1,040 research outputs found
Helium-ignited violent mergers as a unified model for normal and rapidly declining Type Ia Supernovae
The progenitors of Type Ia Supernovae (SNe Ia) are still unknown, despite
significant progress during the last years in theory and observations. Violent
mergers of two carbon--oxygen (CO) white dwarfs (WDs) are one candidate
suggested to be responsible for at least a significant fraction of normal SNe
Ia. Here, we simulate the merger of two CO WDs using a moving-mesh code that
allows for the inclusion of thin helium (He) shells (0.01\,\msun) on top of the
WDs, at an unprecedented numerical resolution. The accretion of He onto the
primary WD leads to the formation of a detonation in its He shell. This
detonation propagates around the CO WD and sends a converging shock wave into
its core, known to robustly trigger a second detonation, as in the well-known
double-detonation scenario for
He-accreting CO WDs. However, in contrast to that scenario where a massive He
shell is required to form a detonation through thermal instability, here the He
detonation is ignited dynamically. Accordingly the required He-shell mass is
significantly smaller, and hence its burning products are unlikely to affect
the optical display of the explosion. We show that this scenario, which works
for CO primary WDs with CO- as well as He-WD companions, has the potential to
explain the different brightness distributions, delay times and relative rates
of normal and fast declining SNe Ia. Finally, we discuss extensions to our
unified merger model needed to obtain a comprehensive picture of the full
observed diversity of SNe Ia.Comment: accepted for publication by ApJL, significant changes to first
version, including addition of merger simulatio
Towards an understanding of Type Ia supernovae from a synthesis of theory and observations
Motivated by the fact that calibrated light curves of Type Ia supernovae (SNe
Ia) have become a major tool to determine the expansion history of the
Universe, considerable attention has been given to, both, observations and
models of these events over the past 15 years. Here, we summarize new
observational constraints, address recent progress in modeling Type Ia
supernovae by means of three-dimensional hydrodynamic simulations, and discuss
several of the still open questions. It will be be shown that the new models
have considerable predictive power which allows us to study observable
properties such as light curves and spectra without adjustable non-physical
parameters. This is a necessary requisite to improve our understanding of the
explosion mechanism and to settle the question of the applicability of SNe Ia
as distance indicators for cosmology. We explore the capabilities of the models
by comparing them with observations and we show how such models can be applied
to study the origin of the diversity of SNe Ia.Comment: 26 pages, 13 figures, Frontiers of Physics, in prin
Monte-Carlo methods for NLTE spectral synthesis of supernovae
We present JEKYLL, a new code for modelling of supernova (SN) spectra and
lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer.
The code assumes spherical symmetry, homologous expansion and steady state for
the matter, but is otherwise capable of solving the time-dependent radiative
transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used
was introduced in a series of papers by Lucy, but the full time-dependent NLTE
capabilities of it have never been tested. Here, we have extended the method to
include non-thermal excitation and ionization as well as charge-transfer and
two-photon processes. Based on earlier work, the non-thermal rates are
calculated by solving the Spencer-Fano equation. Using a method previously
developed for the SUMO code, macroscopic mixing of the material is taken into
account in a statistical sense. In addition, a statistical Markov-chain model
is used to sample the emission frequency, and we introduce a method to control
the sampling of the radiation field. Except for a description of JEKYLL, we
provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good
agreement in the calculated spectra as well as the state of the gas. In
particular, the comparison with CMFGEN, which is similar in terms of physics
but uses a different technique, shows that the Lucy method does indeed converge
in the time-dependent NLTE case. Finally, as an example of the time-dependent
NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a
set of models presented and discussed in detail in an accompanying paper. Based
on this model we investigate the effects of NLTE, in particular those arising
from non-thermal excitation and ionization, and find strong effects even on the
bolometric lightcurve. This highlights the need for full NLTE calculations when
simulating the spectra and lightcurves of SNe.Comment: Accepted for publication by Astronomy & Astrophysic
Optimal decision making for sperm chemotaxis in the presence of noise
For navigation, microscopic agents such as biological cells rely on noisy
sensory input. In cells performing chemotaxis, such noise arises from the
stochastic binding of signaling molecules at low concentrations. Using
chemotaxis of sperm cells as application example, we address the classic
problem of chemotaxis towards a single target. We reveal a fundamental
relationship between the speed of chemotactic steering and the strength of
directional fluctuations that result from the amplification of noise in the
chemical input signal. This relation implies a trade-off between slow, but
reliable, and fast, but less reliable, steering.
By formulating the problem of optimal navigation in the presence of noise as
a Markov decision process, we show that dynamic switching between reliable and
fast steering substantially increases the probability to find a target, such as
the egg. Intriguingly, this decision making would provide no benefit in the
absence of noise. Instead, decision making is most beneficial, if chemical
signals are above detection threshold, yet signal-to-noise ratios of gradient
measurements are low. This situation generically arises at intermediate
distances from a target, where signaling molecules emitted by the target are
diluted, thus defining a `noise zone' that cells have to cross.
Our work addresses the intermediate case between well-studied perfect
chemotaxis at high signal-to-noise ratios close to a target, and random search
strategies in the absence of navigation cues, e.g. far away from a target. Our
specific results provide a rational for the surprising observation of decision
making in recent experiments on sea urchin sperm chemotaxis. The general theory
demonstrates how decision making enables chemotactic agents to cope with high
levels of noise in gradient measurements by dynamically adjusting the
persistence length of a biased persistent random walk.Comment: 9 pages, 5 figure
Type Ia Supernovae and Accretion Induced Collapse
Using the population synthesis binary evolution code StarTrack, we present
theoretical rates and delay times of Type Ia supernovae arising from various
formation channels. These channels include binaries in which the exploding
white dwarf reaches the Chandrasekhar mass limit (DDS, SDS, and helium-rich
donor scenario) as well as the sub-Chandrasekhar mass scenario, in which a
white dwarf accretes from a helium-rich companion and explodes as a SN Ia
before reaching the Chandrasekhar mass limit. We find that using a common
envelope parameterization employing energy balance with alpha=1 and lambda=1,
the supernova rates per unit mass (born in stars) of sub-Chandrasekhar mass SNe
Ia exceed those of all other progenitor channels at epochs t=0.7 - 4 Gyr for a
burst of star formation at t=0. Additionally, the delay time distribution of
the sub-Chandrasekhar model can be divided in to two distinct evolutionary
channels: the `prompt' helium-star channel with delay times < 500 Myr, and the
`delayed' double white dwarf channel with delay times > 800 Myr spanning up to
a Hubble time. These findings are in agreement with recent
observationally-derived delay time distributions which predict that a large
number of SNe Ia have delay times < 1 Gyr, with a significant fraction having
delay times < 500 Myr. We find that the DDS channel is also able to account for
the observed rates of SNe Ia. However, detailed simulations of white dwarf
mergers have shown that most of these mergers will not lead to SNe Ia but
rather to the formation of a neutron star via accretion-induced collapse. If
this is true, our standard population synthesis model predicts that the only
progenitor channel which can account for the rates of SNe Ia is the
sub-Chandrasekhar mass scenario, and none of the other progenitors considered
can fully account for the observed rates.Comment: 6 pages, 1 figure, 1 table, to appear in proceedings for "Binary Star
Evolution: Mass Loss, Accretion and Mergers
Testing for redshift evolution of Type Ia supernovae using the strongly lensed PS1-10afx at
The light from distant supernovae (SNe) can be magnified through
gravitational lensing when a foreground galaxy is located along the line of
sight. This line-up allows for detailed studies of SNe at high redshift that
otherwise would not be possible. Spectroscopic observations of lensed
high-redshift Type Ia supernovae (SNe Ia) are of particular interest since they
can be used to test for evolution of their intrinsic properties. The use of SNe
Ia for probing the cosmic expansion history has proven to be an extremely
powerful method for measuring cosmological parameters. However, if systematic
redshift-dependent properties are found, their usefulness for future surveys
could be challenged. We investigate whether the spectroscopic properties of the
strongly lensed and very distant SN Ia PS1-10afx at deviates from the
well-studied populations of normal SNe Ia at nearby or intermediate distance.
We created median spectra from nearby and intermediate-redshift
spectroscopically normal SNe Ia from the literature at -5 and +1 days from
light-curve maximum. We then compared these median spectra to those of
PS1-10afx. We do not find signs of spectral evolution in PS1-10afx. The
observed deviation between PS1-10afx and the median templates are within what
is found for SNe at low- and intermediate-redshift. There is a noticeable broad
feature centred at ~\AA{}, which is present only to a
lesser extent in individual low and intermediate redshift SN Ia spectra. From a
comparison with a recently developed explosion model, we find this feature to
be dominated by iron peak elements, in particular, singly ionized cobalt and
chromium.Comment: accepted for publication in section 4. Extragalactic astronomy of
Astronomy and Astrophysic
Early light curves for Type Ia supernova explosion models
Upcoming high-cadence transient survey programmes will produce a wealth of
observational data for Type Ia supernovae. These data sets will contain
numerous events detected very early in their evolution, shortly after
explosion. Here, we present synthetic light curves, calculated with the
radiation hydrodynamical approach Stella for a number of different explosion
models, specifically focusing on these first few days after explosion. We show
that overall the early light curve evolution is similar for most of the
investigated models. Characteristic imprints are induced by radioactive
material located close to the surface. However, these are very similar to the
signatures expected from ejecta-CSM or ejecta-companion interaction. Apart from
the pure deflagration explosion models, none of our synthetic light curves
exhibit the commonly assumed power-law rise. We demonstrate that this can lead
to substantial errors in the determination of the time of explosion. In
summary, we illustrate with our calculations that even with very early data an
identification of specific explosion scenarios is challenging, if only
photometric observations are available.Comment: 15 pages, 14 figures, 3 tables, accepted for publication in MNRA
Applying the expanding photosphere and standardized candle methods to Type II-Plateau supernovae at cosmologically significant redshifts: the distance to SN 2013eq
Based on optical imaging and spectroscopy of the Type II-Plateau SN 2013eq,
we present a comparative study of commonly used distance determination methods
based on Type II supernovae. The occurrence of SN 2013eq in the Hubble flow (z
= 0.041 +/- 0.001) prompted us to investigate the implications of the
difference between "angular" and "luminosity" distances within the framework of
the expanding photosphere method (EPM) that relies upon a relation between flux
and angular size to yield a distance. Following a re-derivation of the basic
equations of the EPM for SNe at non-negligible redshifts, we conclude that the
EPM results in an angular distance. The observed flux should be converted into
the SN rest frame and the angular size, theta, has to be corrected by a factor
of (1+z)^2. Alternatively, the EPM angular distance can be converted to a
luminosity distance by implementing a modification of the angular size. For SN
2013eq, we find EPM luminosity distances of D_L = 151 +/- 18 Mpc and D_L = 164
+/- 20 Mpc by making use of different sets of dilution factors taken from the
literature. Application of the standardized candle method for Type II-P SNe
results in an independent luminosity distance estimate (D_L = 168 +/- 16 Mpc)
that is consistent with the EPM estimate.Comment: 12 pages, 4 figures, accepted by A&
Oxygen emission in remnants of thermonuclear supernovae as a probe for their progenitor system
Recent progress in numerical simulations of thermonuclear supernova
explosions brings up a unique opportunity in studying the progenitors of Type
Ia supernovae. Coupling state-of-the-art explosion models with detailed
hydrodynamical simulations of the supernova remnant evolution and the most
up-to-date atomic data for X-ray emission calculations makes it possible to
create realistic synthetic X-ray spectra for the supernova remnant phase.
Comparing such spectra with high quality observations of supernova remnants
could allow to constrain the explosion mechanism and the progenitor of the
supernova. The present study focuses in particular on the oxygen emission line
properties in young supernova remnants, since different explosion scenarios
predict a different amount and distribution of this element. Analysis of the
soft X-ray spectra from supernova remnants in the Large Magellanic Cloud and
confrontation with remnant models for different explosion scenarios suggests
that SNR 0509-67.5 could originate from a delayed detonation explosion and SNR
0519-69.0 from an oxygen-rich merger.Comment: 8 pages, 4 figures, MNRAS accepte
- …