Kepler's SN

K. Tabetha Hole


Visiting Professor
Department of Physics
Weber State University

Research

My research is focused on understanding the structure of supernovae and stellar winds.

Supernovae

Supernovae are the explosions at the end of the lives of certain stars. They are some of the most energetic events in the universe, and are fundamental to its evolution. The large and small-scale structural differences between supernovae provide important information about their composition, and about how nuclear reactions proceed through the star. These in turn give us insights into, and constraints on, how these explosions are triggered. A thorough understanding of supernovae -- aside from being exciting in and of itself -- is also a basis for understanding many other branches of astronomy, from star formation, to galaxy evolution, to the large-scale structure and cosmology of our universe.

My work has focused on investigating inhomogeneities in elemental abundance in the material ejected by supernova explosions, also called "clumps." The presence of clumps is suggested observationally by strong polarization detected in absorption lines at early times in all types of supernovae. This polarization is distinct from that in the continuum and differs in the lines of various elements, indicating changes in abundance rather than in overall density. To understand this mechanism as a source of observed supernova asymmetry, I have built a model of supernovae that predicts how their radiation signatures will be shaped by their structure.

This model, presented in Hole et al. (2010), models supernovae at early times, when the large dispersion of ejecta velocities mean that lines are broad and polarization is necessary for precise detection of asymmetry. Previous work had used elaborate computer models to explore the effects of asymmetric supernova ejecta. However, these codes are computer intensive, making them less practical for sampling large numbers of potential geometries. But the same polarization signature can be created by substantially different host structures.

My modeling code uses strategically simplified physics to model polarized line radiative transfer within a three-dimensional, inhomogeneous rapidly expanding atmosphere. Given a set of parameters, the code generates random sets of clumps in the expanding ejecta and calculates the emergent polarized line profile for each con├×guration. The ensemble of these con├×gurations represents the effects both of various host geometries and of different viewing angles. The results on the effects of the number and size of clumps on the emergent spectrum and Stokes parameters show that random clumpiness can indeed produce line polarization like that observed in supernovae Ia and many core-collapse supernovae. These simulations can also reproduce puzzling polarization features that have been seen in different types of supernovae.

I have also developed a method to connect these simulations to robust observational parameters such as maximum polarization and pseudo-equivalent width of the line in polarized flux. These measures have the potential to eliminate the degeneracy inherent in polarimetry and allow me to constrain the kinds of clumping scenarios that could have produced a given set of observations. In my work with Charla Boom, an undergraduate student, we made the necessary measurements from spectropolarimetric observations of SN2006X. These measurements have shown good agreement with the predictions of my model, and verified its potential to constrain our models of supernova ejecta structure.

Stellar Winds

Stellar winds are made up of particles accelerated off the surface of a star and launched into interstellar space. The Sun's wind (the solar wind) is relatively mild, and does not have a large effect on the evolution of our star. But stellar winds and the resultant loss of mass are stronger and more important in more massive stars. In fact, in extreme cases, massive stars can lose up to half their initial mass via stellar winds while still on the main sequence. This inevitably affects their evolution. Stellar winds also have a significant impact on the star's environment by blowing "wind bubbles" and injecting energy and heavier elements into the surrounding interstellar medium.

My work looks at the wind close to the star, in the region where it is being launched from the stellar atmosphere. These winds are supported and accelerated by radiation pressure, specifically in the absorption lines of certain elements. The structure of the wind near the star can be influenced by variations in density that can create shadowing -- blocking material further from the star from receiving light to accelerate it uniformly. Variations in the star's radiation field will create a time-dependent accelerating force on the wind. And other factors of the star's environment, such as a binary companion, which may have it's own strong wind, can shape the wind structure.

I have created and published a code that looks at the impact of stellar pulsations on the temperature structure of the base of the stellar wind, and compares it to observations of massive stars. Currently I am part of a collaboration working on analyzing and understanding our observations of the Delta Ori massive binary system, modifying my previous code to address other sources of variability, such as starspots, occultation and interaction with a companion.