44 research outputs found
Type Iax SNe as a few-parameter family
We present direct spectroscopic modeling of five Type Iax supernovae (SNe)
with the one dimensional Monte Carlo radiative transfer code TARDIS. The
abundance tomography technique is used to map the chemical structure and
physical properties of the SN atmosphere. Through via fitting of multiple
spectral epochs with self-consistent ejecta models, we can then constrain the
location of some elements within the ejecta. The synthetic spectra of the
best-fit models are able to reproduce the flux continuum and the main
absorption features in the whole sample. We find that the mass fractions of
IGEs and IMEs show a decreasing trend toward the outer regions of the
atmospheres using density profiles similar to those of deflagration models in
the literature. Oxygen is the only element, which could be dominant at higher
velocities. The stratified abundance structure contradicts the well-mixed
chemical profiles predicted by pure deflagration models. Based on the derived
densities and abundances, a template model atmosphere is created for the SN Iax
class and compared to the observed spectra. Free parameters are the scaling of
the density profile, the velocity shift of the abundance template, and the peak
luminosity. The results of this test support the idea that all SNe Iax can be
described by a similar internal structure, which argues for a common origin of
this class of explosions.Comment: 21 pages, 7 tables, 16 figures, accepted by MNRA
Type Ia supernovae from exploding oxygen-neon white dwarfs
The progenitor problem of Type Ia supernovae (SNe Ia) is still unsolved. Most
of these events are thought to be explosions of carbon-oxygen (CO) white dwarfs
(WDs), but for many of the explosion scenarios, particularly those involving
the externally triggered detonation of a sub-Chandrasekhar mass WD (sub-M Ch
WD), there is also a possibility of having an oxygen-neon (ONe) WD as
progenitor. We simulate detonations of ONe WDs and calculate synthetic
observables from these models. The results are compared with detonations in CO
WDs of similar mass and observational data of SNe Ia. We perform hydrodynamic
explosion simulations of detonations in initially hydrostatic ONe WDs for a
range of masses below the Chandrasekhar mass (M Ch), followed by detailed
nucleosynthetic postprocessing with a 384-isotope nuclear reaction network. The
results are used to calculate synthetic spectra and light curves, which are
then compared with observations of SNe Ia. We also perform binary evolution
calculations to determine the number of SNe Ia involving ONe WDs relative to
the number of other promising progenitor channels. The ejecta structures of our
simulated detonations in sub-M Ch ONe WDs are similar to those from CO WDs.
There are, however, small systematic deviations in the mass fractions and the
ejecta velocities. These lead to spectral features that are systematically less
blueshifted. Nevertheless, the synthetic observables of our ONe WD explosions
are similar to those obtained from CO models. Our binary evolution calculations
show that a significant fraction (3-10%) of potential progenitor systems should
contain an ONe WD. The comparison of our ONe models with our CO models of
comparable mass (1.2 Msun) shows that the less blueshifted spectral features
fit the observations better, although they are too bright for normal SNe Ia.Comment: 6 pages, 5 figure
The impact of Type Ia supernova explosions on helium companions in the Chandrasekhar-mass explosion scenario
In the version of the SD scenario of SNe Ia studied here, a CO WD explodes
close to the Chandrasekhar limit after accreting material from a non-degenerate
He companion. In the present study, we employ the Stellar GADGET code to
perform 3D hydrodynamical simulations of the interaction of the SN Ia ejecta
with the He companion taking into account its orbital motion and spin. It is
found that only 2%--5% of the initial companion mass are stripped off from the
outer layers of He companions due to the SN impact. The dependence of the
unbound mass (or the kick velocity) on the orbital separation can be fitted in
good approximation by a power law for a given companion model. After the SN
impact, the outer layers of a He donor star are significantly enriched with
heavy elements from the low-expansion-velocity tail of SN Ia ejecta. The total
mass of accumulated SN-ejecta material on the companion surface reaches about >
10e-3 M_sun for different companion models. This enrichment with heavy elements
provides a potential way to observationally identify the surviving companion
star in SN remnants. Finally, by artificially adjusting the explosion energy of
the W7 explosion model, we find that the total accumulation of SN ejecta on the
companion surface is also dependent on the explosion energy with a power law
relation in good approximation.Comment: 20 figures, 2 tables, accepted for publication in Ap
Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T
The gravitationally confined detonation (GCD) model has been proposed as a
possible explosion mechanism for Type Ia supernovae in the single-degenerate
evolution channel. Driven by buoyancy, a deflagration flame rises in a narrow
cone towards the surface. For the most part, the flow of the expanding ashes
remains radial, but upon reaching the outer, low-pressure layers of the white
dwarf, an additional lateral component develops. This makes the deflagration
ashes converge again at the opposite side, where the compression heats fuel and
a detonation may be launched. To test the GCD explosion model, we perform a 3D
simulation for a model with an ignition spot offset near the upper limit of
what is still justifiable, 200 km. This simulation meets our deliberately
optimistic detonation criteria and we initiate a detonation. The detonation
burns through the white dwarf and leads to its complete disruption. We
determine nucleosynthetic yields by post-processing 10^6 tracer particles with
a 384 nuclide reaction network and we present multi-band light curves and
time-dependent optical spectra. We find that our synthetic observables show a
prominent viewing-angle sensitivity in UV and blue bands, which is in tension
with observed SNe Ia. The strong dependence on viewing-angle is caused by the
asymmetric distribution of the deflagration ashes in the outer ejecta layers.
Finally, we perform a comparison of our model to SN 1991T. The overall
flux-level of the model is slightly too low and the model predicts pre-maximum
light spectral features due to Ca, S, and Si that are too strong. Furthermore,
the model chemical abundance stratification qualitatively disagrees with recent
abundance tomography results in two key areas: our model lacks low velocity
stable Fe and instead has copious amounts of high-velocity 56Ni and stable Fe.
We therefore do not find good agreement of the model with SN 1991T.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic
ASASSN-14lp: two possible solutions for the observed ultraviolet suppression
We test the adequacy of ultraviolet (UV) spectra for characterizing the outer structure of Type Ia supernova (SN) ejecta. For this purpose, we perform spectroscopic analysis for ASASSN-14lp, a normal SN Ia showing low continuum in the mid-UV regime. To explain the strong UV suppression, two possible origins have been investigated by mapping the chemical profiles over a significant part of their ejecta. We fit the spectral time series with mid-UV coverage obtained before and around maximum light by HST, supplemented with ground-based optical observations for the earliest epochs. The synthetic spectra are calculated with the one-dimensional MC radiative transfer code TARDIS from self-consistent ejecta models. Among several physical parameters, we constrain the abundance profiles of nine chemical elements. We find that a distribution of 56Ni (and other iron-group elements) that extends towards the highest velocities reproduces the observed UV flux well. The presence of radioactive material in the outer layers of the ejecta, if confirmed, implies strong constraints on the possible explosion scenarios. We investigate the impact of the inferred 56Ni distribution on the early light curves with the radiative transfer code TURTLS, and confront the results with the observed light curves of ASASSN-14lp. The inferred abundances are not in conflict with the observed photometry. We also test whether the UV suppression can be reproduced if the radiation at the photosphere is significantly lower in the UV regime than the pure Planck function. In this case, solar metallicity might be sufficient enough at the highest velocities to reproduce the UV suppression
Helium as a signature of the double detonation in Type Ia supernovae
The double detonation is a widely discussed mechanism to explain Type Ia
supernovae from explosions of sub-Chandrasekhar mass white dwarfs. In this
scenario, a helium detonation is ignited in a surface helium shell on a
carbon/oxygen white dwarf, which leads to a secondary carbon detonation.
Explosion simulations predict high abundances of unburnt helium in the ejecta,
however, radiative transfer simulations have not been able to fully address
whether helium spectral features would form. This is because helium can not be
sufficiently excited to form spectral features by thermal processes, but can be
excited by collisions with non-thermal electrons, which most studies have
neglected. We carry out a full non-local thermodynamic equilibrium (non-LTE)
radiative transfer simulation for an instance of a double detonation explosion
model, and include a non-thermal treatment of fast electrons. We find a clear
He I {\lambda} 10830 feature which is strongest in the first few days after
explosion and becomes weaker with time. Initially this feature is blended with
the Mg II {\lambda} 10927 feature but over time separates to form a secondary
feature to the blue wing of the Mg II {\lambda} 10927 feature. We compare our
simulation to observations of iPTF13ebh, which showed a similar feature to the
blue wing of the Mg II {\lambda} 10927 feature, previously identified as C I.
Our simulation shows a good match to the evolution of this feature and we
identify it as high velocity He I {\lambda} 10830. This suggests that He I
{\lambda} 10830 could be a signature of the double detonation scenario.Comment: 7 pages, accepted by MNRA
Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with M~0.9 M_sun
Type Ia supernovae (SNe Ia) are thought to result from thermonuclear
explosions of carbon-oxygen white dwarf stars. Existing models generally
explain the observed properties, with the exception of the sub-luminous
1991-bg-like supernovae. It has long been suspected that the merger of two
white dwarfs could give rise to a type Ia event, but hitherto simulations have
failed to produce an explosion. Here we report a simulation of the merger of
two equal-mass white dwarfs that leads to an underluminous explosion, though at
the expense of requiring a single common-envelope phase, and component masses
of ~0.9 M_sun. The light curve is too broad, but the synthesized spectra, red
colour and low expansion velocities are all close to what is observed for
sub-luminous 1991bg-like events. While mass ratios can be slightly less than
one and still produce an underluminous event, the masses have to be in the
range 0.83-0.9 M_sun.Comment: Accepted to Natur