Analyzing Spectra of Type Iin Supernovae and their Host Environments To Determine If Some Arise From Runaway Progenitors

Abstract

We have analyzed observations of Type IIn supernovae taken by the DEIMOS spectrograph on Keck II in order to determine if there is a signicant offset in the peak of the SN H-alpha emission with respect to the predicted H-alpha peak wavelength of the host galaxy at the SN position along the slit. An offset of 30 km/s or greater may imply a runaway progenitor star, ejected via dynamical interaction or as the mass-gaining companion of a binary star system where the primary explodes first. Signicant fractions of runaway progenitors would have an impact on our understanding of the progenitors of SNe IIn, and given that luminous blue variable (LBV) stars are commonly considered as a progenitor star it could also have impacts on the evolutionary path for LBV stars. LBVs are not seen in evolution models as an end-of-life stage but rather a transition stage, and they are also not expected to be the result of a binary supernova kick. Spectra and resulting offset velocities were analyzed and computed for six SNe IIn, of which two returned as possible runaway candidates. Of these two, the site of SN 2010jl had direct observations by the Hubble Space Telescope approximately 10 years prior to explosion. These observations show a luminous blue source in pre-explosion imaging, which was theorized to either be an extremely bright and young star cluster or a massive luminous blue variable star. The other runaway candidate, SN 2013dz, does not appear in the literature but shows signatures of an asymmetrical circumstellar medium. The remaining four SNe IIn show no signicant evidence of runaway behavior along the line of sight. This lack of suffcient offset velocity could imply that the progenitors did not undergo a binary supernova scenario (BSS) ejection, or it could alternatively imply that the progenitor gained so much mass that the resulting ejection velocity was small. With a sample size of six observations, it is diffcult to make any statistically signifcant conclusions. More observations of progenitors, SNe, and post-SNe regions are needed to reach a more robust conclusion.