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Thesis--University of Florida. / Description based on print version record. Typescript. Vita. Includes bibliographical references (vol. 2, leaves 780-785).

Giant stars are believed to lose significant fractions of their total mass over their lifetimes, but the mechanisms responsible for this are ill-understood. One possible mechanism is dynamical mass loss - a hydrodynamical process in which matter is ejected from the stellar surface in ballistic outflows. In this thesis, dynamical mass loss is studied in three stellar regimes: common-envelope objects, asymptotic giant branch stars, and red supergiants. Using hydrodynamical simulations performed with the stellar evolution code MESA, we examine the dynamical behaviour and stability of stars in each of these regimes. We examine the dynamical properties of common-envelope objects during the slow spiral-in phase using a parameterised 1-dimensional model of orbital dissipatory heating. We find that the envelope becomes unstable to high-amplitude dynamical pulsations that can lead to repeated mass-ejection events capable of removing the entire envelope and terminating the common-envelope phase. We estimate this process's α efficiency value and suggest how these results might be employed in parameterised common-envelope models. We employ coupled evolutionary and hydrodynamical simulations of AGB stars to study their dynamical properties as they traverse the TP-AGB and examine their dependence on basic stellar properties and on the thermal pulse cycle. We find that these models experience large amounts of dynamical mass loss, and we construct a parameterised model to estimate its strength. We find that this model is successful at locating the termination of the AGB. We apply a similar approach to a study of RSGs, and find that dynamical mass loss also emerges in this regime. We estimate the conditions under which this occurs and discuss how this mechanism may resolve theoretical problems relating to the Humphreys-Davidson limit and the progenitors of SNe IIn. We conclude that dynamical mass loss is likely to form a vital part of the mass-loss histories of cool giant stars.

We present the results of an extensive observational campaign on the nearby Type Ibn SN 2015G, including data from radio through ultravioletwavelengths. SN2015Gwas asymmetric, showing late-time nebular lines redshifted by similar to 1000 km s(-1). It shared many features with the prototypical SN Ibn 2006jc, including extremely strong He I emission lines and a late-time blue pseudo-continuum. The young SN 2015G showed narrow P-Cygni profiles of He I, but never in its evolution did it showany signature of hydrogen -arguing for a dense, ionized and hydrogenfree circumstellar medium moving outward with a velocity of similar to 1000 km s(-1) and created by relatively recent mass-loss from the progenitor star. Ultraviolet through infrared observations show that the fading SN 2015G (which was probably discovered some 20 d post-peak) had a spectral energy distribution that was well described by a simple, single-component blackbody. Archival HST images provide upper limits on the luminosity of SN 2015G's progenitor, while non-detections of any luminous radio afterglow and optical non-detections of outbursts over the past two decades provide constraints upon its mass-loss history.

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We analyse eight epochs of Hubble Space Telescope H alpha+[N ii] imaging of eta Carinae's outer ejecta. Proper motions of nearly 800 knots reveal that the detected ejecta are divided into three apparent age groups, dating to around 1250 A.D., to around 1550 A.D., and to during or shortly before the Great Eruption of the 1840s. Ejecta from these groups reside in different locations and provide a firm constraint that eta Car experienced multiple major eruptions prior to the nineteenth century. The 1250 and 1550 events did not share the same axisymmetry as the Homunculus; the 1250 event was particularly asymmetric, even one-sided. In addition, the ejecta in the S ridge, which have been associated with the Great Eruption, appear to predate the ejection of the Homunculus by several decades. We detect essentially ballistic expansion across multiple epochs. We find no evidence for large-scale deceleration of the observed knots that could power the soft X-ray shell by ploughing into surrounding material, suggesting that the observed X-rays arise instead from fast, rarefied ejecta from the 1840s overtaking the older dense knots. Early deceleration and subsequent coasting cannot explain the origin of the older outer ejecta - significant episodic mass loss prior to the nineteenth century is required. The time-scale and geometry of the past eruptions provide important constraints for any theoretical physical mechanisms driving eta Car's behaviour. Non-repeating mechanisms such as the merger of a close binary in a triple system would require additional complexities to explain the observations.

In situ burning (ISB) is a practical method of oil spill cleanup in icy conditions. This study investigates one example of a likely oil spill scenario; burning oil in an ice cavity. In this situation, unique and unexplored physical processes come into play compared with the classical problem of confined pool fires in vessels. The icy walls of the cavity create a significant heat sink causing notable lateral heat losses especially for small cavity sizes (5-10 cm). Melting of ice because of the heat from the flame causes the geometry of cavity to change. Specifically, the diameter of the pool fire increases as the burning advances. This widening causes the fuel to stretch laterally thereby reducing its thickness at a faster rate. The melted ice water causes the oil layer to rise which causes the ullage height to decrease. The decrease in ullage and increase in diameter counteract the reduction in thickness because of widening or stretching of the fuel layer. There thus exists a strong coupling between the burning rate and the geometry change of the pool and cavity. To explore the problem, experiments were performed in circular ice cavities of varying diameters (5 - 25 cm). The change in shape of the ice cavity and the oil layer thickness are recorded using a combination of visual images, mass loss, and temperature data along the centerline and edge of the cavity. The average burning rate of crude oil in a cavity is greater than the corresponding burning rate in a vessel of equal diameter, yet the burning efficiency (% of fuel consumed during combustion) is lower. For example, the average mass loss rate in a 10 cm ice cavity is 50% higher than a steel vessel of similar size. However, the burning efficiency is lower by 50%. Widening of cavity (170%) contributes to the increase in the average mass burning rate. At the same time heat losses through fuel layer increase because of decrease in fuel thickness by widening of the fuel layer. This coupling is analyzed using a mathematical model which can predict burning rate and efficiency of crude oil in an ice cavity for the range of cavity diameters examined. Extension of the model to larger sizes comparable to realistic situations in the Arctic is discussed.

The potential for oil exploration on the Arctic Outer Continental Shelf warrants determination of an efficient method to clean up an oil spill. Traditional spill response equipment may not be practical in an Arctic environment; the presence of ice which may prevent proper deployment of equipment. The remoteness of the areas proposed for oil exploration lack the infrastructure and support networks necessary to stage a response to a large oil spill. These difficulties make it necessary to explore alternative means of oil spill cleanup. In situ burning is one method that may be particularly well-suited for arctic and sub-arctic environments due to the minimal amount of equipment required to achieve an efficient burn, i.e. high mass loss. The Arctic and sub- Arctic environments add an additional level of complexity by introducing a spill medium (ice) that is highly unstable at elevated temperatures. Our experiments sought to calculate the mass loss rate of oil mixtures to determine the efficiency with which they burn within ice channels of varying widths. Since fuel layer thickness is a critical factor in determining the effectiveness of an in situ burn the spread rate of oil along an ice channel was studied. Burning of oil in an ice channel yields low efficiencies (10%) primarily due to the geometric changes of the melting ice channel. The spreading was modeled as a constant flux rectilinear buoyancy-inertia governed flow. The melting causes an increase in the surface area and results in the critical thickness of the oil to be reached sooner. Based on the current bench- scale testing, losses due to ice melting cause the efficiencies of the burning process to be excessively low and not viable to full scale clean up. The results warrant future research to understand how varying other parameters, including starting mass of fuel, influence efficiencies.

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<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>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.

In this dissertation, we develop new tools for the study of stellar atmospheres, pulsating stellar atmospheres and mass loss from pulsating stars. These tools provide new insights into the structure and evolution of stars and complement modern observational techniques such as optical interferometry and high resolution spectroscopy. In the first part, a new spherically symmetric version of the Atlas program is developed for modelling extended stellar atmospheres. The program is used to model interferometric observations from the literature and to study limb-darkening for stars with low gravity. It is determined that stellar limb-darkening can be used to constrain fundamental properties of stars. When this is coupled with interferometric or microlensing observations, stellar limb-darkening can predict the masses of isolated stars. The new SAtlas program is combined with the plane-parallel hydrodynamic program Hermes to develop a new spherically-symmetric radiative hydrodynamic program that models radial pulsation in the atmosphere of a star to depths including the pulsation-driving regions of the stars. Preliminary tests of this new program are discussed. In the second part, we study the recent observations of circumstellar envelopes surrounding Cepheids and develop a mass-loss hypothesis to explain their formation. The hypothesis is studied using a modified version of the Castor, Abbott, & Klein theory for radiative-driven winds to contain the effects of pulsation. In the theory, pulsation is found to be a driving mechanism that increases the mass-loss rates of Cepheids by up to four orders of magnitude. These mass-loss rates are large enough to explain the formation of the envelopes from dust forming in the wind at large distances from the surface of the star. The mass-loss rates are found to be plausible explanation for the Cepheid mass discrepancy. We also compute mass-loss rates from optical and infrared observations of Large Magellanic Cloud Cepheids from the infrared excess and find mass loss to be an important phenomena in these stars. The amount of infrared excess is found to potentially affect the structure of the infrared Leavitt law.