The Properties and Environments of Superluminous Supernovae

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The Properties and Environments of Superluminous Supernovae

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Title: The Properties and Environments of Superluminous Supernovae
Author: Lunnan, Ragnhild
Citation: Lunnan, Ragnhild. 2015. The Properties and Environments of Superluminous Supernovae. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: Superluminous supernovae (SLSNe) are a rare class of stellar explosions discovered by wide-field optical transient surveys in the past decade. They are characterized by peak luminosities 10-100 times that of ordinary core-collapse and Type Ia SNe, and radiated energies of order 10^51 erg, comparable to the entire kinetic energy of a canonical supernova explosion. Proposed sources of these tremendous energies include interaction between the supernova ejecta and dense circumstellar material (CSM), energy injection from the spin-down of a rapidly rotating and highly magnetized neutron star, or the pair-instability explosion of a very massive star producing several solar masses of radioactive nickel. In this thesis, I present results from the Pan-STARRS1 Medium Deep Survey (PS1/MDS), which discovered 15 hydrogen-poor SLSNe out to redshift 1.6 over the four years of its operation. I address the nature of SLSNe from two different angles. First, I characterize the SNe themselves, and compare their observed properties to model predictions. The PS1/MDS SLSN sample exhibits a diversity of light curve properties, and a wider range of peak luminosities than previously reported, particularly when accounting for the flux-limited nature of the survey. The light curves can generally be fit with magnetar spin-down models, though our sample also contains one very slowly evolving event that could plausibly be powered by radioactive decay. Second, I present the first comprehensive study of SLSN host galaxy environments and the sub-galactic environments, demonstrating that H-poor SLSNe preferentially occur in low-luminosity, low-mass, low-metallicity galaxies with high specific star formation rates. Their host galaxies are statistically distinct from the hosts of core-collapse SNe, but share many similarities with the galaxies that host long gamma-ray bursts (LGRBs). This suggests that the environmental factors leading to a massive star forming either a SLSN or a LGRB are similar, with a possible common ingredient being a preference for low-metallicity environments through the need of a progenitor with high core angular momentum. In terms of their local environments, resolved Hubble Space Telescope imaging reveals that SLSN locations are correlated with the UV light, though not as strongly as LGRBs are. Although a larger sample size is needed to distinguish them statistically, this trend is also consistent with the interpretation that SLSN progenitors are lower-mass than those of LGRBs, collapsing to form a rapidly spinning neutron star rather than a black hole launching a relativistic jet.
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