Date of Award
Physics and Astronomy
Jennifer L. Hoffman, Ph.D.
Massive stars strongly affect their surroundings through their energetic stellar winds during their lifetime and through their energetic deaths as supernovae. When a stellar wind interacts with the local interstellar medium (ISM), if the relative velocity between wind and ISM is supersonic, then a stellar wind bow shock is formed. Bow shocks and related density enhancements produced by the winds of massive stars moving through the interstellar medium provide important information regarding the motions of the stars, the properties of their stellar winds, and the characteristics of the local medium. Since bow shock nebulae are aspherical structures, light scattering within them produces a net polarization signal even if the region is spatially unresolved. Scattering opacity arising from free electrons and dust leads to a predictable distribution of polarized intensity across the bow shock structure. That polarization encodes information about the shape, composition, opacity, density, and ionization state of the material within the structure.
In my dissertation research, I use a Monte Carlo radiative transfer code that I optimized to simulate the polarization signatures produced by both resolved and unresolved stellar wind bow shocks (SWBS) illuminated by a central star and by emission from the bow shock. I derive bow shock shapes and densities from published analytical calculations and smooth particle hydrodynamic (SPH) models. In the case of the analytical SWBS and electron scattering, I find that higher optical depths produce higher polarization and position angle rotations at specific viewing angles compared to theoretical predictions for low optical depths. This is due to the geometrical properties of the bow shock combined with multiple scattering effects. I also find that the source of illumination plays an important role in determining the distribution of polarization for resolved bow shocks. In the case of dust scattering, the polarization signature is strongly affected by wavelength, dust grain properties, dust temperature, and viewing angle. The behavior of the polarization as a function of wavelength in these cases can distinguish among different dust models. In the case of SPH density structures, I investigate how the polarization changes as a function of the dust grain size and composition present in the SWBS. I present preliminary results of this implementation. In each case, I discuss the observational implications of these models for the stellar winds and interstellar environments of these influential objects, and predict observable signatures that can help constrain quantities of particular interest.
Shrestha, Manisha, "Polarized Bow Shock Nebulae Reveal Features of the Winds and Environments of Massive Stars" (2018). Electronic Theses and Dissertations. 1483.
Received from ProQuest