Two Type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions

TL;DR

This study analyzes two low-metallicity Type Ic supernovae, SN 2007bg and SN 2007bi, highlighting their diverse explosion properties, host environments, and implications for supernova models, including potential pair-instability origins.

Contribution

It presents detailed photometric and spectroscopic data for two supernovae in dwarf galaxies, exploring their explosion characteristics and challenging existing models of supernova luminosity and progenitor mass.

Findings

SN 2007bg has a fast decline rate and high kinetic energy to ejected mass ratio.

SN 2007bi is possibly the most luminous Type Ic supernova, with a large 56Ni mass.

Host galaxy metallicity measurements challenge the pair-instability supernova model.

Abstract

We present BVRI photometry and optical spectroscopy of two Type Ic supernovae SN 2007bg and SN 2007bi discovered in wide-field, non-targeted surveys and associated with sub-luminous blue dwarf galaxies. Neither SNe 2007bg nor 2007bi were found in association with an observed GRB, but are found to inhabit similar low-metallicity environments as GRB associated supernovae. The radio-bright SN 2007bg is hosted by an extremely sub-luminous galaxy of magnitude MB = -12.4+/-0.6 mag with an estimated oxygen abundance of 12+log(O/H) = 8.18+/-0.17. The lightcurve of SN 2007bg displays one of the fastest post-maximum decline rates of all broad-lined Type Ic supernovae known to date and, when combined with its high expansion velocities, a high kinetic energy to ejected mass ratio (E_K/Mej ~ 2.7). We show that SN 2007bi is possibly the most luminous Type Ic known, reaching a peak magnitude of MR ~ 21.3 mag and displays a remarkably slow decline, following the radioactive decay rate of 56Co to 56Fe throughout the course of its observed lifetime. From a simple model of the bolometric light curve of SN 2007bi we estimate a total ejected 56Ni mass of M_Ni = 3.5 - 4.5 solar masses, the largest 56Ni mass measured in the ejecta of a supernova to date. There are two models that could explain the high luminosity and large ejected 56Ni mass. One is a pair-instability supernova (PISN) which has been predicted to occur for massive stars at low metallicities. We measure the host galaxy metallicity of SN 2007bi to be 12 + log(O/H) = 8.15+/-0.15 which is somewhat high to be consistent with the PISN model. An alternative is the core-collapse of a C+O star of 20 - 40 solar masses which is the core of a star of originally 50 - 100 solar masses. (Abridged)