<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)

This thesis describes the development of a red tip/tilt and fringe detection system at the Sydney University Stellar Interferometer (SUSI), modelling the instrumental performance and effects of seeing at SUSI, making observations of Mira variable stars and finally modelling the atmospheres of Mira variables with physically self-consistent models. The new SUSI tip/tilt system is based around a CCD detector and has been successfully used to both track the majority of tip/tilt power in median seeing at an R magnitude of 4.5, and to provide seeing measures for post processing. The new fringe-detection system rapidly scans 33 to 140 $\mu$m in delay and detects the fringes using two avalanche-photodiodes. It has been used to acquire fringe data, provide user feedback and to track the fringe group-delay position. The system visibility (fringe visibility for a point source) and throughput were found to be consistent with models of the SUSI optical beam train. Observations were made of a variety of sources, including the Mira variables R Car and RR Sco, which were observed in two orthogonal polarization states. These measurements were the first successful use of Optical Interferometric Polarimetry (OIP), and enabled scattered light to be separated from bright photospheric flux. Dust scattering was found to originate from a thin shell 2-3 continuum radii from these stars, with an optical depth of 0.1 to 0.2 at 900 nm. Physical models of Mira variables including dust formation were developed, providing consistent explanations for these results as well as many other photometric and interferometric observations.

This thesis describes the development of a red tip/tilt and fringe detection system at the Sydney University Stellar Interferometer (SUSI), modelling the instrumental performance and effects of seeing at SUSI, making observations of Mira variable stars and finally modelling the atmospheres of Mira variables with physically self-consistent models. The new SUSI tip/tilt system is based around a CCD detector and has been successfully used to both track the majority of tip/tilt power in median seeing at an R magnitude of 4.5, and to provide seeing measures for post processing. The new fringe-detection system rapidly scans 33 to 140 $\mu$m in delay and detects the fringes using two avalanche-photodiodes. It has been used to acquire fringe data, provide user feedback and to track the fringe group-delay position. The system visibility (fringe visibility for a point source) and throughput were found to be consistent with models of the SUSI optical beam train. Observations were made of a variety of sources, including the Mira variables R Car and RR Sco, which were observed in two orthogonal polarization states. These measurements were the first successful use of Optical Interferometric Polarimetry (OIP), and enabled scattered light to be separated from bright photospheric flux. Dust scattering was found to originate from a thin shell 2-3 continuum radii from these stars, with an optical depth of 0.1 to 0.2 at 900 nm. Physical models of Mira variables including dust formation were developed, providing consistent explanations for these results as well as many other photometric and interferometric observations.

I have developed a new general-purpose deterministic 2D radiative transfer code for astrophysical dusty environments named LELUYA (www.leluya.org). It can provide the solution to an arbitrary axially symmetric multi-grain dust distribution around an arbitrary heating source. By employing a new numerical method, the implemented algorithm automatically traces the dust density and optical depth gradients, creating the optimal unstructured triangular grid. The radiative transfer equation includes dust scattering, absorption and emission. Unique to LELUYA is also its ability to self-consistently reshape the sublimation/condensation dust cavity around the source to accommodate for the anisotropic diffuse radiation. LELUYAs capabilities are demonstrated in the study of the asymptotic giant branch (AGB) star IRC+10011. The stellar winds emanating from AGB stars are mostly spherically symmetric, but they evolve into largely asymmetric planetary nebulae during later evolutionary phases. The initiation of this symmetry breaking process is still unexplained. IRC+10011 represents a rare example of a clearly visible asymmetry in high-resolution near-infrared images of the circumstellar dusty AGB wind. LELUYA shows that this asymmetry is produced by two bipolar cones with 1/r0.5 density profile, imbedded in the standard 1/r2 dusty wind profile. The cones are still breaking though the 1/r2 wind, suggesting they are driven by bipolar jets. They are about 200 years old, thus a very recent episode in the final phase of AGB evolution before turning into a proto-planetary nebula, where the jets finally break out from the confining spherical wind. IRC+10011 provides the earliest example of this symmetry breaking thus far.

The asymptotic giant branch (AGB) is the last stage of stellar evolution for stars with masses between 0.8-8 M(sun). This phase is characterised by an intense mass loss, which builds up a circumstellar envelope of dust and gas(CSE). It is through this process that the AGB stars contribute to the chemical evolution of galaxies. In addition, a rich and varied chemistry is active within the CSEs.Observations of circumstellar H2O are of particular interest, as it is expected to be one of the most abundant molecules in the inner envelopes of M-type AGB stars (with C/O&lt;1). The first part of this thesis concerns the modelling of water vapour emission lines from CSEs around M-type AGB stars. Using satellite observations and detailed radiative transfer models, H2O abundances in these stars are determined and compared with theoretical chemical models. The importance of resolved H2O line profiles and excitation through different vibrationally excited states are also demonstrated.The second part of the thesis has its focus on the detached shells of dust and gas observed around a handful of carbon AGB stars (with C/O&gt;1), believed to be an effect of highly time-variable mass loss during a thermal pulse. The detached shells around three sources were observed in stellar light scattered by dust and gas in the shells using ground-based and space telescopes. The observations allow a separation of the scattering agents, and reveal information on the detached shells in unprecedented detail. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: In progress.

From the main sequence onwards, stars of intermediate masses (1-8 Solar masses) eject a large portion of their mass with rates as high as 0.0001 Solar masses per year during their transition through the Asymptotical Giant Branch (AGB) stage. The outflows are shaped by the same mechanisms that shape the ejecta, which in turn appear to depart from spherical symmetry as early as the AGB stage. The ejecta are then evolving into asymmetrical structures. Stars like that are giant factories of dust, responsible for the enrichment of their surrounding Galactic medium in metals heavier than helium. Depending on their abundances during the AGB stage, the stars are either oxygen-rich or carbon-rich, and as such, the dust produced in their atmospheres is either O-rich or C-rich. The chemical composition of the ejecta, indicates the stellar chemistry at the moment of ejection. The disruption of the spherical symmetry of the mass loss can be caused by fast rotation, stellar magnetic fields or binarity, the latter being the most efficient and favourable mechanism. Such mechanisms can lead to the creation of circumstellar, equatorial, dusty structures, like discs, torii or spirals. Due to their small relative sizes, compared to their surrounding nebulae, they can be studied at best with the use of infrared interferometric techniques. We report the discovery of three such structures in sources at three different evolutionary stages, respectively, with the use of single- and multi-aperture interferometry. In the C-rich AGB star V Hya we imaged via aperture masking in the near-infrared, a complex and possibly orbiting structure, which is embedded within the star's molecular torus. Our MIDI observations in the mid-infrared have revealed, a silicate disc within the symbiotic nebula M2-9 that is currently being shaped by the central binary system within its core, and a C-rich disc-like structure in the born-again star Sakurai's Object, that is also aligned to an asymmetry found in its surrounding planetary nebula. Finally, we compare the properties of the structures found here with those found in the literature in order to establish a relation between late stellar evolution and the existence of dusty structures.

No description available.

Since the identi cation of proto-planetary nebulae (PPNe) as transition objects between the asymptotic giant branch stars and planetary nebulae more than two decades ago, astronomers have attempted to characterise these exciting objects. Today many questions still elude a conclusive answer, partly due to the sheer diversity observed within this small subset of stellar objects, and partly due to the low numbers detected. Fortunately, many of these objects display a rich spectrum of emission/absorption lines that can be used as diagnostics for these nebulae. This dissertation presents a study of six PPNe using the relatively new (at NIR wavelengths) integral eld spectroscopy technique. This method has allowed the investigation of distinct regions of these nebulae, and in certain cases the application of magneto-hydrodynamic shock models to the data. The goal of this research has been to investigate the evolution of PPNe by detailed examination of a small sample of objects consisting of a full range of evolutionary types. Near-IR ro-vibrational lines were employed as the primary tool to tackle this problem. In all six sources the 1!0S(1) line is used to map the spatial extent of the H2. In three of these objects the maps represent the rst images of their H2 emission nebulae. In the case of the earliest-type object (IRAS 14331-6435) in this sample, the line map gives the rst image of its nebula at any wavelength. In the only M-type object (OH 231.8+4.2) in the sample, high-velocity H2 is detected in discrete clumps along the edges of the bipolar out ow, while a possible ring of slower moving H2 is found around the equatorial region. This is the rst detection of H2 in such a late-type object but due its peculiarities, it is possibly not representative of what is expected of M-type objects. In IRAS 19500-1709, an intermediate-type object, the line map shows the H2 emission to originate in clumpy structures along the edges of a bipolar shell/out ow. The remaining three objects have all been the subject of previous studies but in each case new H2 lines are detected in this work along with other emission lines (Mg ii, Na i &amp; CO). In the case of IRAS 16594-4656, MHD shock models have been used to determine the gas density and shock velocity. Two new python modules/classes have been written. The rst one to deal with the data cubes, extract ux measurements, rebin regions of interest, and produce line maps. The second class allows the easy calculation of many important parameters, for example, excitation temperatures, column density ratio values, extinction estimates from several line-pairs, column density values, and total mass of the H2. The class also allows the production of input les for the shock tting procedure, and simulated shocks for testing this tting process. A new framework to t NIR shock models to data has been developed, employing Monte Carlo techniques and the extensive computing cluster at the University of Hertfordshire (UH). This method builds on the approach used by many other authors, with the added advantages that this framework provides a method of correctly sampling the shock model parameter space, and providing error estimates on the model t. Using this approach, data from IRAS 16594-4656 have been successfully modelled using the shock models. A full description of this class of stellar objects from such a small sample is not possible due to their diverse nature. Although H2 was detected across the full spectral vi range of post-AGB objects, the phase at which H2 emission begins is still not clear. The only M-type object in this work is a peculiar object and may not be representative of a typical post-AGB star. The H2 PPNe appear to be located at lower Galactic latitudes (b 20 ) than the total PPNe population, possibly pointing to an above average mass and hence younger age of these objects.

Modelling stellar structures and comparing them with observationsis a very important step when it comes to verifying our theories aboutstellar evolution. Three-dimensional reconstruction is therefore impor-tant and in the case of certain stellar types it makes for a large portionof the ongoing research.For this project, three dierent objects and their three-dimensionalmodels were selected for 3D-printing. The systems in question areEta Carinae, 1 Gruis and HD 101584 and the reason behind thischoice is the fact that each object showcases a dierent phase and/orprocess of stellar evolution approaching or during the planetary nebula(PN) stage. On top of that, these objects have been observed using atechnique that allows us to deduce their 3D structure.The three-dimensional models and prints allow us to nd features(such as axial symmetries) that give us more information about themovements inside the system and their consequences on how the stellarstructure has and will evolve.

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