society. All rights reserved.


Pignatari M.,Hungarian Academy of Sciences | Pignatari M.,University of Basel | Zinner E.,Washington University in St. Louis | Hoppe P.,Max Planck Institute for Chemistry | And 10 more authors.
Astrophysical Journal Letters | Year: 2015

Carbon-rich grains with isotopic anomalies compared to the Sun are found in primitive meteorites. They were made by stars, and carry the original stellar nucleosynthesis signature. Silicon carbide grains of Type X and C and low-density (LD) graphites condensed in the ejecta of core-collapse supernovae. We present a new set of models for the explosive He shell and compare them with the grains showing 12C/13C and 14N/15N ratios lower than solar. In the stellar progenitor H was ingested into the He shell and not fully destroyed before the explosion. Different explosion energies and H concentrations are considered. If the supernova shock hits the He-shell region with some H still present, the models can reproduce the C and N isotopic signatures in C-rich grains. Hot-CNO cycle isotopic signatures are obtained, including a large production of 13C and 15N. The short-lived radionuclides 22Na and 26Al are increased by orders of magnitude. The production of radiogenic 22Ne from the decay of 22Na in the He shell might solve the puzzle of the Ne-E(L) component in LD graphite grains. This scenario is attractive for the SiC grains of type AB with 14N/15N ratios lower than solar, and provides an alternative solution for SiC grains originally classified as nova grains. Finally, this process may contribute to the production of 14N and 15N in the Galaxy, helping to produce the 14N/15N ratio in the solar system. © 2015. The American Astronomical Society. All rights reserved..


Liu N.,University of Chicago | Liu N.,Chicago Center for Cosmochemistry | Liu N.,Argonne National Laboratory | Savina M.R.,Chicago Center for Cosmochemistry | And 16 more authors.
Astrophysical Journal | Year: 2015

We present strontium, barium, carbon, and silicon isotopic compositions of 61 acid-cleaned 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 88Sr/86Sr and 138Ba/136Ba ratios in mainstream SiC grains, we are able to resolve the effect of 13C concentration from that of 13C-pocket mass on s-process nucleosynthesis, which points toward the existence of large 13C pockets with low 13C concentrations in asymptotic giant branch stars. The presence of such large 13C pockets with a variety of relatively low 13C concentrations seems to require multiple mixing processes in parent asymptotic giant branch stars of mainstream SiC grains. © 2015. The American Astronomical Society. All rights reserved.


Lugaro M.,Monash University | Tagliente G.,National Institute of Nuclear Physics, Italy | Tagliente G.,Ghent University | Karakas A.I.,Australian National University | And 6 more authors.
Astrophysical Journal | Year: 2014

We present model predictions for the Zr isotopic ratios produced by slow neutron captures in C-rich asymptotic giant branch (AGB) stars of masses 1.25-4 M⊙ and metallicities Z = 0.01-0.03, and compare them to data from single meteoritic stardust silicon carbide (SiC) and high-density graphite grains that condensed in the outflows of these stars. We compare predictions produced using the Zr neutron-capture cross sections from Bao et al. and from n-TOF experiments at CERN, and present a new evaluation for the neutron-capture cross section of the unstable isotope 95Zr, the branching point leading to the production of 96Zr. The new cross sections generally present an improved match with the observational data, except for the 92Zr/94Zr ratios, which are on average still substantially higher than predicted. The 96Zr/94Zr ratios can be explained using our range of initial stellar masses, with the most 96Zr-depleted grains originating from AGB stars of masses 1.8-3 M⊙ and the others from either lower or higher masses. The 90, 91Zr/94Zr variations measured in the grains are well reproduced by the range of stellar metallicities considered here, which is the same needed to cover the Si composition of the grains produced by the chemical evolution of the Galaxy. The 92Zr/94Zr versus 29Si/28Si positive correlation observed in the available data suggests that stellar metallicity rather than rotation plays the major role in covering the 90, 91, 92Zr/94Zr spread. © 2014. The American Astronomical Society. All rights reserved.


Liu N.,University of Chicago | Liu N.,Chicago Center for Cosmochemistry | Liu N.,Argonne National Laboratory | Gallino R.,University of Turin | And 9 more authors.
Astrophysical Journal | Year: 2014

We present postprocess asymptotic giant branch (AGB) nucleosynthesis models with different 13C-pocket internal structures to better explain zirconium isotope measurements in mainstream presolar SiC grains by Nicolussi et al. and Barzyk et al. We show that higher-than-solar 92Zr/ 94Zr ratios can be predicted by adopting a 13C-pocket with a flat 13C profile, instead of the previous decreasing-with-depth 13C profile. The improved agreement between grain data for zirconium isotopes and AGB models provides additional support for a recent proposal of a flat 13C profile based on barium isotopes in mainstream SiC grains by Liu et al. © 2014. The American Astronomical Society. All rights reserved..


Stephan T.,University of Chicago | Stephan T.,Chicago Center for Cosmochemistry | Trappitsch R.,University of Chicago | Trappitsch R.,Chicago Center for Cosmochemistry | And 16 more authors.
International Journal of Mass Spectrometry | Year: 2016

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 58Fe and 58Ni 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. © 2016 Elsevier B.V.


Pellin M.J.,Argonne National Laboratory | Pellin M.J.,University of Chicago | Veryovkin I.V.,Argonne National Laboratory | Veryovkin I.V.,Chicago Center for Cosmochemistry | And 14 more authors.
European Journal of Mass Spectrometry | Year: 2010

There are four generally mutually exclusive requirements that plague many mass spectrometric measurements of trace constituents: (1) the small size (limited by the depth probed) of many interesting materials requires high useful yields to simply detect some trace elements, (2) the low concentrations of interesting elements require efficient discrimination from isobaric interferences, (3) it is often necessary to measure the depth distribution of elements with high surface and low bulk contributions, and (4) many applications require precise isotopic analysis. Resonant ionization mass spectrometry has made dramatic progress in addressing these difficulties over the past five years. © IM Publications LLP 2010.

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