101 research outputs found
Quark nova imprint in the extreme supernova explosion SN 2006gy
The extremely luminous supernova 2006gy (SN 2006gy) is among the most
energetic ever observed. The peak brightness was 100 times that of a typical
supernova and it spent an unheard of 250 days at magnitude -19 or brighter.
Efforts to describe SN 2006gy have pushed the boundaries of current supernova
theory. In this work we aspire to simultaneously reproduce the photometric and
spectroscopic observations of SN 2006gy using a quark nova model. This analysis
considers the supernova explosion of a massive star followed days later by the
quark nova detonation of a neutron star. We lay out a detailed model of the
interaction between the supernova envelope and the quark nova ejecta paying
special attention to a mixing region which forms at the inner edge of the
supernova envelope. This model is then fit to photometric and spectroscopic
observations of SN 2006gy. This QN model naturally describes several features
of SN 2006gy including the late stage light curve plateau, the broad H{\alpha}
line and the peculiar blue H{\alpha} absorption. We find that a progenitor mass
between 20Msun and 40Msun provides ample energy to power SN 2006gy in the
context of a QN.Comment: 15 pages, 9 figure
Evidence of Asymmetry in SN 2007rt, a Type IIn Supernova
An optical photometric and spectroscopic analysis of the slowly-evolving Type
IIn SN2007rt is presented, covering a duration of 481 days after discovery. Its
earliest spectrum, taken approximately 100 days after the explosion epoch,
indicates the presence of a dense circumstellar medium, with which the
supernova ejecta is interacting. This is supported by the slowly-evolving light
curve. A notable feature in the spectrum of SN 2007rt is the presence of a
broad He I 5875 line, not usually detected in Type IIn supernovae. This may
imply that the progenitor star has a high He/H ratio, having shed a significant
portion of its hydrogen shell via mass-loss. An intermediate resolution
spectrum reveals a narrow Halpha P-Cygni profile, the absorption component of
which has a width of 128 km/s. This slow velocity suggests that the progenitor
of SN 2007rt recently underwent mass-loss with wind speeds comparable to the
lower limits of those detected in luminous blue variables. Asymmetries in the
line profiles of H and He at early phases bears some resemblance to
double-peaked features observed in a number of Ib/c spectra. These asymmetries
may be indicative of an asymmetric or bipolar outflow or alternatively dust
formation in the fast expanding ejecta. In addition, the late time spectrum, at
over 240 days post-explosion, shows clear evidence for the presence of newly
formed dust.Comment: Submitted to A&A on 4/2/2009. Accepted by A&A on 17/5/2009.15 pages
plus 3 pages of online materia
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
Mitochondria-Ros Crosstalk in the Control of Cell Death and Aging
Reactive oxygen species (ROS) are highly reactive molecules, mainly generated inside mitochondria that can oxidize DNA, proteins, and lipids. At physiological levels, ROS function as “redox messengers” in intracellular signalling and regulation, whereas excess ROS induce cell death by promoting the intrinsic apoptotic pathway. Recent work has pointed to a further role of ROS in activation of autophagy and their importance in the regulation of aging. This review will focus on mitochondria as producers and targets of ROS and will summarize different proteins that modulate the redox state of the cell. Moreover, the involvement of ROS and mitochondria in different molecular pathways controlling lifespan will be reported, pointing out the role of ROS as a “balance of power,” directing the cell towards life or death
SN 2008iy: An Unusual Type IIn Supernova with an Enduring 400 Day Rise Time
We present spectroscopic and photometric observations of the Type IIn
supernova (SN) 2008iy. SN 2008iy showed an unprecedentedly long rise time of
~400 days, making it the first SN to take significantly longer than 100 days to
reach peak optical luminosity. The peak absolute magnitude of SN 2008iy was M_r
~ -19.1 mag, and the total radiated energy over the first ~700 days was ~2 x
10^50 erg. Spectroscopically, SN 2008iy is very similar to the Type IIn SN
1988Z at late times, and, like SN 1988Z, it is a luminous X-ray source (both
supernovae had an X-ray luminosity L_ X > 10^41 erg/s). The Halpha emission
profile of SN 2008iy shows a narrow P Cygni absorption component, implying a
pre-SN wind speed of ~100 km/s. We argue that the luminosity of SN 2008iy is
powered via the interaction of the SN ejecta with a dense, clumpy circumstellar
medium. The ~400 day rise time can be understood if the number density of
clumps increases with distance over a radius ~1.7 x 10^16 cm from the
progenitor. This scenario is possible if the progenitor experienced an episodic
phase of enhanced mass-loss < 1 century prior to explosion or the progenitor
wind speed increased during the decades before core collapse. We favour the
former scenario, which is reminiscent of the eruptive mass-loss episodes
observed for luminous blue variable (LBV) stars. The progenitor wind speed and
increased mass-loss rates serve as further evidence that at least some, and
perhaps all, Type IIn supernovae experience LBV-like eruptions shortly before
core collapse. We also discuss the host galaxy of SN 2008iy, a subluminous
dwarf galaxy, and offer a few reasons why the recent suggestion that unusual,
luminous supernovae preferentially occur in dwarf galaxies may be the result of
observational biases.Comment: 15 pages, 5 figures, MNRAS accepte
SN 2009md: Another faint supernova from a low mass progenitor
We present adaptive optics imaging of the core collapse supernova (SN)
2009md, which we use together with archival \emph{Hubble Space Telescope} data
to identify a coincident progenitor candidate. We find the progenitor to have
an absolute magnitude of mag and a colour of mag, corresponding to a progenitor luminosity of log
/L dex. Using the stellar evolution code STARS,
we find this to be consistent with a red supergiant progenitor with M. The photometric and spectroscopic evolution of
SN 2009md is similar to that of the class of sub-luminous Type IIP SNe; in this
paper we compare the evolution of SN 2009md primarily to that of the
sub-luminous SN 2005cs. We estimate the mass of Ni ejected in the
explosion to be M\ from the luminosity
on the radioactive tail, which is in agreement with the low Ni masses
estimated for other sub-luminous Type IIP SNe. From the lightcurve and spectra,
we show the SN explosion had a lower energy and ejecta mass than the normal
Type IIP SN 1999em. We discuss problems with stellar evolutionary models, and
the discrepancy between low observed progenitor luminosities (log
/L dex) and model luminosities after the
second-dredge-up for stars in this mass range, and consider an enhanced carbon
burning rate as a possible solution. In conclusion, SN 2009md is a faint SN
arising from the collapse of a progenitor close to the lower mass limit for
core-collapse. This is now the third discovery of a low mass progenitor star
producing a low energy explosion and low Ni ejected mass, which
indicates that such events arise from the lowest end of the mass range that
produces a core-collapse supernova (7-8 M).Comment: MNRAS accepted, revised version following referee's comment
SN 2008S: an electron capture SN from a super-AGB progenitor?
We present comprehensive photometric and spectroscopic observations of the
faint transient SN 2008S discovered in NGC 6946. SN 2008S exhibited slow
photometric evolution and almost no spectral variability during the first nine
months, implying a high density CS medium. The light curve is similar in shape
to that of SN 1998S and SN 1979C, although significantly fainter at maximum
light. Our quasi-bolometric lightcurve extends to 300 days and shows a tail
phase decay rate consistent with that of ^{56}Co. We propose that this is
evidence for an explosion and formation of ^{56}Ni (0.0015 +/- 0.0004 M_Sun).
The large MIR flux detected shortly after explosion can be explained by a light
echo from pre-exisiting dust. The late NIR flux excess is plausibly due to a
combination of warm newly-formed ejecta dust together with shock-heated dust in
the CS environment. We reassess the progenitor object detected previously in
Spitzer archive images, supplementing this discussion with a model of the MIR
spectral energy distribution. This supports the idea of a dusty, optically
thick shell around SN 2008S with an inner radius of nearly 90AU and outer
radius of 450AU, and an inferred heating source of 3000 K and luminosity of L ~
10^{4.6} L_Sun. The combination of our monitoring data and the evidence from
the progenitor analysis leads us to support the scenario of a weak electron
capture supernova explosion in a super-AGB progenitor star (of initial mass 6-8
M_sun) embedded within a thick CS gaseous envelope. We suggest that all of main
properties of the electron capture SN phenomenon are observed in SN 2008S and
future observations may allow a definitive answer.Comment: accepted for publication in MNRAS (2009 May 7
Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events
We present new spectroscopic and photometric data of the type Ibn supernovae
2006jc, 2000er and 2002ao. We discuss the general properties of this recently
proposed supernova family, which also includes SN 1999cq. The early-time
monitoring of SN 2000er traces the evolution of this class of objects during
the first few days after the shock breakout. An overall similarity in the
photometric and spectroscopic evolution is found among the members of this
group, which would be unexpected if the energy in these core-collapse events
was dominated by the interaction between supernova ejecta and circumstellar
medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova
explosions which occur within a He-rich circumstellar environment. SNe Ibn are
therefore likely produced by the explosion of Wolf-Rayet progenitors still
embedded in the He-rich material lost by the star in recent mass-loss episodes,
which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet
star could either result from the evolution of a very massive star or be the
more evolved member of a massive binary system. We also suggest that there are
a number of arguments in favour of a type Ibn classification for the historical
SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event
with some resemblance to SN 1991bg.Comment: 17 pages including 12 figures and 4 tables. Slightly revised version,
conclusions unchanged, 1 figure added. Accepted for publication in MNRA
SN 2008S: an electron-capture SN from a super-AGB progenitor?
We present comprehensive photometric and spectroscopic observations of the faint transient SN 2008S discovered in the nearby galaxy NGC 6946. SN 2008S exhibited slow photometric evolution and almost no spectral variability during the first nine months, implying a long photon diffusion time and a high-density circumstellar medium. Its bolometric luminosity (≃1041 erg s−1 at peak) is low with respect to most core-collapse supernovae but is comparable to the faintest Type II-P events. Our quasi-bolometric light curve extends to 300 d and shows a tail phase decay rate consistent with that of 56Co. We propose that this is evidence for an explosion and formation of 56Ni (0.0014 ± 0.0003 M⊙). Spectra of SN 2008S show intense emission lines of Hα, [Ca ii] doublet and Ca ii near-infrared (NIR) triplet, all without obvious P-Cygni absorption troughs. The large mid-infrared (MIR) flux detected shortly after explosion can be explained by a light echo from pre-existing dust. The late NIR flux excess is plausibly due to a combination of warm newly formed ejecta dust together with shock-heated dust in the circumstellar environment. We reassess the progenitor object detected previously in Spitzer archive images, supplementing this discussion with a model of the MIR spectral energy distribution. This supports the idea of a dusty, optically thick shell around SN 2008S with an inner radius of nearly 90 au and outer radius of 450 au, and an inferred heating source of 3000 K. The luminosity of the central star is L≃ 104.6 L⊙. All the nearby progenitor dust was likely evaporated in the explosion leaving only the much older dust lying further out in the circumstellar environment. The combination of our long-term multiwavelength monitoring data and the evidence from the progenitor analysis leads us to support the scenario of a weak electron-capture supernova explosion in a super-asymptotic giant branch progenitor star (of initial mass 6-8 M⊙) embedded within a thick circumstellar gaseous envelope. We suggest that all of main properties of the electron-capture SN phenomenon are observed in SN 2008S and future observations may allow a definitive answe
Direct Determination of Hubble Parameter Using Type IIn Supernovae
We introduce a novel approach, a Dense Shell Method (DSM), for measuring
distances for cosmology. It is based on original Baade idea to relate absolute
difference of photospheric radii with photospheric velocity. We demonstrate
that this idea works: the new method does not rely on the Cosmic Distance
Ladder and gives satisfactory results for the most luminous Type IIn
Supernovae. This allows one to make them good primary distance indicators for
cosmology. Fixing correction factors for illustration, we obtain with this
method the median distance of 68^{+19}_{-15} (68%CL) Mpc to SN 2006gy and
median Hubble parameter 79^{+23}_{-17} (68%CL) km/s/Mpc.Comment: 6 pages, 1 figure, typos correcte
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