This dissertation describes our efforts to use the assembly of matter on nuclear scales as a probe of the assembly of matter on Galactic scales. To investigate the former, we characterize the detailed abundance patterns of the heaviest elements found in ancient, metal-poor stars in the Galaxy. In particular, we place new constraints on and identify several new correlations among the nuclei produced by the rapid nucleosynthetic process, which we use to refine current models of the physical conditions of this process. To investigate the latter, we apply our knowledge of stellar nucleosynthesis to examine correlations between the space motions of stars and their compositions, which retain a record of the composition of the interstellar medium where they formed many billions of years ago. Using new high quality stellar spectra collected from McDonald Observatory and Las Campanas Observatory, we confirm the relative chemical homogeneity of a well-known stellar stream and identify several chemical differences between the two major components of the stellar halo of the Galaxy. Each of these results has significant implications for our understanding of how the Galactic halo formed, grew, and evolved. More profoundly, these results indicate that we have not yet fully characterized the cosmic origins of the heaviest elements in the universe and that we will likely need to examine large samples of metal-poor stars at great distances from the Sun to potentially do so. / text

Tracing the element enrichment in the Universe requires to understand the element production in stellar models which is not well understood, in particular at low metallicity. In this thesis a variety of nucleosynthesis processes in stellar models across initial masses and metallicities is investigated and their relevance for chemical evolution explored. Stellar nucleosynthesis is investigated in asymptotic giant branch (AGB) models and massive star models with initial masses between 1 M⊙ and 25 M⊙ for metal fractions of Z = 0.02, 0.01, 0.006, 0.001, 0.0001. A yield grid with elements from H to Bi is calculated. It serves as an input for chemical evolution simulations. AGB models are computed towards the end of the AGB phase and massive star models are calculated until core collapse followed by explosive core-collapse nucleosynthesis. The simulations include convective boundary mixing in all AGB star models and feature efficient hot-bottom burning and hot dredge-up in AGB models as well the predictions of both heavy elements and CNO species under hot-bottom burning conditions. H-ingestion events in the low-mass low-Z AGB model with initial mass of 1M⊙ at Z = 0.0001 result in the production of large amounts of heavy elements. In super-AGB models H ingestion could potentially lead to the intermediate neutron-capture process. To model the chemical enrichment and feedback of simple stellar populations in hydrodynamic simulations and semi-analytic models of galaxy formation the SYGMA module is created and its functionality is verified through a comparison with a widely adopted code. A comparison of ejecta of simple stellar populations based on yields of this work with a commonly adopted yield set shows up to a factor of 3.5 and 4.8 less C and N enrichment from AGB stars at low metallicity which is attributed to complete stellar models, the modeling of the AGB stage and hot-bottom burning in super- AGB stars. Analysis of two different core-collapse supernova fallback prescriptions show that the total amount of Fe enrichment by massive stars differs by up to two at Z = 0.02. Insights into the chemical evolution at very low metallicity as motivated by the observations of extremely metal poor stars require to understand the H-ingestion events common in stellar models of low metallicity. The occurrence of H ingestion events in super-AGB stars is investigated and identified as a possible site for the production of heavy elements through the intermediate neutron capture process. The peculiar abundance of some C-Enhanced Metal Poor stars are explained with simple models of the intermediate neutron capture process. Initial efforts to model this heavy element production in 3D hydrodynamic simulations are presented. For the first time the nucleosynthesis of interacting convective O and C shells in massive star models is investigated in detail. 1D calculations based on input from 3D hydrodynamic simulations of the O shell show that such interactions can boost the production of odd-Z elements P, Cl, K and Sc if large entrainment rates associated with O-C shell merger are assumed. Such shell merger lead in stellar evolution models to overproduction factors beyond 1 dex and p-process overproduction factors above 1 dex for 130,132Ba and heavier isotopes. Chemical evolution models are able to reproduce the Galactic abundance trends of these odd-Z elements if O-C shell merger occur in more than 50% of all massive stars. / Graduate

<p>The astrophysical ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate is a key quantity used in both classical nova and type I X-ray burst models that predict isotopic abundances produced during nucleosynthesis in the outburst. Currently, uncertainties in ³¹S structure parameters lead to a variation in the reaction rate by a factor of 20 at nova temperatures causing predicted isotopic abundance ratios in the Si-Ar mass region to vary by factors of up to 4. The ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate can be determined indirectly by measuring transfer reactions populating excited states in ³¹S. Nuclear structure information for ³¹S resonant states above the proton threshold of 6131 keV and within the Gamow window that contribute most significantly to the reaction rate can be used to re-evaluate the rate for nova and type I X-ray burst temperatures and reduce current uncertainties. We have performed an experiment in order to study the level structure of ³¹S via the ³²S(<em>d</em>,<em>t</em>)³¹S single-nucleon transfer reaction using the MP tandem accelerator and Q3D magnetic spectrograph at MLL in Munich, Germany. Excited states of ³¹S in the 6-7 MeV region were observed and spin-parity constraints have been suggested. In this work we describe the experimental setup, data analysis and results for both experiments and provide recommendations for further investigation of the ³⁰P(<em>p</em>,<em>γ</em>)³¹S astrophysical reaction rate.</p> / Master of Science (MSc)

The 14N(p,gamma)15O reaction is the slowest stage of the carbon-nitrogen-oxygen cycle of hydrogen burning and thus determines its reaction rate. Precise knowledge of its rate is required to improve the model of hydrogen burning in our sun. The reaction rate is a necessary ingredient for a possible solution of the solar abundance problem that led to discrepancies between predictions of the solar standard model and helioseismology. The solar 13N and 15O neutrino fluxes are used as independent observables that probe the carbon and nitrogen abundances in the solar core. This could settle the disagreement, if the 14N(p,gamma)15O reaction rate is known with high precision. After a review of several measurements its cross section was revised downward due to a much lower contribution by one particular transition, capture to the ground state in 15O. The evaluated total relative uncertainty is still 7.5%, in part due to an unsatisfactory knowledge of the excitation function over a wide energy range. The present work reports experimentally determined cross sections as astrophysical S-factor data at twelve energies between 0.357 - 1.292 MeV for the strongest transition, capture to the 6.79 MeV excited state in 15O with lower uncertainties than before and at ten energies between 0.479 - 1.202 MeV for the second strongest transition, capture to the ground state in 15O. In addition, an R-matrix fit is performed to estimate the impact of the new data on the astrophysical relevant energy range. The recently suggested slight S-factor enhancement at the Gamow window could not be confirmed and differences to previous measurements at energies around 1 MeV were observed. The present extrapolated zero-energy S-factors are S_6.79(0) = (1.19+-0.10) keV b and S_GS(0) = (0.25+-0.05) keV b and they are within the uncertainties consistent with values recommended by the latest review. / Die 14N(p,gamma)15O Reaktion ist die langsamste Phase im Bethe-Weizsäcker-Zyklus des Wasserstoffbrennens und bestimmt deshalb die Reaktionsrate des gesamten Zyklus. Präzise Werte für die Reaktionsrate sind notwendig um das Wasserstoffbrennen in unserer Sonne besser zu verstehen. Besonders das Problem widersprüchlicher Ergebnisse aus Vorhersagen des aktuellen Sonnenmodells und helioseismologischen Experimenten könnte durch genauer bekannte 14N(p,gamma)15O Reaktionsraten aufgelöst werden. Dafür soll der solare 13N und 15O Neutrinofluss von den beta+-Zerfällen als direkter Informationsträger über die Häufigkeit von Stickstoff und Kohlenstoff im Sonneninneren genutzt werden. Der für die Berechnung der Häufigkeiten benötigte Wirkungsquerschnitt der 14N(p,gamma)15O Reaktion wurde in einer Evaluation verschiedener Messungen reduziert, da der Anteil des direkten Protoneneinfang mit Übergang in den Grundzustand deutlich weniger zum gesamten Wirkungsquerschnitt beiträgt als zuvor angenommen. Die evaluierte relative Gesamtunsicherheit ist mit 7.5% dennoch hoch, was zu einem großen Teil an ungenügendem Wissen über die Anregungsfunktion in einem weiten Energiebereich liegt. In der vorliegenden Arbeit werden experimentell ermittelte Wirkungsquerschnitte in Form von astrophysikalischen S-Faktoren für zwei Übergänge vorgestellt. Für den stärksten Übergang, den Protoneneinfang zum angeregten Zustand bei 6.79 MeV in 15O, wurden zwölf S-Faktoren bei Energien zwischen 0.357 – 1.292 MeV mit geringeren Unsicherheiten als zuvor ermittelt und für den direkten Übergang in den Grundzustand zehn Werte zwischen 0.479 – 1.202 MeV. Außerdem wurde ein R-Matrix Fit durchgeführt um den Einfluss der neuen Daten auf Extrapolationen zum astrophysikalisch relevanten Energiebereich zu prüfen. Die kürzlich vorgeschlagene Erhöhung des S-Faktors im Gamow-Fenster konnte nicht bestätigt werden und es wurden auch Unterschiede zu bisherigen Messungen im Energiebereich um 1 MeV deutlich. Die neuen extrapolierten S-Faktoren sind S679(0) = (1.19±0.10) keV b und SGS(0) = (0.25 ± 0.05) keV b und sie stimmen mit den von der Evaluation empfohlenen Werten im Rahmen ihrer Unsicherheiten überein.

Within cosmology, there are two entirely independent pillars which can jointly drive this field towards precision: Astronomical observations of primordial element abundances and the detailed surveying of the cosmic microwave background. However, the comparatively large uncertainty stemming from the nuclear physics input is currently still hindering this effort, i.e. stemming from the 2H(p,γ)3He reaction. An accurate understanding of this reaction is required for precision data on primordial nucleosynthesis and an independent determination of the cosmological baryon density. Elsewhere, our Sun is an exceptional object to study stellar physics in general. While we are now able to measure solar neutrinos live on earth, there is a lack of knowledge regarding theoretical predictions of solar neutrino fluxes due to the limited precision (again) stemming from nuclear reactions, i.e. from the 3He(α,γ)7Be reaction. This thesis sheds light on these two nuclear reactions, which both limit our understanding of the universe. While the investigation of the 2H(p,γ)3He reaction will focus on the determination of its crosssection in the vicinity of the Gamow window for the Big Bang nucleosynthesis, the main aim for the 3He(α,γ)7Be reaction will be a measurement of its γ-ray angular distribution at astrophysically relevant energies. In addition, the installation of an ultra-low background counting setup will be reported which further enables the investigation of the physics of rare events. This is essential for modern nuclear astrophysics, but also relevant for double beta decay physics and the search for dark matter. The presented setup is now the most sensitive in Germany and among the most sensitive ones worldwide. / Innerhalb der Kosmologie gibt es zwei völlig unabhängige Ansätze, die gemeinsam die Präzision in diesem Gebiet weiter vorantreiben können: Astronomische Beobachtungen der primordialen Elementhäufigkeiten und die detaillierte Vermessung des kosmischen Mikrowellenhintergrunds. Dieses Vorhaben wird derzeit allerdings noch durch die vergleichsweise große Unsicherheit des kernphysikalischen Inputs verhindert, vor allem bedingt durch das limitierte Verständnis der 2H(p,γ)3He-Reaktion. Eine präzise Vermessung dieser Reaktion ist sowohl für die Präzisionsdaten zur primordialen Nukleosynthese erforderlich, als auch für die damit einhergehende unabhängige Bestimmung der kosmologischen Baryonendichte. Des Weiteren ist unsere Sonne ein exzellent geeignetes Objekt, um unser theoretisches Verständnis über die Physik von Sternen mit experimentellen Messungen abgleichen zu können. Während wir heutzutage in der Lage sind, solare Neutrinos in Echtzeit auf der Erde messen können, mangelt es noch an der theoretischen Vorhersagekraft von solaren Neutrinoflüssen. Auch hier ist die Präzision (erneut) begrenzt durch das limitierte Verständnis der beteiligten Kernreaktionen, vor allem bedingt durch mangelnde Kenntnis über die 3He(α,γ)7Be-Reaktion. Die vorliegende Arbeit beleuchtet diese zwei Kernreaktionen, die beide unser Verständnis des Universums auf verschiedene Weise einschränken. Während sich die Untersuchung der 2H(p,γ)3He-Reaktion auf die Bestimmung ihres Wirkungsquerschnitts in der Nähe des Gamow-Fensters für die Urknall-Nukleosynthese konzentriert, ist das Hauptanliegen für die 3He(α,γ)7Be-Reaktion eine Messung der Winkelverteilung der dabei emittierten γ-Strahlung bei astrophysikalisch relevanten Energien. Darüber hinaus wird über die Installation eines Messaufbaus zur Untersuchung niedriger Aktivitäten berichtet, das sich durch seine äußerst geringe Untergrundzählrate auszeichnet. Bedingt durch seine hohe Sensitivität kann dieser Aufbau in Zukunft bedeutende Beiträge für die moderne nukleare Astrophysik leisten und ist darüber hinaus beispielsweise auch relevant für die Untersuchung von Doppel-Betazerfällen oder die Suche nach dunkler Materie. Der präsentierte Aufbau ist nun der Sensitivste seiner Art in Deutschland und gehört zu den Sensitivsten weltweit.