4,607 research outputs found
Lower bounds on photometric redshift errors from Type Ia supernovae templates
Cosmology with Type Ia supernovae heretofore has required extensive
spectroscopic follow-up to establish a redshift. Though tolerable at the
present discovery rate, the next generation of ground-based all-sky survey
instruments will render this approach unsustainable. Photometry-based redshift
determination is a viable alternative, but introduces non-negligible errors
that ultimately degrade the ability to discriminate between competing
cosmologies. We present a strictly template-based photometric redshift
estimator and compute redshift reconstruction errors in the presence of
photometry and statistical errors. With reasonable assumptions for a cadence
and supernovae distribution, these redshift errors are combined with systematic
errors and propagated using the Fisher matrix formalism to derive lower bounds
on the joint errors in and relevant to the next
generation of ground-based all-sky survey.Comment: 23 pages, 6 figure
An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate
There is wide agreement that Type Ia supernovae (used as standard candles for
cosmology) are associated with the thermonuclear explosions of white dwarf
stars. The nuclear runaway that leads to the explosion could start in a white
dwarf gradually accumulating matter from a companion star until it reaches the
Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in
a compact binary system. The X-ray signatures of these two possible paths are
very different. Whereas no strong electromagnetic emission is expected in the
merger scenario until shortly before the supernova, the white dwarf accreting
material from the normal star becomes a source of copious X-rays for ~1e7 yr
before the explosion. This offers a means of determining which path dominates.
Here we report that the observed X-ray flux from six nearby elliptical galaxies
and galaxy bulges is a factor of ~30-50 less than predicted in the accretion
scenario, based upon an estimate of the supernova rate from their K-band
luminosities. We conclude that no more than ~5 per cent of Type Ia supernovae
in early type galaxies can be produced by white dwarfs in accreting binary
systems, unless their progenitors are much younger than the bulk of the stellar
population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs
make a significant contribution to the supernova rate.Comment: 10 pages, 1 tabl
Chandra and FUSE spectroscopy of the hot bare stellar core H1504+65
H1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective
temperature close to 200,000 K. We present new FUV and soft X-ray spectra
obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere
primarily composed of carbon and oxygen. The Chandra LETG spectrum (60-160
Angstroem) shows a wealth of photospheric absorption lines from highly ionized
oxygen, neon, and - for the first time identified in this star - magnesium and
suggests relatively high Ne and Mg abundances. This corroborates an earlier
suggestion that H1504+65 represents a naked C/O stellar core or even the C/O
envelope of an O-Ne-Mg white dwarf.Comment: 15 pages, 10 figures, accepted for publication in A&
Double-detonation sub-Chandrasekhar supernovae: synthetic observables for minimum helium shell mass models
Abridged. In the double detonation scenario for Type Ia supernovae (SNe Ia) a
detonation initiates in a shell of He-rich material accreted from a companion
star by a sub-Chandrasekhar-mass White Dwarf (WD). This shell detonation drives
a shock front into the carbon-oxygen (C/O) WD that triggers a secondary
detonation in the core. The core detonation results in a complete disruption of
the WD. Earlier studies concluded that this scenario has difficulties in
accounting for the observed properties of SNe Ia since the explosion ejecta are
surrounded by the products of explosive He burning in the shell. Recently, it
was proposed that detonations might be possible for much less massive He shells
than previously assumed. Moreover, it was shown that even detonations of these
minimum He shell masses robustly trigger detonations of the C/O core. Here we
present time-dependent multi-wavelength radiative transfer calculations for
models with minimum He shell mass and derive synthetic observables for both the
optical and {\gamma}-ray spectral regions. These differ strongly from those
found in earlier simulations of sub-Chandrasekhar-mass explosions in which more
massive He shells were considered. Our models predict light curves which cover
both the range of brightnesses and the rise and decline times of observed SNe
Ia. However, their colours and spectra do not match the observations. In
particular, their B-V colours are generally too red. We show that this
discrepancy is mainly due to the composition of the burning products of the He
shell of our models which contain significant amounts of Ti and Cr. Using a toy
model, we also show that the burning products of the He shell depend crucially
on its initial composition. This leads us to conclude that good agreement
between sub-Chandrasekhar-mass explosions and observed SNe Ia may still be
feasible but further study of the shell properties is required.Comment: 17 pages, 13 figures. Accepted for publication by Ap
Protein annotation and modelling servers at University College London
The UCL Bioinformatics Group web portal offers several high quality protein structure prediction and function annotation algorithms including PSIPRED, pGenTHREADER, pDomTHREADER, MEMSAT, MetSite, DISOPRED2, DomPred and FFPred for the prediction of secondary structure, protein fold, protein structural domain, transmembrane helix topology, metal binding sites, regions of protein disorder, protein domain boundaries and protein function, respectively. We also now offer a fully automated 3D modelling pipeline: BioSerf, which performed well in CASP8 and uses a fragment-assembly approach which placed it in the top five servers in the de novo modelling category. The servers are available via the group web site at http://bioinf.cs.ucl.ac.uk/
Prospect of Studying Hard X- and Gamma-Rays from Type Ia Supernovae
We perform multi-dimensional, time-dependent radiation transfer simulations
for hard X-ray and gamma-ray emissions, following radioactive decays of 56Ni
and 56Co, for two-dimensional delayed detonation models of Type Ia supernovae
(SNe Ia). The synthetic spectra and light curves are compared with the
sensitivities of current and future observatories for an exposure time of 10^6
seconds. The non-detection of the gamma-ray signal from SN 2011fe at 6.4 Mpc by
SPI on board INTEGRAL places an upper limit for the mass of 56Ni of \lesssim
1.0 Msun, independently from observations in any other wavelengths. Signals
from the newly formed radioactive species have not been convincingly measured
yet from any SN Ia, but the future X-ray and gamma-ray missions are expected to
deepen the observable horizon to provide the high energy emission data for a
significant SN Ia sample. We predict that the hard X-ray detectors on board
NuStar (launched in 2012) or ASTRO-H (scheduled for launch in 2014) will reach
to SNe Ia at \sim15 Mpc, i.e., one SN every few years. Furthermore, according
to the present results, the soft gamma-ray detector on board ASTRO-H will be
able to detect the 158 keV line emission up to \sim25 Mpc, i.e., a few SNe Ia
per year. Proposed next generation gamma-ray missions, e.g., GRIPS, could reach
to SNe Ia at \sim20 - 35 Mpc by MeV observations. Those would provide new
diagnostics and strong constraints on explosion models, detecting rather
directly the main energy source of supernova light.Comment: 14 pages, 7 figures, 1 table, accepted for publication in Ap
Time Dependent Monte Carlo Radiative Transfer Calculations For 3-Dimensional Supernova Spectra, Lightcurves, and Polarization
We discuss Monte-Carlo techniques for addressing the 3-dimensional
time-dependent radiative transfer problem in rapidly expanding supernova
atmospheres. The transfer code SEDONA has been developed to calculate the
lightcurves, spectra, and polarization of aspherical supernova models. From the
onset of free-expansion in the supernova ejecta, SEDONA solves the radiative
transfer problem self-consistently, including a detailed treatment of gamma-ray
transfer from radioactive decay and with a radiative equilibrium solution of
the temperature structure. Line fluorescence processes can also be treated
directly. No free parameters need be adjusted in the radiative transfer
calculation, providing a direct link between multi-dimensional hydrodynamical
explosion models and observations. We describe the computational techniques
applied in SEDONA, and verify the code by comparison to existing calculations.
We find that convergence of the Monte Carlo method is rapid and stable even for
complicated multi-dimensional configurations. We also investigate the accuracy
of a few commonly applied approximations in supernova transfer, namely the
stationarity approximation and the two-level atom expansion opacity formalism.Comment: 16 pages, ApJ accepte
Nucleosynthesis in Two-Dimensional Delayed Detonation Models of Type Ia Supernova Explosions
The nucleosynthetic characteristics of various explosion mechanisms of Type
Ia supernovae (SNe Ia) is explored based on three two-dimensional explosion
simulations representing extreme cases: a pure turbulent deflagration, a
delayed detonation following an approximately spherical ignition of the initial
deflagration, and a delayed detonation arising from a highly asymmetric
deflagration ignition. Apart from this initial condition, the deflagration
stage is treated in a parameter-free approach. The detonation is initiated when
the turbulent burning enters the distributed burning regime. This occurs at
densities around g cm -- relatively low as compared to existing
nucleosynthesis studies for one-dimensional spherically symmetric models. The
burning in these multidimensional models is different from that in
one-dimensional simulations as the detonation wave propagates both into
unburned material in the high density region near the center of a white dwarf
and into the low density region near the surface. Thus, the resulting yield is
a mixture of different explosive burning products, from carbon-burning products
at low densities to complete silicon-burning products at the highest densities,
as well as electron-capture products synthesized at the deflagration stage. In
contrast to the deflagration model, the delayed detonations produce a
characteristic layered structure and the yields largely satisfy constraints
from Galactic chemical evolution. In the asymmetric delayed detonation model,
the region filled with electron capture species (e.g., Ni, Fe) is
within a shell, showing a large off-set, above the bulk of Ni
distribution, while species produced by the detonation are distributed more
spherically (abridged).Comment: Accepted by the Astrophysical Journal. 15 pages, 14 figures, 4 table
PTF11kx: A Type-Ia Supernova with a Symbiotic Nova Progenitor
There is a consensus that Type-Ia supernovae (SNe Ia) arise from the
thermonuclear explosion of white dwarf stars that accrete matter from a binary
companion. However, direct observation of SN Ia progenitors is lacking, and the
precise nature of the binary companion remains uncertain. A temporal series of
high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex
circumstellar environment that provides an unprecedentedly detailed view of the
progenitor system. Multiple shells of circumsteller are detected and the SN
ejecta are seen to interact with circumstellar material (CSM) starting 59 days
after the explosion. These features are best described by a symbiotic nova
progenitor, similar to RS Ophiuchi.Comment: 27 pages, 5 figures. In pres
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