Asymmetries in Type IIn Supernovae from Spectropolarimetry

Abstract

In this dissertation, I use spectropolarimetry, spectroscopy, and photometry to study Type IIn supernovae (SNe IIn) and their progenitor environments. In Chapter 1, I first introduce why the environments of massive stars undergoing eruptive mass-loss are interesting to study. I then discuss the classification scheme for SNe and why SNe IIn are of particular importance when studying eruptive mass-loss from massive stars. I describe the benefits of polarization data and how to interpret it in the context of SNe IIn. Lastly, I discuss spectropolarimetric trends in all SNe so far, with an emphasis on results for SNe IIn. In Chapter 2, I show my analysis of pre-SN images over almost 12 years from the Katzman Automatic Imaging Telescope, searching for progenitor outbursts similar to those of SN~2009ip or Eta Car. While no progenitor outbursts were detected, I was able to set limiting magnitudes in the pre-SN images which constrain how faint outbursts must have been in order for there to be no detection. Further, I was able to estimate the probability of detecting at least one outburst within the dataset to be >= 60% for each outburst template used, suggesting that outbursts are typically less luminous than ~1-3% mag or are not common just prior to explosion. This work was published in Bilinski et al. (2015). In Chapter 3, I use spectropolarimetry, spectra, and photometry of SN 2012ab to study its environment. The lack of broad absorption features and the strong intermediate-width emission in Hα is indicative of circumstellar material (CSM) interaction. The spectral evolution for SN 2012ab proves to be interesting as it evolves from having a strong broad blueshifted peak in Hα to a late-time broad red wing with a truncated blue wing. This suggests that SN 2012ab interacted with CSM approaching us early on after explosion and then began to interact with CSM on the far side at later times. Spectropolarimetric results suggest that strong CSM interaction with significant asymmetry tangential to our line of sight turned on by day 76. Mass-loss estimates for SN 2012ab place it among other SNe IIn which likely arise from an LBV-like star in an eccentric binary system undergoing eruptive mass loss. This work was published in Bilinski et al. (2018). In Chapter 4, I use spectropolarimetry, spectra, and photometry of SN 2014ab to study its environment. The spectrum of SN 2014ab remains remarkably unchanged over the course of ~150 d, showing a consistently stronger blueshifted emission component in Hα. This implies obscuration by large dust grains, occultation by optically thick material, or asymmetry between the far and near side of the CSM interaction region. However, spectropolarimetry for SN 2014ab indicates a low polarization level at all epochs, suggesting nearly circular symmetry in the plane perpendicular to our line of sight. SN 2014ab is the first published example of a SN IIn where the polarization is very low, despite there being evidence for asymmetry along the line of sight. This work was published in Bilinski et al. (2020). In Chapter 5, I explore the collective spectropolarimetric properties for a sample of 14 SNe IIn. Past spectropolarimetric studies of SNe IIn have focused on a single object usually with a few epochs of spectropolarimetry at most. I find a tendency for the intrinsic polarization to drop smoothly over hundreds of days after peak and for the intrinsic polarization to be stronger at bluer wavelengths, especially in early epochs. I suggest that scattering of the SN light by dusty CSM may be the source of this wavelength-dependent polarization which is added to the polarization from electron scattering. Changes in the magnitude of the polarization and the wavelength dependence with time are not generally accompanied by changes in the position angle, suggesting a persistent axisymmetric geometry in the environment of SNe IIn. This result favors progenitors for SNe IIn that are undergoing binary interaction prior to death. This work will soon be submitted for publication in the Monthly Notices of the Royal Astronomical Society.