50 research outputs found
On the Progenitors of Core-Collapse Supernovae
Theory holds that a star born with an initial mass between about 8 and 140
times the mass of the Sun will end its life through the catastrophic
gravitational collapse of its iron core to a neutron star or black hole. This
core collapse process is thought to usually be accompanied by the ejection of
the star's envelope as a supernova. This established theory is now being tested
observationally, with over three dozen core-collapse supernovae having had the
properties of their progenitor stars directly measured through the examination
of high-resolution images taken prior to the explosion. Here I review what has
been learned from these studies and briefly examine the potential impact on
stellar evolution theory, the existence of "failed supernovae", and our
understanding of the core-collapse explosion mechanism.Comment: 7 Pages, invited review accepted for publication by Astrophysics and
Space Science (special HEDLA 2010 issue
Supernova 2007bi as a pair-instability explosion
Stars with initial masses 10 M_{solar} < M_{initial} < 100 M_{solar} fuse
progressively heavier elements in their centres, up to inert iron. The core
then gravitationally collapses to a neutron star or a black hole, leading to an
explosion -- an iron-core-collapse supernova (SN). In contrast, extremely
massive stars (M_{initial} > 140 M_{solar}), if such exist, have oxygen cores
which exceed M_{core} = 50 M_{solar}. There, high temperatures are reached at
relatively low densities. Conversion of energetic, pressure-supporting photons
into electron-positron pairs occurs prior to oxygen ignition, and leads to a
violent contraction that triggers a catastrophic nuclear explosion. Tremendous
energies (>~ 10^{52} erg) are released, completely unbinding the star in a
pair-instability SN (PISN), with no compact remnant. Transitional objects with
100 M_{solar} < M_{initial} < 140 M_{solar}, which end up as iron-core-collapse
supernovae following violent mass ejections, perhaps due to short instances of
the pair instability, may have been identified. However, genuine PISNe, perhaps
common in the early Universe, have not been observed to date. Here, we present
our discovery of SN 2007bi, a luminous, slowly evolving supernova located
within a dwarf galaxy (~1% the size of the Milky Way). We measure the exploding
core mass to be likely ~100 M_{solar}, in which case theory unambiguously
predicts a PISN outcome. We show that >3 M_{solar} of radioactive 56Ni were
synthesized, and that our observations are well fit by PISN models. A PISN
explosion in the local Universe indicates that nearby dwarf galaxies probably
host extremely massive stars, above the apparent Galactic limit, perhaps
resulting from star formation processes similar to those that created the first
stars in the Universe.Comment: Accepted version of the paper appearing in Nature, 462, 624 (2009),
including all supplementary informatio
Two Type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions
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)Comment: Minor changes. 19 pages, 21 Figures. Accepted by A&