Thesis (Ph.D.) - University of St Andrews, August 2008.

Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: John E. Gizis, Dept. of Physics & Astronomy. Includes bibliographical references.

Mira and semi-regular variable stars have been studied for centuries but continue to be enigmatic. One unsolved mystery is the presence of polarization from these stars. In particular, we present 40 years of polarization measurements for the prototype o Ceti and V CVn and find very different phenomena for each star. The polarization fraction and position angle for Mira is found to be small and highly variable. On the other hand, the polarization fraction for V CVn is large and variable, from 2-7%, and its position angle is approximately constant, suggesting a long-term asymmetric structure. We suggest a number of potential scenarios to explain these observations.

Dissertation (Ph.D.)--University of Toledo, 2009. / Typescript. "Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics ." Bibliography: leaves 71-74.

Stars on the asymptotic giant branch (AGB) lose a significant fraction of their mass to their surroundings through stellar winds. As a result, they are surrounded by circumstellar shells of gas and dust. This stellar mass loss replenishes and enriches the interstellar medium (ISM) with the products of stellar nucleosynthesis, progressively increasing its metallicity and thereby driving galactic chemical evolution. In this thesis I present a comprehensive study of oxygen-rich (O-rich) AGB stars and red supergiants (RSG) observed with the Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer in the Milky Way, the Large and Small Magellanic Clouds, and Galactic globular clusters; focusing on the composition of the dust in the circumstellar envelopes surrounding these stars. Combining spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates, I detect crystalline silicates in stars with dust mass-loss rates which span over a factor of 1000, down to rates of ~10^{-9} Msun/yr. Detections of crystalline silicates are more prevalent in higher mass-loss rate objects, and our results indicate that the dust mass-loss rate has a greater influence on the crystalline fraction than the gas mass-loss rate, suggesting that thermal annealing of amorphous silicate grains is the primary formation mechanism of crystalline silicates in such environments rather than the direct condensation of crystalline silicates from the gas phase. I also find that metallicity influences the composition of crystalline silicates, with enstatite seen increasingly at low metallicity, while forsterite becomes depleted at these metallicities due to the different chemical composition of the gas. To trace the evolution of alumina and silicate dust along the AGB, I present an alternative grid of MODUST radiative transfer-models for a range of dust compositions, mass-loss rates, dust shell inner radii and stellar parameters. Our analysis shows that the AKARI [11]-[15] versus [3.2]-[7] colour is a robust indicator of the fractional abundance of alumina in O-rich AGB stars. From the modelling, I show that a grain mixture consisting primarily of amorphous silicates, with contributions from amorphous alumina and metallic iron provides a good fit to the observed spectra of O-rich AGB stars in the LMC. In agreement with previous studies, we find a correlation between the dust composition and mass-loss rate; the lower the mass-loss rate the higher the percentage of alumina in the shell. Finally, I present mid-infrared observations of the Local Group dwarf elliptical galaxy M32; where I find a large population of dust-enshrouded stars. These observations will act as a pathfinder for future observations with the JWST and SPICA.

Malgré leur rareté et leur vie éphémère, les étoiles massives, par l’entremise de leur forte émission de rayonnements ultraviolet et de leurs vents stellaires importants, ont un fort impact sur l’enrichissement du milieu interstellaire en éléments lourds, ainsi que sur son ionisation, sa structure et sa cinématique. L’étude des étoiles massives et de leur perte de masse est essentielle pour mieux comprendre l’évolution stellaire. En particulier en fin de vie, lors de la phase Wolf-Rayet, on constate parfois la présence d’une nébuleuse sous forme de bulle autour de ces étoiles. Dans le cadre de cette étude, nous analysons la présence et les caractéristiques de ces bulles autour d’étoiles Wolf-Rayet dans la galaxie M33, avec les données de SITELLE, un spectromètre imageur installé au télescope Canada-France-Hawaii. Sur les 211 étoiles Wolf-Rayet connues dans cette galaxie, nous avons pu en observer 178 et détecter la présence de bulles autour de 43 d’entre elles grâce à des critères que nous avons déterminés à partir de nos données. Nous avons analysé chacune d’entre elles et nous présentons notre analyse et les résultats obtenus. Nous remarquons notamment l’absence de biais favorisant la présence d’une bulle autour d’un type spectral par rapport aux autres, et la présence de bulles intriquées autour de certaines étoiles WR pouvant attester de l’évolution des vents stellaires durant les différentes phases de la vie de l’étoile. / Despite their scarcity and their short lifetime, massive stars have a strong impact on the enrichment of the interstellar medium via their heavy element production, ultraviolet radiation and stellar wind emission. Hence, the study of massive stars and their mass-loss history is essential for the understanding of stellar evolution. Especially at the end of their lives, during the Wolf-Rayet stage, we often observe bubble-like nebulae surrounding massive stars. In this study, we analyze the presence and characteristics of these Wolf-Rayet bubbles in M33, with data from SITELLE, an instrument mounted on the Canada-France-Hawaii telescope. Of the 211 known Wolf-Rayet stars in M33, we were able to observe 178 of them and we detected the presence of a bubble around 43, according to the criteria we adopted based on our data. We present here the analysis and the results of our study. We find notably that the presence of bubbles doesn’t seem to be correlated with the spectral type of the progenitor star, and we observe a structure of nested bubbles around some of the WR stars, enlightening us on the evolution of stellar winds during the different phases of a massive star’s life.

We review the recent discoveries of magnetic fields in different types of massive stars and briefly discuss strategies for spectropolarimetric observations to be carried out in the future.