Thesis (Ph. D.)--University of Missouri-Columbia, 1999. / Typescript. Vita. Includes bibliographical references (leaves 116-120). Also available on the Internet.

The absorption line profiles of Spica (α Virginis, HD116658, B1.5IV, m=0.97) show features, at about the 1% level, moving from the blue wavelengths towards the red wavelengths. A series of spectra were taken, at the 1.22 m telescope at the Dominion Astrophysical Observatory on three nights in April, 1982 and two nights in April, 1984, to study these moving features. As Spica is a member of a binary system, the effect of the secondary had to be removed from the observations. This was done by subtracting a template spectrum which had been scaled, broadened and shifted to match the secondary, from each observation. The required shifts were determined using the orbital elements on blended nights and using the Fahlman-Glaspy small-shifts technique on unblended nights. An average of all the spectra was then subtracted from each observation. The resulting series of residuals clearly show the motion of the features seen in the line profiles. The acceleration of the features was estimated to be between 0.0055 and 0.0068 kms⁻². Assuming the features are due to nonradial oscillations, this acceleration corresponds to waves moving slowly, about 5 to 20 kms⁻¹, in a prograde direction. The angular frequency of the oscillations, after accounting for the effects of rotation, would be about 3.4X10⁻⁵ rads⁻¹. A computer model that produces line profiles, under the assumption of a single nonradial oscillation, was used to produce profiles for comparison with observations. Using an [formula omitted]=8 and m=-8 mode, an intrinsic frequency of 3.4X10⁻⁵ rads⁻¹ and a stellar rotation rate of 190 kms⁻¹, the model produced profiles similar to those observed. The change in the model profiles with time was also similar to that observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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

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.

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.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21575号 / 理博第4482号 / 新制||理||1643(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)教授 柴田 一成, 准教授 岩室 史英, 教授 一本 潔 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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