This thesis explores the chemical signatures of galaxy formation and evolution using a software package designed specifically for this investigation. We describe the development of this multi-zone chemical evolution code, which simulates the spacetime evolution of stars, gas and a vast array of chemical elements within galactic systems. We use this tool to analyse observations of a wide range of astrophysical systems. The chemical evolution code is first calibrated using empirical constraints from the Milky Way. These simulations help shed light on the nature of the gas accretion processes that fueled the formation of our Galaxy. We demonstrate the importance of low- and intermediate-mass stars in explaining the elemental and isotopic abundance patterns measured in Galactic stars. An intriguing question in astrophysics is whether pollution from intermediate-mass stellar winds is responsible for anomalous abundances in globular cluster stars. We test this scenario by modelling the formation and chemical evolution of a globular cluster. Recently, the most detailed abundance pattern ever measured beyond the local universe was obtained for a high-redshift quasar absorption cloud, providing an exciting opportunity to explore early conditions of galaxy formation. We compare the chemical abundances in this distant object with predictions from a series of models, in order to gain insight into the protogalaxy's age and star formation history. We continue investigating the high-redshift universe, turning our attention to the issue of space-time variations in the fine-structure constant, as suggested by quasar absorption-line constraints. An excess abundance of heavy Mg isotopes in the absorbing clouds could partly account for the data, without needing to invoke variations in fundamental constants of nature. An enhanced early population of intermediate-mass stars could lead to such extreme Mg isotopic ratios, but we show that additional chemical consequences of this scenario conflict with observations.

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京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22249号 / 理博第4563号 / 新制||理||1655(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 前田 啓一, 講師 LEE Shiu Hang, 教授 長田 哲也 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Where did the elements come from? Why are they found in the abundance that they are? These are two of the fundamental questions that the field of astrophysics has sought to answer. The first major studies of elemental synthesis were done in the 1950's and 1960's. Most notable among them was the Burbidge, Burbidge, Fowler, and Hoyle paper [Clayton 73]. This paper set forth the general theory of elemental synthesis in stars and supernovae by means of nuclear reactions. It remains the leading theory for elemental abundance today.As with most theories, the picture of elemental synthesis remains incomplete. While it is thought that the overall theory is correct, there are still many mysteries in the details. There are several kinds of nuclear reactions that occur in stars and supernova that create the elements heavier than iron. They include the r-process, s-process, and p-process, along with several others. However, there are some elements whose creation is not fully understood. There are a variety of reasons for this, which will be discussed.In our experiment we studied the nuclear reaction properties of an isotope of Copper (65Cu). It is theorized that it is produced by the p-process during a supernova explosion. The p-process can be described simply as the collision of an alpha particle with a large atomic nucleus with a proton byproduct. Little actual experimental data has been taken involving the p-process, which is why we chose this reaction. The experiment was done using the Tandem Van de Graaff Accelerator at Ohio University. / Department of Physics and Astronomy

Thesis (Ph. D.)--University of Canterbury, 2006. / Typescript (photocopy). Includes bibliographical references (p. 195-204). Also available via the World Wide Web.

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<p> In this thesis, a variety of topics are investigated. Part I discusses asymptotic giant branch (AGB) stars. We review their evolution and their contribution to the galactic chemical evolution. We particularly pay attention to the nucleosynthesis in different layers of the AGB stars, and discuss diverse chains of reactions that can happen under different circumstances. </p> <p> Out of many of such reactions, three are the subjects of our special attention. The 23Na(p,α)20Ne, 23Na(p, γ)24Mg and 26YAl(p, γ)27Si reactions are important reactions that are part of the NeNa and MgAl cycles. Their reaction rates used to be uncertain by orders of magnitude, and thus have been subjects of investigation. Recently, there has been new experimental information released on these reactions. In this project, we have used this new information, and have calculated the new reaction rates for those reactions. The results show less uncertainty range in all three reaction rates compared to the prior measurements. </p> <p> We then have used these new less uncertain rates to calculate the AGB yields of hydrogen through to 62Ni. However, these reaction rates only affect the yields of Ne to Si isotopes noticeably, which are presented in Appendix A. Dr. Karakas has calculated the AGB yields by computing stellar evolution and nucleosynthesis models for a 6 M (symbol) AGB star with three different metallicities (Z = 0.02, 0.004 and 0.008) using the new reaction rates. The results show that the changes in the yields due to individually using the updated 23Na(p, γ)24Mg or 23Na(p,α)20Ne reaction rate are noticeable for some isotopes. However, these new reaction rates result in completely opposite changes in most of the yields; moreover, the updated 26gAl(p, γ)27Si reaction rate has no effect on any of the stellar yields except on the yield of 28 Si obtained by the Z = 0.02 model. Thus, by using all three new reaction rates simultaneously in the nucleosynthesis network, we only see major changes for a few isotopes, e.g. significant destruction of 20Ne and considerable production of 23 Na, 24Mg and 28Si. There is no noticeable effect on any of the remaining AGB yields. </p> <p> Part II of this project discusses the significance of studying the nuclear structure of 26Si and 30S, which are not yet well understood. We discuss classical novae and their nucleosynthesis. We pay attention to some reactions, whose rates are still uncertain, e.g. the 25 Al(p, γ)26 Si, and 29 P(p, γ)30S reactions. To lower the uncertainty range in such reaction rates, the structure of 26Si and 30S should be better understood. </p> <p> We have carried out an experiment at Wright Nuclear Structure Laboratory (WNSL) at Yale University to be able to determine whether or not further studies of the structure of 26Si and 308 can be pursued by the (12C,6He) reaction mechanism. We investigated the 20 NeC2C,6He)26 Si and 12C(24 Mg,6He)30 S reactions. The time for collecting the data for the whole experiment was only about five days. Taking into consideration the number of experiments that were done in five days, some of them resulted in low statistics. The 20 NeC2C,6He)26 Si experiment gave a null result. This is due to the fact that the target that was used was old, and the 20 Ne in that target has been diffused out. Thus, we could not determine whether the (12C,6He) reaction mechanism proves to be a good method to study the structure of 26 Si. As for the nuclear structure of 30 8, we could see the ground state and the first excited state. The time was not enough to collect enough data to be able to determine this structure; however, the (12C,6He) reaction mechanism for studying the structure of 30 S looks promising. </p> / Thesis / Master of Science (MSc)

A thorough understanding of nucleosynthesis and element formation in stars of all evolutionary phases is of vital importance in stellar astrophysics. It provides information about internal structure, conditions and nuclear processes occurring in the stellar interior. The heavy elements formed in a star throughout its life are returned to the interstellar medium through mass loss processes. New populations of stars are then formed from this previously enriched material. This continues the cycle of element recycling in the Universe and has great consequence for galactic chemical evolution. As both modelling and observing techniques advance, more surveys are required to ensure there is agreement between the two. It is hoped that when a thorough understanding of the internal processes in giant stars is reached, the evolutionary models will reproduce the observed elemental yields. This work provides an internally self-consistent analysis of the element abundances produced via nucleosynthesis and s-process element formation occurring in giant stars in different stellar environments. High resolution spectroscopic observations have been taken of Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) stars in three different stellar environments. Spectrum synthesis has been used to determine s-process element abundances for RGB stars in the Hyades open cluster, RGB and AGB stars in the globular cluster, 47 Tucanae, and AGB stars in the galactic field. It was found that the two Hyades giant studied showed solar, or near-solar, abundances of s-process elements. Enhancements in the light s-process elements, Y and Zr, of +0.02 to +0.11 were observed, while enhancements in the heavy s-process elements, La, Pr and Nd, ranged from +0.06 to +0.16. These results are consistent with previous findings of enhancements in Y of ~+0.12, and of ~+0.15 for the heavy s-process elements. The results from 47 Tucanae suggest a genuine star-to-star scatter in the s-process element abundances in the giant stars of this globular cluster. This is unexpected due to the fact that stars in a globular cluster are thought to have the same formation and chemical history. However, spreads in s-process element abundances of as much as +-0.7 dex are observed between this study and three other studies of similar stars in the same cluster. A range of field stars along the AGB phase, ranging from M to MS to S to SC, have been analysed for s-process enrichment. The observed element abundances are compared with those predicted by recent modelling of the AGB phase of evolution. Enhancements in s-process element abundances range from [s/Fe]~0.00 for M stars, to ~+0.50 for MS stars, through to ~+0.95 for S stars. The comparison of these enhancements with those predicted by modelling provides an indication of the success of these models and will enable theoreticians to further refine their understanding of the internal nucleosynthetic processes present in giant stars.

The detection of γ-rays from explosive astrophysical scenarios such as novae provides an excellent opportunity for the study of on-going nucleosynthesis in the Universe. Within this context, this work has addressed an uncertainty in the destruction rate of the 18F nucleus, thought to be the primary source of 511 keV γ-rays from novae. A direct measurement of the 18F(p,α )15O cross section has provided the opportunity to extract resonance parameters through the R-Matrix formalism. The inferred parameters of populated states in 19Ne include the observation of a broad 1/2+ state, consistent with a recent theoretical prediction, which will have a significant impact on the rate of destruction of this γ-ray producing radioisotope. The 18O(p,α )15N reaction follows similar nuclear and kinematic processes and is expected to occur in the hydrogen burning layers of AGB stars. Resonance widths have been extracted from a direct measurement in the region around a poorly constrained broad state close to the Gamow window. This has produced a new parameter set for future reference and provides new information on the reaction rate. The complex R-Matrix formalism used in these analyses is a crucial tool in the study of nuclear astrophysics reactions, and many codes have been written to implement the complex mathematics. This thesis presents a comparison of two publicly available codes from the JINA collaboration and a code used extensively by the University of Edinburgh. For this, the recent results of the 18F destruction reaction, presented here, have been used. A minor error was found within one of the codes, and corrected. The final parameters extracted, and the resulting cross sections calculations, are shown to be consistent between the three codes. A further γ-ray line of interest at 1.809 MeV, characteristic of 26Al decay, has been observed throughout the interstellar medium. If, however, this isotope is formed in a known isomeric state, its decay bypasses the emission of this γ-ray, thus complicating the interpretation of observed γ-ray fluxes. To this end, an experiment has been carried out, providing proof of principle of a direct measurement of the 26mAl(p,γ)27Si reaction. The calculation of the isomeric intensity is presented here.

Using two techniques not previously applied to Cassiopeia A (Cas A), we measure the reddening toward its expansion center. An estimate of AV from the near-IR [Fe II] lines is hampered by uncertain atomic data, though the spatial variation in their flux ratio allows relative measurement of the extinction in regions without previous optical estimates. We use a second technique based on the broad-band IR shape of the synchrotron emission, and find Aᵥ = 6.2 ± 0.6 for a knot 13" from the expansion center. Assuming a plausible lower limit on the apparent magnitude of the SN in outburst, the ⁵⁶Ni yield was 0.058 < M(Ni) < 0.16M⊙. With the ⁴⁴Ti mass from published gamma-ray observations, this implies a ⁴⁴Ca/ ⁵⁶Fe ratio consistent with the solar abundance. Recently published Spitzer Space Telescope IRS observations detect dust and line emission from cold gas interior to Cas A’s reverse shock. Using simple physical arguments and new hydrodynamic, non-equilibrium photoionization calculations, we infer the physical conditions in this material. We find that the mid-IR bright clumps are photoionized by the SNR shocks, over-dense relative to the expected average in the interior of the remnant, and have abundances consistent with incomplete oxygen burning. The lack of detectable iron lines indicates that any Si-burning material still interior to the reverse shock must be far more tenuous than the clumps of O-burning ashes. Finally, we present calculations from a new multi-dimensional hydrodynamics and non-equilibrium ionization and cooling code designed to model the emission from SNR shocks. Two-dimensional simulations of a shock-cloud interaction in a pure-oxygen plasma, with flow parameters relevant to Cas A, show a wider range of temperatures and ionization states than is typical in single-zone or 1D calculations, indicating that fluid and cooling instabilities play a role in producing the observed spectra of radiative shocks in metal-rich gas.