Search results for: pulsation-and-mass-loss-in-stars

Pulsation and Mass Loss in Stars

Author : R. Stalio
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Stellar mass loss is an essential part of the cycling of material from the interstellar medium into stars and back, and must be understood if we are to model processes on galactic to cosmological scales. The study of stellar winds and the effects of stellar mass loss has reached a particularly exciting stage where observational capabilities are increasingly able to provide interesting constraints on models and theories. Recent resu1ts from theoretical and observational work for both hot and cool stars with substantial winds have led to the suggestion that a combination of pulsation with other mechanisms makes for particularly efficient mass loss from stars. This provided the original motivation for the organization of this workshop. The conference was organized along relatively conventional lines according to the types of objects being scrutinized. However the true unity of the proceedings comes from the interplay of the mechanisms involved. For example, for the cool, luminous Mira variables, pulsation leads to shock waves that extend the atmosphere, enhancing dust formation; radiation pressure on dust drives the wind, cooling the atmosphere and in some cases suppressing the shocks. Similarly for the Be stars, both pulsation (in this case, non-radial) and radiation pressure (due to UV resonance lines) are expected to be important, and this expectation is at least qualitatively borne out by the observations.

Pulsation Rotation and Mass Loss in Early Type Stars

Author : Luis A. Balona
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In this Symposium, researchers specializing in pulsation, rotation, magnetic fields and stellar winds are brought together for the first time in order to broaden our understanding of O and B stars. Thanks to advances in digital spectroscopy, new types of pulsating B stars have been discovered. The pulsations can be understood in terms of the recent revision of metal opacities, but the effects of rapid rotation and magnetic fields need further study. Observations in the UV and X-ray regions demonstrate that many B and Be stars show other activity, besides pulsation which is not yet understood. The reason for the enhanced mass loss in Be stars is a question which dominates the Symposium and which remains unanswered, although it is surely to be found in activity at or near the photosphere coupled with rotation. It is shown that the geometry of the circumstellar envelopes around Be stars is indeed a flattened disk as they can now be optically resolved. The variability of radiatively-driven winds from O and B stars are likely related to the rotation of the star. This underlines the central theme of the book: that the various phenomena seen in these stars cannot be studied in isolation.

Pulsation and Mass Loss in Stars

Author : R Stalio
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Angular Momentum and Mass Loss for Hot Stars

Author : L.A. Willson
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Fundamental unsolved problems of stellar astrophysics include the effects of angular momentum on stellar structure and evolution, the nature and efficiency of the processes by which angular momentum is redistributed within and lost from stars, and the role that stellar rotation plays in enhancing or driving stellar mass loss. There appears to be a qualitative change in the nature and efficiency of these mechanisms near spectral type FO: hotter (more massive) stars typically retain more angular momentum at least until they reach the main sequence, while cooler stars typically spin down quickly. For the hotter stars, recent work suggests a strong link between the type of pulsation behavior, the mass loss rates, and the rotation velocity. If the same mechanisms are able to drive mass loss from the main sequence A stars, as has recently been proposed, then the current interpretations of a number of observations will be drastically affected: e. g. the ages of clusters may be incorrect by up to a factor of two, and the surface abundances of isotopes of He, Li and Be may no longer give constraints on cosmological nucleosynthesis. There are also effects on the evolution of the abundances of elements in the interstellar medium and on the general evolution of populations of stars. Thus the questions of the mechanisms of angular momentum and mass loss of stars more massive than the sun is important not only for stellar studies but for the foundations of much of modern astrophysics.

Pulsation Rotation and Mass Loss in Early Type Stars

Author : Luis A. Balona
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On Shaky Ground

Author : Jacqueline Goldstein
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The Hertzsprung-Russell Diagram (HRD) is a monumental visual narrative of stellar structure and evolution depicted by the surface properties of stars. The subsurface story, however, is more uncertain. Angular momentum and mass loss, for example, can alter a star's path through the HRD, but the prescriptions for incorporating angular momentum transport and mass loss into stellar models are incomplete. To improve prescriptions and stellar models, more information is needed on the subsurface properties of stars. Fortunately, stars across the HRD harbor instabilities in the form of stellar pulsations that can reveal stellar structure that is otherwise hidden from view. Instabilities can also influence stellar evolution, transporting angular momentum and driving mass loss. In this thesis I present new methods and models for exploring instabilities in stars, with an emphasis on massive stars. I present a new model for an anelastic Tayler Instability, which could play a role in angular momentum transport; a new contour method for calculating fast-growing stellar pulsations, which have previously been a challenge to calculate; and a new massive main-sequence instability strip mapping fast-growing pulsations, which could play a role in pulsation-driven mass loss. These new methods and models provide a solid foundation for the further exploration of instabilities in stars with the goal of improving the narrative of stellar structure and evolution.

International Conference on Magnetic Fields in O B and A Stars

Author : Luis A. Balona
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Mass Losing Pulsating Stars and their Circumstellar Matter

Author : Y. Nakada
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Editing the proceedings of a scientific meeting is not an easy task. Sometimes people who give an excellent talk do not send the manuscript by the deadline. However, this time, thanks to the punctuality of all the participants, we have this excellent volume for the workshop on mass losing pulsating stars and their circumstellar matter prepared in time. Almost all of the oral presentations including the summary are collected in this volume. We regret that we cannot put in this volume a few posters that we failed to receive before the editorial work. The workshop was planned as a small meeting with less than fifty attendants because the city of Sendai was far from the most of the active institutions. However, the number of submitted papers exceeded the SOC's expectation; many interesting contributions had to be scheduled in the poster session. Still, the oral sessions were so tight that many participants might have felt frustrated for the shortage of discussions. The organizers of the workshop have to apologize to the attendants for the inconvenience caused from such a happy underestimate about the size of the workshop.

Precision Asteroseismology IAU S301

Author : Joyce A. Guzik
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IAU Symposium 301 highlights the recent advances in the field of asteroseismology and was the twenty-first in a series of pulsation meetings started in Los Alamos in 1971 and held every two years. Topics discussed centred around seismic studies of all types of pulsating stars, which - in the era of space observations made by MOST, CoRoT and Kepler - use data of unprecedented precision. The Symposium was also the opportunity to honour Wojtek Dziembowski, one of the world's leaders in the study of solar and stellar pulsations. Highlights include contributions on observing from space and the ground, techniques of analysis and mode identification, astrophysical applications of pulsations, pulsation-convection interaction, mass loss, microphysics, pulsations in main-sequence stars, compact stars and supergiants, and solar-like oscillations. Containing many excellent reviews, this volume is an important reference source for researchers on solar and stellar pulsations.

Pulsating Stars

Author : Márcio Catelan
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This book surveys our understanding of stars which change in brightness because they pulsate. Pulsating variable stars are keys to distance scales inside and beyond the Milky Way galaxy. They test our understanding not only of stellar pulsation theory but also of stellar structure and evolution theory. Moreover, pulsating stars are important probes of the formation and evolution of our own and neighboring galaxies. Our understanding of pulsating stars has greatly increased in recent years as large-scale surveys of pulsating stars in the Milky Way and other Local Group galaxies have provided a wealth of new observations and as space-based instruments have studied particular pulsating stars in unprecedented detail.

Pulsation and Mass Loss in Stars

Author : R. Stalio
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Stellar mass loss is an essential part of the cycling of material from the interstellar medium into stars and back, and must be understood if we are to model processes on galactic to cosmological scales. The study of stellar winds and the effects of stellar mass loss has reached a particularly exciting stage where observational capabilities are increasingly able to provide interesting constraints on models and theories. Recent resu1ts from theoretical and observational work for both hot and cool stars with substantial winds have led to the suggestion that a combination of pulsation with other mechanisms makes for particularly efficient mass loss from stars. This provided the original motivation for the organization of this workshop. The conference was organized along relatively conventional lines according to the types of objects being scrutinized. However the true unity of the proceedings comes from the interplay of the mechanisms involved. For example, for the cool, luminous Mira variables, pulsation leads to shock waves that extend the atmosphere, enhancing dust formation; radiation pressure on dust drives the wind, cooling the atmosphere and in some cases suppressing the shocks. Similarly for the Be stars, both pulsation (in this case, non-radial) and radiation pressure (due to UV resonance lines) are expected to be important, and this expectation is at least qualitatively borne out by the observations.

Stellar Physics

Author : G.S. Bisnovatyi-Kogan
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"Stellar Physics" is a rather unique book in the growing literature on star formation and evolution. Not only does the author, a leading expert in the field, give a very thorough description of the current knowledge about stellar physics, but he handles with equal care the many problems that this field of research still faces. A bibliography with well over 650 entries makes this book an unparalleled source of references. "Stellar Evolution and Stability" is the second volume and can be read, as can the first volume, as a largely independent work. It traces in great detail the evolution of the protostar towards the main sequence and beyond this to the last stage of stellar evolution, with the corresponding vast range from white dwarfs to the mighty supernovae explosions and blackhole formation. The book concludes with special chapters on the dynamical, thermal and pulsing stability of stars.

Evolution of Cepheids with Pulsationally driven Mass Loss

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Models have been run of intermediate mass stars (5, 6, 7, and 8 M/sub theta/ with Y = 0.28, Z = 0.02) with pulsationally-driven mass loss occurring in the Cepheid instability strip. The new 12C(.cap alpha., .gamma.)16O rates of Caughlan were used. The enhanced rate extends the tip of the blue loop, allowing the 5 and 6 M/sub theta/ models to re-enter the Cepheid strip, unlike the models calculated using the old rates (Becker, 1981). An investigation was conducted to see if mass loss during the Cepheid stage could redden the tip of the blue loop sufficiently to place it inside the instability strip, thereby ''trapping'' the star, and allowing it to lose mass for a period of time significantly longer than the normal crossing time. Results show that this mechanism does in fact work for a 7 M/sub theta/ star with mass loss rates as low as approx. 5 x 10−7 M/sub theta/ yr−1. Observations of P-Cygni profiles in Cepheids indicate that this rate is not unreasonable. This behavior acts to reduce the discrepancy between the evolutionary and pulsation-derived masses for Cepheids. Another consequence is that the rates of period change are decreased, bringing them into better agreement with observed values. 7 refs., 2 tabs.

Stellar Pulsations

Author : J.C. Suárez
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Analyses of photometric time series obtained from the MOST, CoRoT and Kepler space missions were presented at the 20th conference on Stellar Pulsations (Granada, September 2011). These results are leading to a re-appraisal of our views on stellar pulsation in some stars and posing some new and unexpected challenges. The very important and exciting role played by innovative ground-based observational techniques, such as interferometric measurements of giant pulsating stars and high-resolution spectroscopy in the near infrared, is also discussed. These Proceedings are distinguished by the format of the conference, which brings together a variety of related but different topics not found in other meetings of this nature.

Multiepoch Infrared Interferometric Observations of Evolved Stars at the VLTI

Author : Iva Karovicova
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Cette thèse exploite des mesures interférométriques multi-époques d'étoiles évoluées de la branche horizontale asymptotique (AGB) du diagramme HR. Il s'agit d'étoiles de masses petites à intermédiaires (0.6 - 10 Mo) en phase finale de leur évolution. Ces étoiles pulsantes subissent une perte de masse très importante via un vent stellaire dense et poussiéreux qui enrichit l'environnement stellaire de gaz et de poussières. Ces processus de perte de masse sont encore mal compris et font l'objet de beaucoup d'investigations. Grâce à leur luminosité et diamètre très importants, ces étoiles sont des cibles privilégiées d'observations à Haute Résolution Angulaire. Les étoiles AGB étudiées ici ont été mesurées pendant plusieurs cycles d'oscillation au moyen des deux interféromètres AMBER et MIDI du VLTI, donc en proche et moyen infrarouge. Le but de cette investigation est la connexion entre le mécanisme de pulsation, la condensation et la chimie des poussières dans le but de mieux comprendre leur perte de masse. J'ai étudié un échantillon de quatre étoiles évoluées, les variables de type Mira riches en oxygène RR Aql, S Ori, Gx Mon et R Cnc, au moyen de donnée MIDI multi-époque et de données AMBER individuelles. Ces données sont modélisées par des modèles standards de disques uniformes ou gaussiens. Les données AMBER sont modélisées par une atmosphère auto-excitée libre de poussière incluant les couches moléculaires juste supérieures responsables du continuum spectral (modèles de série P et M, Ireland et al. 2004a,b). Les données MIDI permettent de rajouter à ce modèle l'enveloppe gazeuse radiative. Le transfert radiatif de l'enveloppe utilise le code mcsim_mpi (Ohnaka et al., 2006) et prolonge le travail de Wittkowski et al. (2007). Deux types de poussière sont envisagés, silicate et Al2O3. Les modèles ont été simulés à différentes phases et avec différents paramètres de l'enveloppe de poussière pour étudier la variabilité de la photométrie et de la dimension interférométrique en infrarouge moyen. Les visibilité spectrales n'ont pas permis, pour aucune de nos étoiles, de mettre en évidence de variation significative intra-cycle ou d'un cycle à l'autre, dans nos barres d'erreur de 5 à 20%. Notre étude montre que les spectres de visibilité et de photométrie des quatre étoiles sont bien décrits par le modèle d'enveloppe radiative au-dessus d'une source centrale décrite par un modèle d'atmosphère dynamique. L'enveloppe de poussière optiquement mince de RR Aql est bien modélisée par des grains de silicate. L'addition de grains de Al2O3 n'améliore pas l'ajustement du modèle, mais n'exclut pas la présence d'une enveloppe interne optiquement plus mince que celle de silicate. GX Mon est bien modélisé par une combinaison de A Al2O3 et de silicates, alors que les enveloppes d nS Ori et R Cncn sont modélisables par des grains de Al2O3 seul sans contribution de silicate. Les rayons inférieurs des enveloppes sont de l'ordre de 2 à 2.5 rayons photosphériques pour Al2O3 et de 4 à 5 pour le silicate. Les modélisations effectuées dans ce travail de thèse confirment que les variations inter et intra-cycle attendues ne sont pas détectables dans la précision de nos mesures. Cette étude est la première comparaison entre des mesures interférométriques couvrant plusieurs cycles d'oscillation et des modèles couvrant une large gamme de phases des oscillations.

Stars Stellar evolution

Author : Klaas de Boer
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The diverse forms that stars assume in the course of their lives can all be derived from the initial conditions : the mass and the original chemical composition. In this textbook Stars and Stellar Evolution the basic concepts of stellar structure and the main roads of stellar evolution are described. First, the observable parameters are presented, which are based on the radiation emerging from a stellar atmosphere. Then the basic physics is described, such as the physics of gases, radiation transport, and nuclear processes, followed by essential aspects of modelling the structure of stars. After a chapter on star formation, the various steps in the evolution of stars are presented. This leads us to brown dwarfs, to the way a star changes into the red-giant state and numerous other stages of evolution and ultimately to the stellar ashes such as white dwarfs, supernovae and neutron stars. Stellar winds, stellar rotation and convection all influence the way a star evolves. The evolution of binary stars is included by using several canonical examples in which interactive processes lead to X-ray binaries and supernovae of type Ia. Finally, the consequences of the study of stellar evolution are tied to observed mass and luminosity functions and to the overall evolution of matter in the universe. The authors aim at reaching an understanding of stars and their evolution by both graduate students and astronomers who are not themselves investigating stars. To that end, numerous graphs and sketches, among which the Hertzsprung-Russell diagram is the dominant one, help trace the ways of stellar evolution. Ample references to specialised review articles as well as to relevant research papers are included.

A Nonlinear Study of Luminous Blue Variables and Possible Outbursts

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Linear pulsation analysis of luminous blue variable models shows instability to pulsations in multiple radial and nonradial strange modes (see Glatzel, these proceedings). These modes have large linear growth rates, sometimes exceeding several hundred percent per period, which prompted us to investigate the nonlinear behavior of envelope models. While the nonradial modes are predicted in the linear analysis to have higher growth rates than the radial modes, nonlinear nonradial pulsations are beyond the capabilities of pulsation hydrodynamics codes developed to date. As for relevant radial nonlinear calculations, Stothers & Chin (1993) report briefly on nonlinear hydrodynamic calculations of one dynamically unstable massive star envelope model. Aikawa & Sreenivasan (1996) have done nonlinear oscillation modeling of strange modes in low-mass AGB stars. Kiriakidis et al. (these proceedings) present nonlinear models (not including convection) of two types of strange-mode pulsators, massive stars and Wolf-Rayet stars. They find periodic or irregular pulsations, and suggest that pulsation drives mass loss. Here we present new nonlinear hydrodynamic calculations to explore the link between strange-mode pulsations and LBV outbursts.

Physics Formation and Evolution of Rotating Stars

Author : Andre Maeder
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Rotation is ubiquitous at each step of stellar evolution, from star formation to the final stages, and it affects the course of evolution, the timescales and nucleosynthesis. Stellar rotation is also an essential prerequisite for the occurrence of Gamma-Ray Bursts. In this book the author thoroughly examines the basic mechanical and thermal effects of rotation, their influence on mass loss by stellar winds, the effects of differential rotation and its associated instabilities, the relation with magnetic fields and the evolution of the internal and surface rotation. Further, he discusses the numerous observational signatures of rotational effects obtained from spectroscopy and interferometric observations, as well as from chemical abundance determinations, helioseismology and asteroseismology, etc. On an introductory level, this book presents in a didactical way the basic concepts of stellar structure and evolution in "track 1" chapters. The other more specialized chapters form an advanced course on the graduate level and will further serve as a valuable reference work for professional astrophysicists.

Structure and Evolution of Asymptotic Giant Branch Stars

Author : Kawthar Rashid Mehio
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In this thesis, we focus on the study of structure and evolution of low mass sta rs of masses 2M (solar) and 3M (solar) and initial solar-like composition. The m ost interesting evolutionary phase of such stars is the Asymptotic Giant Branch (AGB) stage. On the Asymptotic Giant Branch, these stars exhibit remarkably high lum inosities and suffer the so called Thermal Pulsation. During these pulsations, s uch stars are considered to synthesize the bulk of heavy elements by the so call ed s-process nucleosynthesis. Also, other elements are produced like Carbon, Oxy gen, and Fluorine. Our study will deal with the influence of mass loss and convection tre atment on the evolution through the AGB phase. Since such stars are numerous and very bright, they can be observed in external galaxies. Their association with a fundamental site of nucleosynthesis in the un iverse illuminates their importance for the chemical evolution of the galaxy. We studied the 2M (solar) star up to the Asymptotic Giant Branch phase. We foll owed its evolution through the 'Helium Flash' and the thermal pulsating phase. Y et the third dredge up was not obtained. So, we adopted a new mass loss formula that is believed to affect the occurrence of the third dredge up. We applied tha t on a 3M (solar) star but our results were not as expected, but we studied in t he process the effect of neutrino losses on the thermal pulsating phase and the interpulse period. Eventually, we applied convective mixing in to radiative regi ons by two methods, instantaneous mixing and exponential mixing. In the exponent ial mixing, we used two different mixing parameters and we obtained that as the parameter increases, the earlier we obtain the third dredge up, the longer is th e interpulse period, and the more dredged up materials to the surface. This is t o be expected, since longer interpulse period means the more materials are proce ssed, thus more dredged up. As for the instantaneous overshooting, the third dre dge up was obtained and compared to the exponential overshooting.

Mass Loss from Red Giants

Author : Mark Morris
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Red giant and supergiant stars have long been favorites of professional 6 and amateur astronomers. These enormous stars emit up to 10 times more energy than the Sun and, so, are easy to study. Some of them, specifically the pulsating long-period variables, significantly change their size, brightness, and color within about a year, a time scale of interest to a single human being. Some aspects of the study of red giant stars are similar to the study of pre-main-sequence stars. For example, optical astronomy gives us a tantalizing glimpse of star forming regions but to really investi gate young stars and protostars requires infrared and radio astronomy. The same is true of post-main-sequence stars that are losing mass. Optical astronomers can measure the atomic component of winds from red giant stars that are undergoing mass loss at modest rates 6 (M $ 10- M9/yr.). But to see dust grains and molecules properly, 5 especially in stars with truly large mass loss rates, ~ 10- M9/yr, one requires IR and radio astronomy. As this stage of copious mass loss only lasts for ~105 years one might be tempted to ask, "who cares?".