Type Ia Supernova Explosions Research Articles

THE CLASSIFICATION OF MAGNETOHYDRODYNAMIC REGIMES OF THERMONUCLEAR COMBUSTION

Physical properties of magnetohydrodynamic (MHD) reaction fronts are studied as functions of the thermodynamic conditions, and the strength and orientation of the magnetic field in the unburned matter through which the fronts propagate. We determine the conditions for the existence of the various types of MHD reaction fronts and the character of the changes in physical quantities across these reaction fronts. The analysis is carried out in general for a perfect gas equation of state and a constant energy release, and then extended to thermonuclear reaction fronts in degenerate carbon-oxygen mixtures and degenerate helium in conditions typical of Type Ia supernova explosions. We find that as unburned matter enters perpendicular to a reaction front, the release of energy through burning generates shear velocity in the reacting gas that, depending on the type of reaction front, strengthens or weakens the magnetic field. In addition, we find that the steady-state propagation of a reaction front is impossible for certain ranges of magnetic field direction. Our results provide insight into the phenomena of MHD thermonuclear combustion that is relevant to the interpretation of future simulations of SN Ia explosions that have magnetic fields systematically incorporated.

General relativistic white dwarfs and their astrophysical implications

We consider applications of general relativistic uniformly-rotating white dwarfs to several astrophysical phenomena related to the spin-up and the spin-down epochs and to delayed type Ia supernova explosions of super-Chandrasekhar white dwarfs, where we estimate the “spinning down” lifetime due to magnetic-dipole braking. In addition, we describe the physical properties of Soft Gamma Repeaters and Anomalous X-Ray Pulsars as massive rapidly-rotating highly-magnetized white dwarfs. Particularly we consider one of the so-called low-magnetic-field magnetars SGR 0418+5729 as a massive rapidly-rotating highly-magnetized white dwarf and give bounds for the mass, radius, moment of inertia, and magnetic field by requiring the general relativistic uniformly-rotating configurations to be stable.

Early X-ray emission from Type Ia supernovae originating from symbiotic progenitors or recurrent novae

One of the key observables for determining the progenitor nature of Type Ia supernovae is provided by their immediate circumstellar medium, which according to several models should be shaped by the progenitor binary system. So far, X-ray and radio observations indicate that the surroundings are very tenuous, producing severe upper-limits on the mass loss from winds of the progenitors. In this study, we perform numerical hydro-dynamical simulations of the interaction of the SN ejecta with circumstellar structures formed by possible mass outflows from the progenitor systems and we estimate numerically the expected numerical X-ray luminosity. We consider two kinds of circumstellar structures: a) A circumstellar medium formed by the donor star's stellar wind, in case of a symbiotic binary progenitor system; b) A circumstellar medium shaped by the interaction of the slow wind of the donor star with consecutive nova outbursts for the case of a symbiotic recurrent nova progenitor system. For the hydro-simulations we used well-known Type Ia supernova explosion models, as well as an approximation based on a power law model for the density structure of the outer ejecta. We confirm the strict upper limits on stellar wind mass loss, provided by simplified interpretations of X-ray upper limits of Type Ia supernovae. However, we show that supernova explosions going off in the cavities created by repeated nova explosions, provide a possible explanation for the lack of X-ray emission from supernovae originating from symbiotic binaries. Moreover, the velocity structure of circumstellar medium, shaped by a series of nova explosion matches well with the Na absorption features seen in absorption toward several Type Ia supernovae.

A 3D numerical model for Kepler's supernova remnant

We present new 3D numerical simulations for Kepler's supernova remnant. In this work we revisit the possibility that the asymmetric shape of the remnant in X-rays is the product of a Type Ia supernova explosion which occurs inside the wind bubble previously created by an AGB companion star. Due to the large peculiar velocity of the system, the interaction of the strong AGB wind with the interstellar medium results in a bow shock structure. In this new model we propose that the AGB wind is anisotropic, with properties such as mass-loss rate and density having a latitude dependence, and that the orientation of the polar axis of the AGB star is not aligned with the direction of motion. The ejecta from the Type Ia supernova explosion is modelled using a power-law density profile, and we let the remnant evolve for 400 yr. We computed synthetic X-ray maps from the numerical results. We find that the estimated size and peculiar X-ray morphology of Kepler's supernova remnant are well reproduced by considering an AGB mass-loss rate of 10−5 M⊙ yr−1, a wind terminal velocity of 10 km s−1, an ambient medium density of 10−3 cm−3 and an explosion energy of 7 × 1050 erg. The obtained total X-ray luminosity of the remnant in this model reaches 6 × 1050 erg, which is within a factor of 2 of the observed value, and the time evolution of the luminosity shows a rate of decrease in recent decades of ∼2.4 per cent yr−1 that is consistent with the observations.

One possible solution of peculiar type Ia supernovae explosions caused by a charged white dwarf

Recent astrophysics observation reveals the existence of some super luminous type Ia supernovae. One natural explanation of such a peculiar phenomenon is to require the progenitor of such a supernova to be a highly super-Chandrasekhar mass white dwarf. Along this line, in this paper, we propose a possible mechanism to explain this phenomenon based on a charged white dwarf. In particular, by choosing suitable new variables and a representative charge distribution, an analytic solution is obtained. The stability issue is also discussed, remarkably, it turns out that the charged white dwarf configuration can be dynamically stable. Moreover, we investigate the general relativistic effects and it is shown that the general relativistic effects can be negligible when the mass of the charged white dwarf is below about $3M_\odot$.

Three-dimensional pure deflagration models with nucleosynthesis and synthetic observables for Type Ia supernovae

We investigate whether pure deflagration models of Chandrasekhar-mass carbon-oxygen white dwarf stars can account for one or more subclass of the observed population of Type Ia supernova (SN Ia) explosions. We compute a set of 3D full-star hydrodynamic explosion models, in which the deflagration strength is parametrized using the multispot ignition approach. For each model, we calculate detailed nucleosynthesis yields in a post-processing step with a 384 nuclide nuclear network. We also compute synthetic observables with our 3D Monte Carlo radiative transfer code for comparison with observations. For weak and intermediate deflagration strengths (energy release E_nuc <~ 1.1 x 10^51 erg), we find that the explosion leaves behind a bound remnant enriched with 3 to 10 per cent (by mass) of deflagration ashes. However, we do not obtain the large kick velocities recently reported in the literature. We find that weak deflagrations with E_nuc ~ 0.5 x 10^51 erg fit well both the light curves and spectra of 2002cx-like SNe Ia, and models with even lower explosion energies could explain some of the fainter members of this subclass. By comparing our synthetic observables with the properties of SNe Ia, we can exclude the brightest, most vigorously ignited models as candidates for any observed class of SN Ia: their B - V colours deviate significantly from both normal and 2002cx-like SNe Ia and they are too bright to be candidates for other subclasses.

A subgrid-scale model for deflagration-to-detonation transitions in Type Ia supernova explosion simulations

A promising model for normal Type Ia supernova (SN Ia) explosions are delayed detonations of Chandrasekhar-mass white dwarfs, in which the burning starts out as a subsonic deflagration and turns at a later phase of the explosion into a supersonic detonation. The mechanism of the underlying deflagration-to-detonation transition (DDT) is unknown in detail, but necessary conditions have been determined recently. The region of detonation initiation cannot be spatially resolved in multi-dimensional full-star simulations of the explosion. We develop a subgrid-scale (SGS) model for DDTs in thermonuclear supernova simulations that is consistent with the currently known constraints. The probability for a DDT to occur is calculated from the distribution of turbulent velocities measured on the grid scale in the vicinity of the flame and the fractal flame surface area that satisfies further physical constraints, such as fuel fraction and fuel density. The implementation of our DDT criterion provides a solid basis for simulations of thermonuclear supernova explosions in the delayed detonation scenario. It accounts for the currently known necessary conditions for the transition and avoids the inclusion of resolution-dependent quantities in the model. The functionality of our DDT criterion is demonstrated on the example of one three-dimensional thermonuclear supernova explosion simulation.

Solar abundance of manganese: a case for near Chandrasekhar-mass Type Ia supernova progenitors

Context: Manganese is predominantly synthesised in Type Ia supernova (SN Ia) explosions. Owing to the entropy dependence of the Mn yield in explosive thermonuclear burning, SNe Ia involving near Chandrasekhar-mass white dwarfs (WDs) are predicted to produce Mn to Fe ratios significantly exceeding those of SN Ia explosions involving sub-Chandrasekhar mass primary WDs. Of all current supernova explosion models, only SN Ia models involving near-Chandrasekhar mass WDs produce [Mn/Fe] > 0.0. Aims: Using the specific yields for competing SN Ia scenarios, we aim to constrain the relative fractions of exploding near-Chandrasekhar mass to sub-Chandrasekhar mass primary WDs in the Galaxy. Methods: We extract the Mn yields from three-dimensional thermonuclear supernova simulations referring to different initial setups and progenitor channels. We then compute the chemical evolution of Mn in the Solar neighborhood, assuming SNe Ia are made up of different relative fractions of the considered explosion models. Results: We find that due to the entropy dependence of freeze-out yields from nuclear statistical equilibrium, [Mn/Fe] strongly depends on the mass of the exploding WD, with near-Chandraskher mass WDs producing substantially higher [Mn/Fe] than sub-Chandrasekhar mass WDs. Of all nucleosynthetic sources potentially influencing the chemical evolution of Mn, only explosion models involving the thermonuclear incineration of near-Chandrasekhar mass WDs predict solar or super-solar [Mn/Fe]. Consequently, we find in our chemical evolution calculations that the observed [Mn/Fe] in the Solar neighborhood at [Fe/H] > 0.0 cannot be reproduced without near-Chandrasekhar mass SN Ia primaries. Assuming that 50 per cent of all SNe Ia stem from explosive thermonuclear burning in near-Chandrasekhar mass WDs results in a good match to data.

Type Ia supernovae inside planetary nebulae: shaping by jets

Using 3D numerical hydrodynamical simulations we show that jets launched prior to type Ia supernova (SN Ia) explosion in the core-degenerate scenario can account for the appearance of two opposite lobes ('Ears') along the symmetry axis of the SN remnant (SNR). In the double-degenerate and core-degenerate scenarios the merger of the two degenerate compact objects is very likely to lead to the formation of an accretion disk, that might launch two opposite jets. In the CD scenario these jets interact with the envelope ejected during the preceding common envelope phase. If explosion occurs shortly after the merger process, the exploding gas and the jets will collide with the ejected nebula, leading to SNR with axisymmetric components including 'Ears'. We also explore the possibility that the jets are launched by the companion white dwarf prior to its merger with the core. This last process is similar to the one where jets are launched in some pre-planetary nebulae. The SNR 'Ears' in this case are formed by a spherical SN Ia explosion inside an elliptical planetary nebula-like object. We compare our numerical results with two SNRs - Kepler and G299.2-2.9.

COMPARING THE LIGHT CURVES OF SIMULATED TYPE Ia SUPERNOVAE WITH OBSERVATIONS USING DATA-DRIVEN MODELS

We propose a robust, quantitative method to compare the synthetic light curves of a Type Ia Supernova (SNIa) explosion model with a large set of observed SNeIa, and derive a figure of merit for the explosion model's agreement with observations. The synthetic light curves are fit with the data-driven model SALT2 which returns values for stretch, color, and magnitude at peak brightness, as well as a goodness-of-fit parameter. Each fit is performed multiple times with different choices of filter bands and epoch range in order to quantify the systematic uncertainty on the fitted parameters. We use a parametric population model for the distribution of observed SNIa parameters from large surveys, and extend it to represent red, dim, and bright outliers found in a low-redshift SNIa data set. We discuss the potential uncertainties of this population model and find it to be reliable given the current uncertainties on cosmological parameters. Using our population model, we assign each set of fitted parameters a likelihood of being observed in nature, and a figure of merit based on this likelihood. We define a second figure of merit based on the quality of the light curve fit, and combine the two measures into an overall figure of merit for each explosion model. We compute figures of merit for a variety of 1D, 2D and 3D explosion models and show that our evaluation method allows meaningful inferences across a wide range of light curve quality and fitted parameters.

Chemical evolution of Local Group dwarf galaxies in a cosmological context – I. A new modelling approach and its application to the Sculptor dwarf spheroidal galaxy

We present a new approach for chemical evolution modelling, specifically designed to investigate the chemical properties of dwarf galaxies in a full cosmological framework. In particular, we focus on the Sculptor dwarf spheroidal galaxy as a test bed for our model. We select four candidate Sculptor-like galaxies from the satellite galaxy catalogue generated by implementation of a version of the Munich semi-analytic model for galaxy formation on the level 2 Aquarius dark matter simulations. We follow explicitly the evolution of several chemical elements, both in the cold gas out of which the stars form and in the hot medium residing in the halo. We take into account in detail the lifetimes of stars of different initial masses, the distribution of the delay times for type Ia supernova explosions and the dependency of the stellar yields from the initial metallicity of the stars. We allow large fractions of metals to be deposited into the hot phase, either directly as stars die or through reheated gas flows powered by supernova explosions. In order to reproduce both the observed metallicity distribution function and the observed abundance ratios of long-lived stars of Sculptor, large fractions of the reheated metals must never re-enter regions of active star formation. Our analysis sets further constraints on the semi-analytical models and, at large, on possible metal enrichment scenarios for the Sculptor dwarf spheroidal galaxy.

GRAVITATIONALLY UNSTABLE FLAMES: RAYLEIGH-TAYLOR STRETCHING VERSUS TURBULENT WRINKLING

In this paper, we provide support for the Rayleigh-Taylor-(RT)-based subgrid model used in full-star simulations of deflagrations in Type Ia supernovae explosions. We use the results of a parameter study of two-dimensional direct numerical simulations of an RT unstable model flame to distinguish between the two main types of subgrid models (RT or turbulence dominated) in the flamelet regime. First, we give scalings for the turbulent flame speed, the Reynolds number, the viscous scale, and the size of the burning region as the non-dimensional gravity (G) is varied. The flame speed is well predicted by an RT-based flame speed model. Next, the above scalings are used to calculate the Karlovitz number (Ka) and to discuss appropriate combustion regimes. No transition to thin reaction zones is seen at Ka = 1, although such a transition is expected by turbulence-dominated subgrid models. Finally, we confirm a basic physical premise of the RT subgrid model, namely, that the flame is fractal, and thus self-similar. By modeling the turbulent flame speed, we demonstrate that it is affected more by large-scale RT stretching than by small-scale turbulent wrinkling. In this way, the RT instability controls the flame directly from the large scales. Overall, these results support the RT subgrid model.

Rotation of surviving companion stars after type Ia supernova explosions in the WD+MS scenario

In the SD scenario of SNe Ia the companion survives the SN explosion and thus should be visible near the center of the SN remnant and may show some unusual features. A promising approach to test progenitor models of SNe Ia is to search for the companion in SNRs. Here we present the results of 3D hydrodynamics simulations of the interaction between the SN Ia blast wave and a MS companion taking into consideration its orbital motion and spin. The primary goal of this work is to investigate the rotation of surviving companions after SN Ia explosions in the WD+MS scenario. We use Eggleton's code including the optically thick accretion wind model to obtain realistic models of companions. The impact of the SN blast wave on these companions is followed in 3D hydrodynamic simulations employing the SPH code GADGET3. We find that the rotation of the companion does not significantly affect the amount of stripped mass and the kick velocity caused by the SN impact. However, in our simulations, the rotational velocity of the companion is significantly reduced to about 14% to 32% of its pre-explosion value due to the expansion of the companion and the fact that 55%-89% of the initial angular momentum is carried away by the stripped matter. Compared with the observed rotational velocity of the presumed companion star of Tycho's SN, Tycho G, of 6 km/s the final rotational velocity we obtain is still higher by at least a factor of two. Whether this difference is significant, and may cast doubts on the suggestion that Tycho G is the companion of SN 1572, has to be investigated in future studies. Based on binary population synthesis results we present, for the first time, the expected distribution of rotational velocities of companions after the explosion which may provide useful information for the identification of the surviving companion in observational searches in other historical SNRs.

A SUPER-SOLAR METALLICITY FOR THE PROGENITOR OF KEPLER'S SUPERNOVA

We have performed deep X-ray observations of the remnant of Kepler's supernova (SN 1604) as a Key Project of the Suzaku Observatory. Our main goal is to detect secondary Fe-peak elements in the SN ejecta to gain insights into the Type Ia supernova explosion mechanism and the nature of the progenitor. Here we report our initial results. We made a conclusive detection of X-ray emission lines from highly ionized Mn, Cr, and Ni as well as Fe. The observed Mn-to-Cr line flux ratio is ~0.60, ~30% larger than that measured in Tycho's remnant. We estimate a Mn-to-Cr mass ratio of ~0.77, which is strongly suggestive of a large neutron excess in the progenitor star before the onset of the thermonuclear runaway. The observed Ni-to-Fe line flux ratio (~0.03) corresponds to a mass ratio of ~0.06, which is generally consistent with the products of explosive Si-burning regime in Type Ia explosion models, and rules out contamination from the products of neutron-rich nuclear statistical equilibrium in the shocked ejecta. Together with the previously suggested luminous nature of the explosion, these mass ratios provide strong evidence for a super-solar metallicity in the SN progenitor (~3 Z_sun). Kepler's supernova was likely the thermonuclear explosion of a white dwarf formed in the recent past that must have exploded through a relatively prompt channel.

Insights into thermonuclear supernovae from the incomplete Si-burning process

Type Ia supernova (SNIa) explosions synthesize a few tenths to several tenths of a solar mass, whose composition is the result of incomplete silicon burning that reaches peak temperatures of 4 GK to 5 GK. The elemental abundances are sensitive to the physical conditions in the explosion, making their measurement a promising clue to uncovering the properties of the progenitor star and of the explosion itself. Using a parameterized description of the thermodynamic history of matter undergoing incomplete silicon burning, we computed the final composition for a range of parameters wide enough to encompass current models of SNIa. Then, we searched for combinations of elemental abundances that trace the parameters values and are potentially measurable. For this purpose, we divide the present study into two epochs of SNIa, namely the optical epoch, from a few weeks to several months after the explosion, and the X-ray epoch, which refers to the time period in which the supernova remnant is young, starting one or two hundred years age and ending a thousand years after the event. During the optical epoch, the only SNIa property that can be extracted from the detection of incomplete silicon burning elements is the neutron excess of the progenitor white dwarf at thermal runaway, which can be determined through measuring the ratio of the abundance of manganese to that of titanium, chromium, or vanadium. Conversely, in the X-ray epoch, any abundance ratio built using a couple of elements from titanium, vanadium, chromium, or manganese may constrain the initial neutron excess. Furthermore, measuring the ratio of the abundances of vanadium to manganese in the X-ray might shed light on the timescale of the thermonuclear explosion.

SELF-SHIELDING OF SOFT X-RAYS IN TYPE Ia SUPERNOVA PROGENITORS

There are insufficient super soft (~ 0.1 keV) X-ray sources in either spiral or elliptical galaxies to account for the rate of explosion of Type Ia supernovae in either the single degenerate or the double degenerate scenarios. We quantify the amount of circumstellar matter that would be required to suppress the soft X-ray flux by yielding a column density in excess of 10^{23} cm^{-2}. We summarize evidence that appropriate quantities of matter are extant in SN Ia and in recurrent novae that may be supernova precursors. The obscuring matter is likely to have a large, but not complete, covering factor and to be substantially non-spherically symmetric. Assuming that much of the absorbed X-ray flux is re-radiated as black-body radiation in the UV, we estimate that fewer than 100 sources might be detectable in the GALEX all sky survey.

Three-dimensional delayed-detonation models with nucleosynthesis for Type Ia supernovae

We present results for a suite of fourteen three-dimensional, high resolution hydrodynamical simulations of delayed-detonation modelsof Type Ia supernova (SN Ia) explosions. This model suite comprises the first set of three-dimensional SN Ia simulations with detailed isotopic yield information. As such, it may serve as a database for Chandrasekhar-mass delayed-detonation model nucleosynthetic yields and for deriving synthetic observables such as spectra and light curves. We employ a physically motivated, stochastic model based on turbulent velocity fluctuations and fuel density to calculate in situ the deflagration to detonation transition (DDT) probabilities. To obtain different strengths of the deflagration phase and thereby different degrees of pre-expansion, we have chosen a sequence of initial models with 1, 3, 5, 10, 20, 40, 100, 150, 200, 300, and 1600 (two different realizations) ignition kernels in a hydrostatic white dwarf with central density of 2.9 x 10^9 gcc, plus in addition one high central density (5.5 x 10^9 gcc), and one low central density (1.0 x 10^9 gcc) rendition of the 100 ignition kernel configuration. For each simulation we determined detailed nucleosynthetic yields by post-processing 10^6 tracer particles with a 384 nuclide reaction network. All delayed detonation models result in explosions unbinding the white dwarf, producing a range of 56Ni masses from 0.32 to 1.11 solar masses. As a general trend, the models predict that the stable neutron-rich iron group isotopes are not found at the lowest velocities, but rather at intermediate velocities (~3,000 - 10,000 km/s) in a shell surrounding a 56Ni-rich core. The models further predict relatively low velocity oxygen and carbon, with typical minimum velocities around 4,000 and 10,000 km/s, respectively.

DIVERSITY OF TYPE Ia SUPERNOVAE IMPRINTED IN CHEMICAL ABUNDANCES

A time delay of Type Ia supernova (SN Ia) explosions hinders the imprint of their nucleosynthesis on stellar abundances. However, some occasional cases give birth to stars that avoid enrichment of their chemical compositions by massive stars and thereby exhibit a SN Ia-like elemental feature including a very low [Mg/Fe] (~-1). We highlight the elemental feature of Fe-group elements for two low-Mg/Fe objects detected in nearby galaxies, and propose the presence of a class of SNe Ia that yield the low abundance ratios of [Cr,Mn,Ni/Fe]. Our novel models of chemical evolution reveal that our proposed class of SNe Ia (slow SNe Ia) is associated with ones exploding on a long timescale after their stellar birth, and gives a significant impact on the chemical enrichment in the Large Magellanic Cloud (LMC). In the Galaxy, on the other hand, this effect is unseen due to the overwhelming enrichment by the major class of SNe Ia that explode promptly (prompt SNe Ia) and eject a large amount of Fe-group elements. This nicely explains the different [Cr,Mn,Ni/Fe] features between the two galaxies as well as the puzzling feature seen in the LMC stars exhibiting very low Ca but normal Mg abundances. Furthermore, the corresponding channel of slow SN Ia is exemplified by performing detailed nucleosynthesis calculations in the scheme of SNe Ia resulting from a 0.8+0.6 solar mass white dwarf merger.

Nuclear Physics Constraints on the Characteristics of Astrophysical Thermonuclear Flashes

We review the nuclear physics that is associated with the outbursts of Type Ia (thermonuclear) supernova explosions and with the thermonuclear runaway events that define the outbursts of both classical novae and recurrent novae. We describe how distinguishing characteristics of these two classes of astrophysical explosion are strongly dependent both upon fuel ignition in degenerate matter and upon the rates of critical charged-particle reaction rates and weak interaction rates. In this centennial celebration of the important contributions of Rutherford and his collaborators to our understanding of the structure of the nucleus of an atom, it is quite interesting to note the evolution of the α-particle scattering experiments described in Rutherford's seminal paper (Rutherford 1911) to current studies of α-particle induced reactions and their defining roles in studies of stellar, nova, and supernova nucleosynthesis. We identify and discuss for example: (1) the manner in which (α, p) reactions in proximity to the Z = N line carry the major flows from 12C and 16O to 56Ni in Type Ia supernovae; and (2) the critical role of the 15O(α, γ)19Ne reaction in possibly effecting "breakout" of the Hot CNO cycles at the highest temperatures achievable in Classical Novae. In this contribution, we first review the current status our understanding of Type Ia supernova events and then that of Classical Novae.

Monte Carlo radiation hydrodynamics: methods, tests and application to Type Ia supernova ejecta

In astrophysical systems, radiation-matter interactions are important in transferring energy and momentum between the radiation field and the surrounding material. This coupling often makes it necessary to consider the role of radiation when modelling the dynamics of astrophysical fluids. During the last few years, there have been rapid developments in the use of Monte Carlo methods for numerical radiative transfer simulations. Here, we present an approach to radiation hydrodynamics that is based on coupling Monte Carlo radiative transfer techniques with finite-volume hydrodynamical methods in an operator-split manner. In particular, we adopt an indivisible packet formalism to discretize the radiation field into an ensemble of Monte Carlo packets and employ volume-based estimators to reconstruct the radiation field characteristics. In this paper the numerical tools of this method are presented and their accuracy is verified in a series of test calculations. Finally, as a practical example, we use our approach to study the influence of the radiation-matter coupling on the homologous expansion phase and the bolometric light curve of Type Ia supernova explosions.