A1 Refereed original research article in a scientific journal
Two type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions
Authors: Young DR, Smartt SJ, Valenti S, Pastorello A, Benetti S, Benn CR, Bersier D, Botticella MT, Corradi RLM, Harutyunyan AH, Hrudkova M, Hunter I, Mattila S, de Mooij EJW, Navasardyan H, Snellen IAG, Tanvir NR, Zampieri L
Publisher: EDP SCIENCES S A
Publication year: 2010
Journal: Astronomy and Astrophysics
Journal name in source: ASTRONOMY & ASTROPHYSICS
Journal acronym: ASTRON ASTROPHYS
Article number: ARTN A70
Volume: 512
Number of pages: 19
ISSN: 0004-6361
DOI: https://doi.org/10.1051/0004-6361/200913004
Abstract
Results. Neither SNe 2007bg nor 2007bi were found in association with an observed GRB, but from estimates of the metallicities of their host-galaxies they 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 M(B) = -12.4 +/- 0.6 mag and an estimated oxygen abundance of 12 + log(O/H) = 8.18 +/- 0.17 (on the Pettini & Pagel 2004 scale). The early lightcurve evolution of SN 2007bg matches the fast-pace decline of SN 19941 giving it 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)/M(ej) similar to 2.7). We also show that SN 2007bi is possibly the most luminous type Ic known, reaching a peak magnitude of M(R) similar to -21.3 mag and displays a remarkably slow decline, following the radioactive decay rate of (56)Co to (56)Fe throughout the course of its observed lifetime. SN 2007bi also displays an extreme longevity in its spectral evolution and is still not fully nebular at approximately one year post-maximum brightness. From a simple model of the bolometric light curve of SN 2007bi we estimate a total ejected (56)Ni mass of M(Ni) = 3.5-4.5 M(circle dot), the largest (56)Ni mass measured in the ejecta of a supernova to date. There are two models that could explain the high luminosity and large ejected (56)Ni 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 (on the McGaugh 1991 scale) 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 M(circle dot) which is the core of a star of originally 50-100 M(circle dot).
Results. Neither SNe 2007bg nor 2007bi were found in association with an observed GRB, but from estimates of the metallicities of their host-galaxies they 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 M(B) = -12.4 +/- 0.6 mag and an estimated oxygen abundance of 12 + log(O/H) = 8.18 +/- 0.17 (on the Pettini & Pagel 2004 scale). The early lightcurve evolution of SN 2007bg matches the fast-pace decline of SN 19941 giving it 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)/M(ej) similar to 2.7). We also show that SN 2007bi is possibly the most luminous type Ic known, reaching a peak magnitude of M(R) similar to -21.3 mag and displays a remarkably slow decline, following the radioactive decay rate of (56)Co to (56)Fe throughout the course of its observed lifetime. SN 2007bi also displays an extreme longevity in its spectral evolution and is still not fully nebular at approximately one year post-maximum brightness. From a simple model of the bolometric light curve of SN 2007bi we estimate a total ejected (56)Ni mass of M(Ni) = 3.5-4.5 M(circle dot), the largest (56)Ni mass measured in the ejecta of a supernova to date. There are two models that could explain the high luminosity and large ejected (56)Ni 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 (on the McGaugh 1991 scale) 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 M(circle dot) which is the core of a star of originally 50-100 M(circle dot).
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