One of the most important questions regarding the progenitor systems of Type Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to explosions that reproduce observations of normal ...events. Here we present a fully three-dimensional simulation of a violent merger of two carbon-oxygen white dwarfs with masses of 0.9 M sub(middot in circle) and 1.1 M sub(middot in circle) combining very high resolution and exact initial conditions. A well-tested combination of codes is used to study the system. We start with the dynamical inspiral phase and follow the subsequent thermonuclear explosion under the plausible assumption that a detonation forms in the process of merging. We then perform detailed nucleosynthesis calculations and radiative transfer simulations to predict synthetic observables from the homologously expanding supernova ejecta. We find that synthetic color light curves of our merger, which produces about 0.62 M sub(middot in circle) of super(56)Ni, show good agreement with those observed for normal SNe Ia in all wave bands from U to K. Line velocities in synthetic spectra around maximum light also agree well with observations. We conclude that violent mergers of massive white dwarfs can closely resemble normal SNe Ia. Therefore, depending on the number of such massive systems available these mergers may contribute at least a small fraction to the observed population of normal SNe Ia.
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.
Context. The origin of subluminous Type Ia supernovae (SNe Ia) has long eluded any explanation, because all Chandrasekhar-mass models have severe problems reproducing them. Recently, it has been ...proposed that violent mergers of two white dwarfs of 0.9 M⊙ could lead to subluminous SNe Ia events that resemble 1991bg-like SNe Ia. Aims. Here we investigate whether this scenario still works for mergers of two white dwarfs with a mass ratio below one. We aim to determine the range of mass ratios for which a detonation still forms during the merger, as only those events will lead to an SN Ia. This range is an important ingredient for population synthesis and one decisive point for judging the viability of the scenario. In addition, we perform a resolution study of one of the models. Finally we discuss the connection between violent white dwarf mergers with a primary mass of 0.9 M⊙ and 1991bg-like SNe Ia. Methods. The latest version of the smoothed particle hydrodynamics code Gadget3 was used to evolve binary systems with different mass ratios until they merge. We analyzed the result and looked for hot spots in which detonations can form. Results. We show that mergers of two white dwarfs with a primary white dwarf mass of ≈ 0.9 M⊙ and a mass ratio more than about 0.8 robustly reach the conditions we require for igniting a detonation and thus produce thermonuclear explosions during the merger itself. We also find that, while our simulations do not yet completely resolve the hot spots, increasing the resolution leads to conditions that are even more likely to ignite detonations. Additionally, we compare the abundance structure of the ejecta of the thermonuclear explosion of two merged white dwarfs with data inferred from observations of a 1991bg-like SN Ia (SN 2005bl). The abundance distributions of intermediate mass and iron group elements in velocity space agree qualitatively, and our model reproduces the lack of material at high velocities inferred from the observations. Conclusions. The violent merger scenario constitutes a robust possibility for two merging white dwarfs to produce a thermonuclear explosion. Mergers with a primary white dwarf mass of ≈ 0.9 M⊙ are very promising candidates for explaining subluminous SNe Ia. This would imply that subluminous SNe Ia form a distinct class of objects, which are not produced in the standard single white dwarf scenario for SNe Ia, but instead arise from a different progenitor channel and explosion mechanism.
Stellar evolution models predict the existence of hybrid white dwarfs (WDs) with a carbon–oxygen core surrounded by an oxygen–neon mantle. Being born with masses ∼1.1 M⊙, hybrid WDs in a binary ...system may easily approach the Chandrasekhar mass (M
Ch) by accretion and give rise to a thermonuclear explosion. Here, we investigate an off-centre deflagration in a near-M
Ch hybrid WD under the assumption that nuclear burning only occurs in carbon-rich material. Performing hydrodynamics simulations of the explosion and detailed nucleosynthesis post-processing calculations, we find that only 0.014 M⊙ of material is ejected while the remainder of the mass stays bound. The ejecta consist predominantly of iron-group elements, O, C, Si and S. We also calculate synthetic observables for our model and find reasonable agreement with the faint Type Iax SN 2008ha. This shows for the first time that deflagrations in near-M
Ch WDs can in principle explain the observed diversity of Type Iax supernovae. Leaving behind a near-M
Ch bound remnant opens the possibility for recurrent explosions or a subsequent accretion-induced collapse in faint Type Iax SNe, if further accretion episodes occur. From binary population synthesis calculations, we find the rate of hybrid WDs approaching M
Ch to be of the order of 1 per cent of the Galactic
SN Ia rate.
H and He features in photospheric spectra have seldom been used to infer quantitatively the properties of Type IIb, Ib and Ic supernovae (SNe IIb, Ib and Ic) and their progenitor stars. Most ...radiative transfer models ignored non-local thermodynamic equilibrium (NLTE) effects, which are extremely strong especially in the He-dominated zones. In this paper, a comprehensive set of model atmospheres for low-mass SNe IIb/Ib/Ic is presented. Long-standing questions, such as how much He can be contained in SNe Ic, where He lines are not seen, can thus be addressed. The state of H and He is computed in full NLTE, including the effect of heating by fast electrons. The models are constructed to represent iso-energetic explosions of the same stellar core with differently massive H/He envelopes on top. The synthetic spectra suggest that 0.06-0.14 M⊙ of He and even smaller amounts of H suffice for optical lines to be present, unless ejecta asymmetries play a major role. This strongly supports the conjecture that low-mass SNe Ic originate from binaries where progenitor mass loss can be extremely efficient.
Abstract
A non-local-thermodynamic-equilibrium (NLTE) level population model of the first and second ionisation stages of iron, nickel and cobalt is used to fit a sample of XShooter optical + ...near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia). From the ratio of the NIR lines to the optical lines limits can be placed on the temperature and density of the emission region. We find a similar evolution of these parameters across our sample. Using the evolution of the Fe ii 12 570 Å to 7 155 Å line as a prior in fits of spectra covering only the optical wavelengths we show that the 7200 Å feature is fully explained by Fe ii and Ni ii alone. This approach allows us to determine the abundance of Ni ii/Fe ii for a large sample of 130 optical spectra of 58 SNe Ia with uncertainties small enough to distinguish between Chandrasekhar mass (MCh) and sub-Chandrasekhar mass (sub-MCh) explosion models. We conclude that the majority (85%) of normal SNe Ia have a Ni/Fe abundance that is in agreement with predictions of sub-MCh explosion simulations of ∼Z⊙ progenitors. Only a small fraction (11%) of objects in the sample have a Ni/Fe abundance in agreement with MCh explosion models.
We present results of simulations of stellar collapse and explosions in spherical symmetry for progenitor stars in the 8–$10\,M_\odot$ range with an O-Ne-Mg core. The simulations were continued until ...nearly one second after core bounce and were performed with the Prometheus/Vertex code with a variable Eddington factor solver for the neutrino transport, including a state-of-the-art treatment of neutrino-matter interactions. Particular effort was made to implement nuclear burning and electron capture rates with sufficient accuracy to ensure a smooth continuation, without transients, from the progenitor evolution to core collapse. Using two different nuclear equations of state (EoSs), a soft version of the Lattimer & Swesty EoS and the significantly stiffer Wolff & Hillebrandt EoS, we found no prompt explosions, but instead delayed explosions, powered by neutrino heating and the neutrino-driven baryonic wind which sets in about 200 ms after bounce. The models eject little nickel (${<} 0.015~M_\odot$), explode with an energy of ${\ga}0.1\times 10^{51}\,$erg, and leave behind neutron stars (NSs) with a baryonic mass near $1.36\,M_\odot$. Different from previous models of such explosions, the ejecta during the first second have a proton-to-baryon ratio of $Y_{\rm{e}} \ga 0.46$, which suggests a chemical composition that is not in conflict with galactic abundances. No low-entropy matter with $Y_{\rm{e}} \ll 0.5$ is ejected. This excludes such explosions as sites of a low-entropy r-process. The low explosion energy and nucleosynthetic implications are compatible with the observed properties of the Crab supernova, and the small nickel mass supports the possibility that our models explain some subluminous type II-P supernovae.
Abstract
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.
Context. The colour fluctuations of type Ia supernovae (SNe Ia) include intrinsic and extrinsic components, which both contribute to the observed variability. Previous works proposed a statistical ...separation of these two contributions, but the individual intrinsic colour contributions of each SN Ia were not extracted. In addition, a large uncertainty remains on the value of the parameter RV, which characterises the dust extinction formula. Aims. Leveraging the known parameterisation of the extinction formula for dust in our Galaxy, and applying it to the host galaxy of SNe Ia, we propose a new method of separation –valid for each SN– using the correlations between colour fluctuations. This also allows us to derive a well-constrained value of the extinction parameter RV with different, possibly smaller systematic errors. We also define a three-dimensional space of intrinsic colour fluctuations. Methods. The key ingredients in this attempt at separating the intrinsic and extinction colour components for each SN –and subsequently measuring RV– are the assumption of a linearized dependence of magnitude on the extinction component of colour, a one-dimensional extra-intrinsic colour space (in addition to Ca II H&Kλ3945 and Si IIλ4131 contributions) over four independent colours, and the absence of correlation between the intrinsic and extrinsic variabilities. Results. We show that a consistent solution is found under the previous assumptions, but the observed systematic trends point to a (small) inadequacy of the extinction formula. Once corrected, all systematic extinction effects can be cancelled by choosing a single scaling of the extinction colour component as well as an appropriate value of RV = 2.181 ± 0.117. The observed colours are described within an accuracy of 0.025 mag. The resulting magnitude variability is 0.13 over all UBVRI bandpasses, and this fluctuation is shown to be independent of the bandpass to within 0.02 mag.
We investigate the consequences of fairly normal Type Ia supernovae being embedded in compact and dense envelopes of carbon- and oxygen-rich circumstellar material by means of detailed radiation ...hydrodynamic simulations. Our main focus rests on exploring the effects of the interaction between ejecta and circumstellar material on the ejecta evolution and the broad-band light curve. In our calculations, we find that a strong reverse shock efficiently decelerates and compresses the ejecta material. This leads to a significant broadening of the optical light curve, a longer rise to maximum and a slower decline in the tail phase. During the interaction, substantial radiative energy is generated, which mostly emerges in the extreme ultraviolet and X-ray regime. Only if reprocessing due to radiation-matter interactions is very efficient, a significant boost in the optical light curve is observed. We discuss these findings in particular in the context of the superluminous event SN 2009dc. As our calculations are able to reproduce a number of its peculiar properties, we conclude that the flavour of the interaction scenario investigated in this work constitutes a promising candidate to explain such 'Super-Chandrasekhar' supernovae.
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