The post-asymptotic giant branch (post-AGB) phase is a short episode in the life of a star with mass between 0.8 to 8 M⊙. It comes after the AGB phase, and before the planetary nebula phase. A rapid change in many physical properties of a star is suggested to happen in this phase, for example the onset of jets. However, a lot of details are still unknown. In this thesis, three major problems are addressed: insufficient samples of post-AGB stars, identification of post-AGB stars, and the true status of a special class of objects called the “water fountains (WFs)”. WFs are evolved stars associated with high velocity collimated bipolar jets that can be traced by H2O maser emissions. For the first two problems, new searching criteria are introduced with two new maser surveys on oxygen-rich post-AGB stars. It is necessary to collect more samples of post-AGB stars for further studies. Nonetheless, there has been no systematic searching method because most of the post-AGB stars are dim in optical and near-infrared wavelengths, which increases the difficulty in identification. Maser thus becomes a good alternative tool. In the first survey which focused only on H2O masers, over 200 AGB or post-AGB star candidates have been selected and observed. Those candidates were mainly chosen by new colour criteria with the far-infrared AKARI data. In particular, four characteristic maser sources were found, and they are currently suggested as possible very young post-AGB stars. In the second survey, another 100 objects were observed in OH and/or H2O masers. Three possible high velocity objects were discovered, including a new rare member of WFs. The colour criteria are proved to be quite sensitive in distinguishing post-AGB stars from AGB stars or other types of objects, even though there are still some contamination from young stellar objects. A follow-up study shows that the Q-parameters are effective in isolating objects with spherical or aspherical envelopes, which are also useful in finding post-AGB stars. Regarding the third problem, one-dimensional radiative transfer models have been used to study the spectral energy distributions (SEDs) of all known WFs. Owing to the young dynamical ages of the WF jets, it has been suggested that WFs represent the very early post-AGB phase, when the circumstellar envelopes started to depart from spherical symmetry. However, it is shown from the present SED study that WFs in fact could have various form of morphologies, some of them are even spherical which resemble the AGB stars. It implies that WFs do not necessary belong to the very early post-AGB phase, and the present interpretation on the WF status may not be entirely correct. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Thesis (Ph. D.)--University of Washington, 1996. / Vita. Includes bibliographical references (p. [101]-107).

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. August 2015. / This work investigates the impact of the extragalactic background light fluctuations on very high energy !-ray spectra from distant blazars. We calculate the extragalactic background light spectral energy distribution using a model that extends those proposed by Razzaque et al. (2009ApJ.697.483R) and Finke et al. (2010ApJ.712.238F). We introduce a model for fluctuations in the extragalactic background light based on fluctuations in the star formation rate density, since these two fluctuations can reasonably be expected to be correlated. Fluctuations in the star formation rate are estimated from the semi-analytical galaxy catalogue of Guo et al. (2013MNRAS.428.1351G), we use his model to derive the resulting opacities for !-rays from distant sources. We determine the mean, lower and upper limits for the scatter of the star formation rate density, which then allow us to compute corresponding limits on the extragalactic background light spectrum. We then calculate the impact of these fluctuations limits on the !-ray optical depth. This appears to be the first detailed analytical model that aims to account for the impact of extragalactic background light fluctuations on the !-ray opacity. The model predicts relatively high variations ( 15%) on the opacity in the energy range less than 100 GeV for nearby sources. The impact is found to be smaller (⇠ 5%) for very high energy !-rays from distant sources.

The acoustic resonance phenomena of metal halide lamps are investigated. A measuring system is set up to examine the effects of the acoustic resonance to the shape of the lamp arc, the lighting energy spectrum, the color temperature, the light output as well as electrical characteristics. Two ballast circuits are built to drive the lamps with the sine-wave current and the square-wave current, respectively. One ballast employs the series resonant inverter to output the sinusoidal lamp current. The other makes use of the full-bridge inverter to drive the lamps with the square-wave current. They are operated over a high-frequency range from 20kHz to 30kHz. For both ballast circuits, the operating frequency and the magnitude of the lamp current can be controlled independently. ¡@¡@The experiments are conducted on the 70-W metal halide lamps. The experimental results show that the spectral energy and the color temperature change more significantly as the acoustic resonance becomes more serious. These effects become less significant when the lamps are driven by the square-wave current. The degree of the acoustic resonance can be identified by detecting the variation of the lamp voltage or the output light. The investigated results of the thesis can provide useful information when a standard of the acoustic resonance is considered.

Techn. Univ., Habil.-Schr.--Chemnitz, 2006.

No description available.

Thesis (M.S.)--California Polytechnic State University, 2008. / "June 2nd, 2008." "In partial fulfillment of the requirements for the degree [of] Master of Science in Engineering with Specialization in Materials Engineering." "Presented to the faculty of California Polytechnic State University, San Luis Obispo." Major professor: Richard Savage, Ph.D. Includes bibliographical references (leaves 98-100). Also available online and on microfiche (2 sheets).

AN ABSTRACT ON THE THESIS OFJose Benavides Pantoja, for the Master of Arts degree in Linguistics presented on April 30, 2020, at Southern Illinois University Carbondale. TITLE: PHONETIC VARIATION IN KAMËNTS̈Á: AN ANALYSIS OF STOPS AND AFFRICATESMAJOR PROFESSOR: Dr. Karen Baertsch This study contributes to the limited body of research concerned with documenting and studying the minority and indigenous languages of the world. More specifically, it focuses on Kamënts̈á, a language isolate spoken by less than 500 people in southern Colombia. According to UNESCO (2003), this language has been categorized as “definitely endangered” which makes its documentation even more critical. Although there have been initial investigations undertaken by Juajibioy, Howard, and McDowell, the scarce accessible research has relied on purely impressionistic reviews of the language, leading to unreliable conclusions and descriptions of the language. This thesis constitutes the first phonetic acoustic analysis of Kamënts̈á language in order to substantiate what has been described in the current literature. Additionally, the geographic location where Kamënts̈á emerged, together with Spanish, and Inga Kichwa has proven to be an intriguing sociohistorical and linguistic landscape for the study of these languages being an area worth of investigating. Given that there is no existing documentation of language variation in the language, the primary goal of this thesis is to analyze the production of plosive and affricate segments at a word-level to examine language variation at a phonetic level. Considering that Kamënts̈á has been poorly documented, it was not surprising for differences in the production to be found. One difference fell in the phonemic status of the segment between younger and older speakers of the language. The production of the segments in three linguistic environments at word-level attested for variation of some phonemes. For this study, data were collected from two experimental tasks. For the first task, eight Kamënts̈á-Spanish bilinguals (N=8) were recorded producing words in isolation in a word-elicitation exercise. The purpose of this task was to gather raw data on the production of the segments at word level. For the second task, the consultants were asked to take a sociolinguistic questionnaire. This instrument was designed to account for the linguistic ethnography and language ecology of the Kamënts̈á. The purpose of this task was also to tie sociolinguistic data from the speakers with any phonemic differences in the realization of the segments. Results supported the VOT framework proposed by Lisker and Abramson (1964) as an effective metric to measure the voicing status of plosives. For the analysis of affricates, the center of gravity and spectral peak location demonstrated accurate acoustic correlates to capture frication in the phonemes. Equally, amplitude provided valuable information for the analysis of prenasalization. What is more, consistent resemblances were found comparing Kamënts̈á with other studies regarding the acoustic cues, place and manner of articulation, and voicing status of the phonemes.

No description available.