Si Isotopic Ratios in Mainstream Presolar SiC Grains Revisited

Although mainstream SiC grains, the major group of presolar SiC grains found in meteorites, are believed to have originated in the expanding envelope of asymptotic giant branch (AGB) stars during their late carbon-rich phases, their Si isotopic ratios show a distribution that cannot be explained by nucleosynthesis in this kind of stars. Previously, this distribution has been interpreted to be the result of contributions from many AGB stars of different ages whose initial Si isotopic ratios vary due to the Galactic chemical evolution of the Si isotopes. This paper presents a new interpretation based on local heterogeneities of the Si isotopes in the interstellar medium at the time the parent stars of the mainstream grains were born. Recently, several authors have presented inhomogeneous chemical evolution models of the Galactic disk in order to account for the well known evidence that F and G dwarfs of similar age show an intrinsic scatter in their elemental abundances.Comment: Accepted for publication by ApJ. 19 pages of text + 17 figures and 4 table

The Sixth Torino Workshop

Nuclear astrophysics is about 50 years old. It grew from nuclear physics and was nurtured by those with an interest in astrophysics. A milestone was the publication of the seminal paper of Geoffrey Burbdige, Margaret Burbidge, Willy Fowler and Fred Hoyle in 1957. Since then, it has become a mainstay of modern astrophysics, and has a natural and especially close link to stellar astrophysics. But there has been a revolution in recent years, which has seen the isolation and analysis of circumstellar dust grains, recovered from meteorites. These 'pre-solar'grains (indicating that they predate the solar system, and survived the various processes associated with its formation) have provided incomparable and revolutionary data of exceptional quality. It is now common to have access to isotopic abundance ratios for major elements as well as for trace elements, and these can be magnificent indications for processes active in the parent star. Much of the presolar grain analysis has been performed on samples of the Murchison meteorite, which fell around the little town of Murchison, near Melbourne, in 1969. The time was right for a workshop dedicated to the astrophysics learned from this analysis, and we are pleased to present some of the papers in this issue of PASA. This workshop was also the sixth in a series of workshops on nuclear astrophysics, initiated at Torino University. We are proud to continue the name and the tradition of these workshops. This was only the second Torino workshop held outside of Italy. The first was organised by Manuel Forestini, in France. Tragically, Manuel suffered a fatal heart attack on March 11, 2003. Manuel worked in many areas of stellar evolution, and was the author of a textbook on the subject. But his main area was AGB stars, and he has contributed much to our understanding of these complex objects, and especially their nucleosynthesis. Manuel did everything with passion and good humour, and was a very dear friend and a valued collaborator. We wish to dedicate this meeting to his memory, his science and his friendship

Iron and Nickel Isotopic Ratios in Presolar SiC Grains

We report the first Fe isotopic anomalies and the first Ni isotopic ratio measurements in presolar SiC grains of separate KJG from the Murchison meteorite. With NanoSIMS, we analyzed Fe and Ni in 37 X grains from Type II supernovae and 53 SiC grains of other types. The Ni/Fe and Co/Fe ratios in grains of all types are much higher than in the gas from which the grains are believed to have condensed. A majority of the X grains and a couple of mainstream grains contain Fe-rich subgrains. Most X grains have large excesses in 57Fe,61Ni, and 62Ni.60Ni excesses are small and the 54Fe/56Fe ratios of almost all X grains are normal. These isotopic compositions are best explained by mixing of material from the He/N zone of Type II supernovae with material from the He/C zone. The lack of any 54Fe excesses is puzzling in view of the fact that the Si/S zone, whose contribution resulted in the 28Si excesses in X grains, is very rich in 54Fe. It has yet to be seen whether elemental fractionation between Si and Fe is an explanation. The 57Fe deficits observed in a few X grains remain unexplained. In comparison to the X grains, fewer mainstream and AB grains have anomalies. Observed 62Ni excesses in some mainstream grains are larger than predicted for AGB stars of solar metallicity and are not accompanied by corresponding 61Ni excesses. A Y grain and a Z grain have excesses in 54Fe and 62Ni, but close to normal 57Fe/56Fe and 60,61Ni/58Ni ratios. These isotopic compositions are not expected for grains from low-metallicity AGB stars

Sulfur isotopic compositions of submicrometer SiC grains from the Murchison meteorite

We report C, Si, N, S, Mg-Al, and Ca-Ti isotopic compositions of presolar silicon carbide (SiC) grains from the SiC-rich KJE size fraction (0.5-0.8 μm) of the Murchison meteorite. One thousand one hundred thirteen SiC grains were identified based on their C and Si isotopic ratios. Mainstream, AB, C, X, Y, and Z subtypes of SiC, and X-type silicon nitride (Si₃N₄) account for 81.4%, 5.7%, 0.1%, 1.5%, 5.8%, 4.9%, and 0.4%, respectively. Twenty-five grains with unusual Si isotopic ratios, including one C grain, 16 X grains, 1 Y grain, 5 Z grains, and 2 X-type Si₃N₄ grains were selected for N, S, Mg-Al, and Ca-Ti isotopic analysis. The C grain is highly enriched in ²⁹Si and ³⁰Si (δ²⁹Si = 1345‰ ± 19‰, δ³⁰Si = 1272‰ ± 19‰). It has a huge ³²S excess, larger than any seen before, and larger than that predicted for the Si/S supernova (SN) zone, providing evidence against the elemental fractionation model by Hoppe et al. Two SN models investigated here present a more satisfying explanation in terms of a radiogenic origin of ³²S from the decay of short-lived ³²Si (τ1/2 = 153 yr). Silicon-32 as well as ²⁹Si and ³⁰Si can be produced in SNe by short neutron bursts; evidence for initial 44Ti (τ1/2 = 60 yr) in the C grain is additional evidence for an SN origin. The X grains have marginal ³²S excesses, much smaller than expected from their large ²⁸Si excesses. Similarly, the Y and Z grains do not show the S-isotopic anomalies expected from their large Si isotopic anomalies. Low intrinsic S contents and contamination with isotopically normal S are the most likely explanations

Constraints on the Nature of the s- and r-processes

AbstractNeutron-capture (Z > 30) elements are detected in many very metal-poor halo stars, and so they must have been manufactured by some of the earliest element donors in our Galaxy's history. The bulk amounts of neutron-capture elements with respect to the iron group vary by several orders of magnitude from star to star at low metallicities. Additionally, abundance distributions among these elements are often strikingly different from that of the solar system. Some stars exhibit abundances that must have been made purely in "rapid" neutron-capture events (the r-process), some in "slow" events (the s-process), and some have hybrid mixes. Here we summarize the major observed categories of the neutron-capture abundances in metal-poor stars, and discuss their implications for early Galactic nucleosynthesis

New constraints on the major neutron source in low-mass AGB stars

We compare updated Torino postprocessing asymptotic giant branch (AGB) nucleosynthesis model calculations with isotopic compositions of mainstream SiC dust grains from low-mass AGB stars. Based on the data-model comparison, we provide new constraints on the major neutron source, 13C({\alpha},n)16O in the He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba grain data can only be consistently explained by the Torino model calculations that adopt the recently proposed magnetic-buoyancy-induced 13C-pocket. This observation provides strong support to the suggestion of deep mixing of H into the He-intershell at low 13C concentrations as a result of efficient transport of H through magnetic tubes.Comment: ApJ, accepte

Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models

Asymptotic Giant Branch (AGB) stars play a fundamental role in the s-process nucleosynthesis during their thermal pulsing phase. The theoretical predictions obtained by AGB models at different masses, s-process efficiencies, dilution factors and initial r-enrichment, are compared with spectroscopic observations of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s), collected from the literature. We discuss here five stars as example, CS 22880-074, CS 22942-019, CS 29526-110, HE 0202-2204, and LP 625-44. All these objects lie on the main-sequence or on the giant phase, clearly before the TP-AGB stage: the hypothesis of mass transfer from an AGB companion, would explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are CEMP(s+r) objects, and are interpreted assuming that the molecular cloud, from which the binary system formed, was already enriched in r-process elements by SNII pollution. In several cases, the observed s-process distribution may be accounted for AGB models of different initial masses with proper 13C-pocket efficiency and dilution factor. Na (and Mg), produced via the neutron capture chain starting from 22Ne, may provide an indicator of the initial AGB mass.Comment: 8 pages, 6 figures, 2 table