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Intermediate mass stars ( %0 .8 - 8 solar masses), at the asymptotic giant branch phase (AGB) suffer intensive mass loss, which leads to the formation of a circumstellar shell (s) of gas and dust in their circumstellar envelope. At the end of the AGB phase, the mass-loss decreases or stops and the circumstellar envelope begins to drift away from the star. If the velocity of the AGB phase wind has been relatively constant, then dust or molecular emission furthest from the star represents the oldest mass loss, while material closer to the star represents more recent mass loss. Therefore, the history of mass loss during the AGB phase is imprinted on the dust shells of the post-AGB envelope. Thus, by studying the distribution of material in the form of dust emission in the circumstellar shells of late evolved stars (i.e. the post AGB phases are pre - planetary nebula (PPN) and the planetary nebula (PN)) we can gain a better understanding of the mass-loss processes involved in the evolution of intermediate mass stars. I studied two groups of intermediate mass stars, namely six oxygen rich and six carbon rich candidates. In this thesis a study of evolution of intermediate mass stars is confronted by means of observations, in which far - infrared (FIR) images, are used to study the physical properties and the material distribution of dust shells of AGB and post AGB circumstellar envelope. Infrared radiation from thermal dust emission can be used to probe the entire dust shell because, near to mid-infrared radiation arises solely from the hottest regions close to the star; while the outer regions away from the star are cool such that they emit at longer infrared wavelengths. Essentially, radiation in the FIR to sub-millimeter wavelengths is emitted by the entire dust shell and hence can be used to probe the entire dusty envelope. Therefore far-infrared emission by late evolved stars can be used to probe the large scale - structure of AGB and post - AGB circumstellar shells.Our results from space observations indicated the following: The sizes of the circumstellar dust shell observed in oxygen rich stars are within %1 pc. We derived the dust masses derived from far infrared ISO PHT 32 observation of oxygen rich stars that are between 1.7 -- 4 x 10-4 solar masses. These results provides us with a lower limit in the progenitor masses of stars estimated to be within 0.56 -- 0.76 solar masses. These results indicate that the initial main sequence masses fro oxygen rich stars are within 1 - 3.5 solar masses. The time scales derived since the oldest mass was ejected during the evolution of oxygen rich stars are 4 - 13 x 104 years. For the carbon rich stars: The size of the circumstellar dust shells are within 1 - 1.6 pc. The masses of dust in the shells are a approximated to be within 0.1 - 1.44 x 10-4 solar masses. A lower limit to the progenitor masses of carbon rich stars that are between 0.61 - 0.9 solar mass, suggesting that these stars evolved from the main sequence masses between 2. - 6 solar masses.
This book explores why dust forms around stars, and how to model stellar dust formation and dust-forming environments consistently.
The term proto-planetary nebulae (PPNe) in the context of the late stages of stellar evolution was created only slightly more than 20 years ago to express the belief that in the near future these objects will become planetary nebulae (PNe). The first proto-planetary nebulae (called also post-Asymptotic Giant Branch, or shortly post-AGB objects) AFGL 2688 and AFGL 618 were discovered in mid seventies in course of the Air Force Sky Survey. Investigation of this phase of stellar evolution developed very rapidly in 1980's after the IRAS mission when it became clear that proto-planetary nebulae emit a significant part of their energy in the mid-and far-infrared. Hundreds of new candidates have been proposed but the recognition of the real proto-planetary nebulae is not a simple task and needs a substantial effort to exclude cases that represent different evolutionary stages. High resolution spectroscopy of stellar atmospheres is of much importance in this respect. Surprisingly, only a small group of central stars, the so called 21 11m emitters, show chemical signatures of the 3 rd dredge up process. Very recently, a more detailed studies of mid-infrared spectra from the Infrared Space Observatory (ISO) allow for a better understanding of chemical composition and evolution of circumstellar material around these stars. A new impetus in the field of proto-planetary nebulae research was started in the 1990's with high spatial resolution imaging in mid-infrared and optical wavelength ranges.
From the reviews: Astronomy and Astrophysics Abstracts has appeared in semi-annual volumes since 1969 and it has already become one of the fundamental publications in the fields of astronomy, astrophysics and neighbouring sciences. It is the most important English-language abstracting journal in the mentioned branches. ... The abstracts are classified under more than hundred subject categories, thus permitting a quick survey of the whole extended material. The AAA is a valuable and important publication for all students and scientists working in the fields of astronomy and related sciences. As such it represents a necessary ingredient of any astronomical library all over the world." Space Science Reviews #1 "Dividing the whole field plus related subjects into 108 categories, each work is numbered and most are accompanied by brief abstracts. Fairly comprehensive cross-referencing links relevant papers to more than one category, and exhaustive author and subject indices are to be found at the back, making the catalogues easy to use. The series appears to be so complete in its coverage and always less than a year out of date that I shall certainly have to make a little more space on those shelves for future volumes." The Observatory Magazine #1
An understanding of how stars evolve is central to astrophysics. The basic theory is well established. However, the subject has undergone a renaissance in recent years as powerful computers have become widely available and allowed complex evolutionary models to be developed and compared in great detail with observations from the latest instruments. This timely volume presents the review articles from an international meeting in Elba, Italy, where experts gathered to review how our understanding of stellar evolution has advanced. Topics covered include fundamentals of stellar evolution, star clusters, variable stars, asymptotic giant branch stars, degenerate stars, the evolution of binary stars, and chemical and galactic evolution. Throughout, theory and observation are closely compared. The book also emphasises the critical role stars have on our understanding of how galaxies evolve. In this book we are provided with both the fundamentals and the latest research. In this way, it will provide an invaluable supplement for graduate students, and a timely review for researchers.
The most comprehensive and up-to-date survey available on stellar structure and evolution, with a special emphasis on currently unsolved problems.
Analysis of the IRAS low resolution spectra show that the 8-22 micron spectral range show a variety of emission features. The strongest features in spectra of M stars are the 10 and 18 micron silicate emission features. In addition a three-component feature with peaks at 10, 11 and 13.1 micron and a weak, broad 9-15 micron feature is present in many M variable stars. Most carbon stars show the 11.2 micron SiC emission feature as well as, in some cases, an unidentified 8-9 micron emission feature. The MS, S and SC stars show a range of emission features whose peaks range from 10 to 11.2 micron. The excess emission above the underlying photospheric continuum in the 8-22 micron region for S Mira variables shows a sharp increase for period greater than about 370 days.
Over the last decade we have witnessed a rapid change in our understanding of the late stages of stellar evolution. A major stimulus to this has been the synthesis of observational data from different wavebands of the electromagnetic spectrum. The advent of infrared astronomy has led to the discovery of many luminous. late-type stars obscured by their circumstellar dust envelope. Sources discovered in the IRC and AFGL infrared sky surveys were followed up by radio observa tions, leading to the widespread use of the OH and CO molecules as probes of the circumstellar envelopes. Advances in the technique of aperture synthesis have made possible observations with unprecedent resolving power, both in spectral-line and continuum. The success of the recent IRAS sky survey, with the detection of over 250,000 sources, brings the promise of even more exciting years ahead. This area of astronomical research is also blessed with the close collaboration between theorists and observers. New ideas are constantly being quantitatively tested by new data. Theoretical predictions are eagerly used as guides for further observations. This conference was initiated with the following objective: bring together workers in optical, infrared, radio and theoretical astronomy and let them confront each other. Based on the post-conference res ponses we received, many of the participants have indeed found this Workshop a stimulating experience. The Workshop on the Late Stages of Stellar Evolution was held from 2-5 June 1986 in Calgary, Canada.