Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 84-91). Available also in a digital version from Dissertation Abstracts.

This thesis presents observational studies of long-term and extreme variability of stars with the Digital Access to a Sky Century@Harvard (DASCH) project. Stellar variations over decades are poorly explored. With the unique 100 years coverage of DASCH, for the first time, we are able to study the variable sky over long timescales in a systematic way. I have developed photometric calibration and variable search algorithms for DASCH. I have discovered exciting new types of long-term variables, which do not match any of the common classes, and studied the physical processes involved. Following a brief introduction on variable stars and DASCH in the first chapter, I describe my work on DASCH pipeline, including photometric development and defect filtering in chapter 2. I present our discovery of a group of peculiar long-term K giant variables with \(\sim1\) mag variations over decades in chapter 3. Follow-up observations show that they consist of two subgroups, including a subgroup of RS CVn binaries with strong magnetic activity, and another subgroup of single stars. In both cases, the variation amplitudes and timescales are abnormal, and may be related to either ultra strong star spots, or novel dust formation processes. In chapter 4, I present the discovery of a 5 yr dip around 1900 in the eclipsing binary KU Cyg consisting of a F star and a K giant, which is related to the accretion disk surrounding the F star. It showed a slow fading \((\sim 4 yr)\), which is probably caused by increases in dust extinction in the disk, and a relatively fast brightening \((\sim 1 yr)\), which may be due to the evaporation of dust transported inward through the disk. The extinction excess which caused the fading may arise from an increased mass transfer rate in the system or from dust clump ejections from the K giant, in accordance with K giant “dimming” as discussed above. In chapter 5, I present a 10 yr nova-like outburst in a peculiar symbiotic system. With P = 119 days, it is interestingly located in the period gap region between classical novae and symbiotic novae. The most probable explanation of the outburst is hydrogen shell-burning on the white dwarf (WD) without significant mass loss, which suggests a promising new channel for Type Ia Supernovae (SNe). In chapter 6, I present the DASCH light curves of Kepler planet-candidate host stars. We found no variation for these host stars. In chapter 7, I present my variable search algorithms and the resulting DASCH variable catalog for the Kepler field. The conclusion is presented in chapter 8. / Astronomy

published_or_final_version / Physics / Master / Master of Philosophy

Diss. (sammanfattning) Uppsala : Univ., 2003. / Härtill 5 uppsatser.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisors: Beverley J. Wills and Neal J. Evans, II. Vita. Includes bibliographical references. Also available from UMI.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 103-105). Available also in a digital version from Dissertation Abstracts.

Variability of pre-main-sequence stars observed at optical wavelengths has been attributed to fluctuations in the mass accretion rate from the circumstellar disk onto the forming star. Detailed models of accretion disks suggest that young deeply em- bedded protostars should also exhibit variations in their accretion rates, and that these changes can be tracked indirectly by monitoring the response of the dust enve- lope at mid-IR to millimeter wavelengths. Interferometers such as ALMA offer the resolution and sensitivity to observe small fluctuations in brightness at the scale of the disk where episodic accretion may be driven. In this thesis, novel methods for comparing interferometric observations are presented and applied to CARMA and ALMA 1.3mm observations of deeply embedded protostars in Serpens taken 9 years apart. No brightness variation is found above the limits of the analysis of a factor of ~>50%, due to the limited sensitivity of the CARMA observations and small number of sources common to both epochs. It is further shown that follow up ALMA observa- tions with a similar sample size and sensitivity may be able to uncover variability at the level of a few percent, and the implications of this for future work are discussed. / Graduate

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With the aim of better understanding the history of ocean closure and suturing between India and Asia, we conducted a geologic investigation of a siliciclastic matrix tectonic melange within the western Yarlung suture zone of southern Tibet (Lopu Range region, similar to 50 km northwest of Saga). The siliciclastic matrix melange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone. Metapelite and metabasite blocks in the melange exhibit lower greenschist fades mineral assemblages, indicating that they were not deeply subducted. We obtained detrital zircon U-Pb geochronologic and sandstone petrographic data from sandstone blocks in the melange and sandstone beds from Tethyan Himalayan strata exposed to the south of the suture. The sandstones from both units are all similar in U-Pb detrital zircon age spectra and petrography to the nearby Tethyan Cretaceous-Paleocene Sangdanlin section, which records the earliest appearance (at similar to 59 Ma) of arc-affinity strata deposited conformably on Indian-affinity strata. Two Paleocene sandstones, one of which is a schistose block incorporated in the siliciclastic matrix melange, yielded indistinguishable maximum depositional ages of similar to 59 Ma. Mesozoic Asian-affinity sandstone blocks previously documented in the siliciclastic matrix melange 200-500 km along strike to the east are notably absent in the Lopu Range region. We documented a gradational transition in structural style from the block-in-matrix melange in the northeast to the south-vergent Tethyan thrust belt in the southwest. Blocks of Tethyan Himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest. We conclude that no arc-affinity sandstone blocks were incorporated into the subduction complex until India-Asia collision at similar to 59 Ma when the Xigaze forearc basin became overfilled and Tethyan Himalayan strata entered the trench. As collision progressed, there was a gradual transition in structural style from block-in-matrix melange formation to imbricate-style thrust belt formation. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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