Aspherical Explosions Research Articles

The aspherical explosions of the 03fg-like Type Ia supernovae 2021zny and 2022ilv revealed by polarimetry

Context. A peculiar subtype of Type Ia supernovae (SNe), 03fg-like (super-Chandrasekhar) SNe, show different observational properties from prototypical Type Ia SNe: they typically have high luminosities at the light-curve peak, low expansion velocities, and strong carbon features. The origin of this class of Type Ia SNe has been actively debated. Recent nebular-phase infrared observations of the 03fg-like Type Ia SN 2022pul using the James Webb Space Telescope revealed large-scale asymmetries in the ejecta and the presence of the strong [Ne II] line at 12.81 μm, suggesting a violent merger of two white dwarfs as its origin. Aims. Polarimetry is another powerful tool for studying the overall ejecta asymmetries of spatially unresolved SNe. Here, we aim to check the universality of the violent merger scenario as the origin of 03fg-like Type Ia SNe, by studying their explosion geometries using polarimetry. Methods. In this Letter we present imaging-polarimetric observations of the two 03fg-like Type Ia SNe 2021zny and 2022ilv. Results. SNe 2021zny and 2022ilv show high intrinsic polarization (∼1%–∼2%), which might be composed of multiple components with different polarization angles. This indicates that they have complex aspherical structures in their ejecta, supporting the violent merger scenario for their origin. Our observations provide the first clear evidence from polarimetry for such aspherical structures.

Evidence for bipolar explosions in Type IIP supernovae

Aims. Recent observations of core-collapse supernovae (SNe) suggest aspherical explosions. Globally, aspherical structures in SN explosions are thought to encode information regarding the underlying explosion mechanism. However, the exact explosion geometries from the inner cores to the outer envelopes are poorly understood. Methods. Here, we present photometric, spectroscopic, and polarimetric observations of the Type IIP SN 2021yja and discuss its explosion geometry in comparison to those of other Type IIP SNe that show large-scale aspherical structures in their hydrogen envelopes (SNe 2012aw, 2013ej and 2017gmr). Results. During the plateau phase, SNe 2012aw and 2021yja exhibit high continuum polarization characterized by two components with perpendicular polarization angles. This behavior can be interpreted as being due to a bipolar explosion, where the SN ejecta is composed of a polar (energetic) component and an equatorial (bulk) component. In such a bipolar explosion, an aspherical axis created by the polar ejecta would dominate at early phases, while the perpendicular axis along the equatorial ejecta would emerge at late phases after the photosphere in the polar ejecta has receded. Our interpretation of the explosions in SNe 2012aw and 2021yja as bipolar is also supported by other observational properties, including the time evolution of the line velocities and the line shapes in the nebular spectra. The polarization of other Type IIP SNe that show large-scale aspherical structures in the hydrogen envelope (SNe 2013ej and 2017gmr) is also consistent with the bipolar-explosion scenario, although this is not conclusive.

Observability of flashes from ejecta crashes in aspherical supernovae, with application to SN 2008D

ABSTRACT A new class of transient, which has been hypothesized to accompany the explosion of an aspherical compact supernova, would arise when streams of ejecta collide outside the star. However, conditions that favour the prompt release of radiation from the collision, such as a diffuse stellar envelope, disfavour the creation of non-radial ejecta in the first place. To determine whether the collision can both occur and be visible, we simulate aspherical explosions using the HUJI-RICH moving-mesh hydrodynamics code and analyse them in terms of diffusion measures defined for individual fluid elements. While our simulations are highly idealized, they connect to realistic explosions via a single dimensionless parameter. Defining two measures of the importance of diffusivity (two versions of the inverse Péclet number), we find that one varies in a way that indicates colliding ejecta can release a photon flash, while the other does not. Examining the X-ray transient XT 080109 associated with supernova SN 2008D, we find that its fluence and duration are consistent with the properties of an ejecta collision in the aspherical model that is most likely to emit a flash. Our results give tentative evidence for the possibility of collision-induced flashes for a narrow and radius-dependent range of asphericity, and motivate future radiation hydrodynamics simulations.

Evidence for multiple origins of fast declining Type II supernovae from spectropolarimetry of SN 2013ej and SN 2017ahn

ABSTRACT The origin of the diverse light-curve shapes of Type II supernovae (SNe), and whether they come from similar or distinct progenitors, has been actively discussed for decades. Here, we report spectropolarimetry of two fast declining Type II (Type IIL) SNe: SN 2013ej and SN 2017ahn. SN 2013ej exhibited high continuum polarization from very soon after the explosion to the radioactive tail phase with time-variable polarization angles. The origin of this polarimetric behaviour can be interpreted as the combination of two different aspherical structures, namely an aspherical interaction of the SN ejecta with circumstellar matter (CSM) and an inherently aspherical explosion. Aspherical explosions are a common feature of slowly declining Type II (Type IIP) SNe. By contrast, SN 2017ahn showed low polarization not only in the photospheric phase but also in the radioactive tail phase. This low polarization in the tail phase, which has never before been observed in other Type IIP/L SNe, suggests that the explosion of SN 2017ahn was nearly spherical. These observations imply that Type IIL SNe have, at least, two different origins: they result from stars that have different explosion properties and/or different mass-loss processes. This fact might indicate that 13ej-like Type IIL SNe originate from a similar progenitor to those of Type IIP SNe accompanied by an aspherical CSM interaction, while 17ahn-like Type IIL SNe come from a more massive progenitor with less hydrogen in its envelope.

Explosion energies for core-collapse supernovae I: analytic, spherically symmetric solutions

ABSTRACT Recent multidimensional simulations of core-collapse supernovae are producing successful explosions and explosion-energy predictions. In general, the explosion-energy evolution is monotonic and relatively smooth, suggesting a possible analytic solution. We derive analytic solutions for the expansion of the gain region under the following assumptions: spherical symmetry, one-zone shell, and powered by neutrinos and α particle recombination. We consider two hypotheses: (I) explosion energy is powered by neutrinos and α recombination and (II) explosion energy is powered by neutrinos alone. Under these assumptions, we derive the fundamental dimensionless parameters and analytic scalings. For the neutrino-only hypothesis (II), the asymptotic explosion energy scales as $E_{\infty } \approx 1.5 M_\mathrm{ g}\nu _0^2 \eta ^{2/3}$, where Mg is the gain mass, $\nu _0$ is the free-fall velocity at the shock, and η is a ratio of the heating and dynamical time-scales. Including both neutrinos and recombination (hypothesis I), the asymptotic explosion energy is $E_{\infty } \approx M_g \nu _0^2 (1.5\eta ^{2/3} + \beta f(\rho _0))$, where β is the dimensionless recombination parameter. We use Bayesian inference to fit these analytic models to simulations. Both hypotheses fit the simulations of the lowest progenitor masses that tend to explode spherically. The fits do not prefer hypothesis I or II; however, prior investigations suggest that α recombination is important. As expected, neither hypothesis fits the higher mass simulations that exhibit aspherical explosions. In summary, this explosion energy theory is consistent with the spherical explosions of low progenitor masses; the inconsistency with higher progenitor-mass simulations suggests that a theory for them must include aspherical dynamics.

Monte Carlo simulations of fast Newtonian and mildly relativistic shock breakout from a stellar wind

ABSTRACT Strong explosion of a compact star surrounded by a thick stellar wind drives a fast (>0.1c) radiation mediated shock (RMS) that propagates in the wind, and ultimately breaks out gradually once photons start escaping from the shock transition layer. In exceptionally strong or aspherical explosions, the shock velocity may even be relativistic. The properties of the breakout signal depend on the dynamics and structure of the shock during the breakout phase. Here we present, for the first time, spectra and light curves of the breakout emission of fast Newtonian and mildly relativistic shocks, that were calculated using self-consistent Monte Carlo simulations of finite RMS with radiative losses. We find a strong dependence of the νFν peak on shock velocity, ranging from ∼1 keV for vs/c = 0.1 to ∼100 keV for vs/c = 0.5, with a shift to lower energies as losses increase. For all cases studied the spectrum below the peak exhibits a nearly flat component (Fν ∼ ν0) that extends down to the break frequency below which absorption becomes important. This implies much bright optical/ultraviolet emission than hitherto expected. The computed light curves show a gradual rise over tens to hundreds of seconds for representative conditions. The application to SN 2008D/XRT 080109 and the detectability limits are also discussed. We predict a detection rate of about one per year with eROSITA.

Matter Mixing in Aspherical Core-collapse Supernovae: Three-dimensional Simulations with Single-star and Binary Merger Progenitor Models for SN 1987A

Abstract We perform three-dimensional hydrodynamic simulations of aspherical core-collapse supernovae, focusing on the matter mixing in SN 1987A. The impacts of four progenitor (pre-supernova) models and parameterized aspherical explosions are investigated. The four pre-supernova models include a blue supergiant (BSG) model based on a slow-merger scenario developed recently for the progenitor of SN 1987A (Urushibata et al. 2018). The others are a BSG model based on a single-star evolution and two red supergiant (RSG) models. Among the investigated explosion (simulation) models, a model with the binary merger progenitor model and an asymmetric bipolar-like explosion, which invokes a jetlike explosion, best reproduces constraints on the mass of high-velocity 56Ni, as inferred from the observed [Fe ii] line profiles. The advantage of the binary merger progenitor model for the matter mixing is the flat and less extended ρ r 3 profile of the C+O core and the helium layer, which may be characterized by the small helium core mass. From the best explosion model, the direction of the bipolar explosion axis (the strongest explosion direction) and the neutron star (NS) kick velocity and direction are predicted. Other related implications and future prospects are also given.

Nucleosynthesis in Primordial Hypernovae

We investigate the relationship between explosion energy and nucleosynthesis in Population III supernovae and provide nucleosynthetic results for the explosions of stars with progenitor masses of $15\,\mathrm{M}_\odot$, $20\,\mathrm{M}_\odot$, $30\,\mathrm{M}_\odot$, $40\,\mathrm{M}_\odot$, $60\,\mathrm{M}_\odot$, and $80\,\mathrm{M}_\odot$, and explosion energies between approximately $10^{50}$ erg and $10^{53}$ erg. We find that the typical abundance pattern observed in metal-poor stars are best matched by supernovae with progenitor mass in the range $15\,\mathrm{M}_\odot$ - $30\,\mathrm{M}_\odot$, and explosion energy of $\sim$ ($5$ - $10$) $\times$ $10^{51}$ erg. In these models, a reverse shock caused by jumps in density between shells of different composition serves to decrease synthesis of chromium and manganese, which is favourable to matching the observed abundances in metal-poor stars. Spherically symmetric explosions of our models with progenitor mass $\geq$ $40\,\mathrm{M}_\odot$ do not provide yields that are compatible with the iron-peak abundances that are typically observed in metal-poor stars, however, by approximating the yields that we might expect from these models in highly aspherical explosions, we find indications that explosions of stars $40\,\mathrm{M}_\odot$ - $80\,\mathrm{M}_\odot$ with bipolar jets may be good candidates for the enrichment sources of metal-poor stars with enhanced carbon abundances.

X-Ray Light Curve and Spectra of Shock Breakout in a Wind

Abstract We investigate the properties of X-ray emission from shock breakout of a supernova in a stellar wind. We consider a simple model describing aspherical explosions, in which the shock front with an ellipsoidal shape propagates into the dense circumstellar matter. For this model, both X-ray light curves and spectra are simultaneously calculated using a Monte Carlo method. We show that the shock breakout occurs simultaneously in all directions in a steady and spherically symmetric wind. As a result, even for the aspherical explosion, the rise and decay timescales of the light curve do not significantly depend on the viewing angles. This fact suggests that the light curve of the shock breakout may be used as a probe of the wind mass-loss rate. We compare our results with the observed spectrum and light curve of X-ray outburst 080109/SN 2008D. The observation can be reproduced by an explosion with a shock velocity of 60% of the speed of light and circumstellar matter with a mass-loss rate of yr−1.

Type Ic core-collapse supernova explosions evolved from very massive stars

We investigate the possibility of a super-luminous Type Ic core-collapse supernovae producing a large amount of 56Ni. Very massive stars with a main-sequence mass larger than 100 Msun and a metallicity 0.001 < Z < 0.004 are expected to explode as super-luminous Type Ic supernovae. Stars with ~ 110 - 150 Msun and Z < 0.001 would explode as Type Ic pulsational pair-instability supernovae if the whole H and He layers has been lost by the mass loss during pulsational pair-instability. We evaluate the total ejecta mass and the yields of 56Ni, O, and Si in core-collapse supernovae evolved from very massive stars. We adopt 43.1 and 61.1 Msun WO stars with Z=0.004 as supernova progenitors expected to explode as Type Ic core-collapse supernovae. These progenitors have masses of 110 and 250 Msun at the zero-age main sequence. Spherical explosions with an explosion energy larger than 2e52 erg produce more than 3.5 Msun 56Ni, enough to reproduce the light curve of SN 2007bi. Asphericity of the explosion affects the total ejecta mass as well as the yields of 56Ni, O, and Si. Aspherical explosions of the 110 and 250 Msun models reproduce the 56Ni yield of SN 2007bi. These explosions will also show large velocity dispersion. An aspherical core-collapse supernova evolved from a very massive star is a possibility of the explosion of SN 2007bi.

MATTER MIXING IN ASPHERICAL CORE-COLLAPSE SUPERNOVAE: A SEARCH FOR POSSIBLE CONDITIONS FOR CONVEYING56Ni INTO HIGH VELOCITY REGIONS

We perform two-dimensional axisymmetric hydrodynamic simulations of matter mixing in aspherical core-collapse supernova explosions of a 16.3 $M_{\odot}$ star with a compact hydrogen envelope. Observations of SN 1987A have provided evidence that $^{56}$Ni synthesized by explosive nucleosynthesis is mixed into fast moving matter ($\gtrsim$ 3,500 km s$^{-1}$) in the exploding star. In order to clarify the key conditions for reproducing such high velocity of $^{56}$Ni, we revisit matter mixing in aspherical core-collapse supernova explosions. Explosions are initiated artificially by injecting thermal and kinetic energies around the interface between the iron core and the silicon-rich layer. Perturbations of 5% or 30% amplitude in the radial velocities are introduced at several points in time. We found that no high velocity $^{56}$Ni can be obtained if we consider bipolar explosions with perturbations (5% amplitude) of pre-supernova origins. If large perturbations (30% amplitude) are introduced or exist due to some unknown mechanism in a later phase just before the shock wave reaches the hydrogen envelope, $^{56}$Ni with a velocity of 3,000 km s$^{-1}$ can be obtained. Aspherical explosions that are asymmetric across the equatorial plane with clumpy structures in the initial shock waves are investigated. We found that the clump sizes affect the penetration of $^{56}$Ni. Finally, we report that an aspherical explosion model that is asymmetric across the equatorial plane with multiple perturbations of pre-supernova origins can cause the penetration of $^{56}$Ni clumps into fast moving matter of 3,000 km s$^{-1}$. We show that both aspherical explosion with clumpy structures and perturbations of pre-supernova origins may be necessary to reproduce the observed high velocity of $^{56}$Ni. To confirm this, more robust three-dimensional simulations are required.

GENERAL-RELATIVISTIC SIMULATIONS OF THREE-DIMENSIONAL CORE-COLLAPSE SUPERNOVAE

We study the three-dimensional (3D) hydrodynamics of the post-core-bounce phase of the collapse of a 27-solar-mass star and pay special attention to the development of the standing accretion shock instability (SASI) and neutrino-driven convection. To this end, we perform 3D general-relativistic simulations with a 3-species neutrino leakage scheme. The leakage scheme captures the essential aspects of neutrino cooling, heating, and lepton number exchange as predicted by radiation-hydrodynamics simulations. The 27-solar-mass progenitor was studied in 2D by B. Mueller et al. (ApJ 761:72, 2012), who observed strong growth of the SASI while neutrino-driven convection was suppressed. In our 3D simulations, neutrino-driven convection grows from numerical perturbations imposed by our Cartesian grid. It becomes the dominant instability and leads to large-scale non-oscillatory deformations of the shock front. These will result in strongly aspherical explosions without the need for large-scale SASI shock oscillations. Low-l-mode SASI oscillations are present in our models, but saturate at small amplitudes that decrease with increasing neutrino heating and vigor of convection. Our results, in agreement with simpler 3D Newtonian simulations, suggest that once neutrino-driven convection is started, it is likely to become the dominant instability in 3D. Whether it is the primary instability after bounce will ultimately depend on the physical seed perturbations present in the cores of massive stars. The gravitational wave signal, which we extract and analyze for the first time from 3D general-relativistic models, will serve as an observational probe of the postbounce dynamics and, in combination with neutrinos, may allow us to determine the primary hydrodynamic instability.

Chemical Yields from Supernovae and Hypernovae

AbstractWe review the final stages of stellar evolution, supernova properties, and chemical yields as a function of the progenitor's mass. (1) 8 - 10M⊙stars are super-AGB stars when the O+Ne+Mg core collapses due to electron capture. These AGB-supernovae may constitute an SN 2008S-like sub-class of Type IIn supernovae. These stars produce little α-elements and Fe-peak elements, but are important sources of Zn and light p-nuclei. (2) 10 - 90M⊙stars undergo Fe-core collapse. Nucleosynthesis in aspherical explosions is important, as it can well reproduce the abundance patterns observed in extremely metal-poor stars. (3) 90 - 140M⊙stars undergo pulsational nuclear instabilities at various nuclear burning stages, including O and Si-burning. (4) Very massive stars withM≳ 140M⊙either become pair-instability SNe, or undergo core-collapse to form intermediate mass black holes if the mass loss is small enough.

Asphericity in Supernova Explosions from Late-Time Spectroscopy

Core-collapse supernovae (CC-SNe) are the explosions that announce the death of massive stars. Some CC-SNe are linked to long-duration gamma-ray bursts (GRBs) and are highly aspherical. One important question is to what extent asphericity is common to all CC-SNe. Here we present late-time spectra for a number of CC-SNe from stripped-envelope stars and use them to explore any asphericity generated in the inner part of the exploding star, near the site of collapse. A range of oxygen emission-line profiles is observed, including a high incidence of double-peaked profiles, a distinct signature of an aspherical explosion. Our results suggest that all CC-SNe from stripped-envelope stars are aspherical explosions and that SNe accompanied by GRBs exhibit the highest degree of asphericity.

Explosion of very massive stars and the origin of intermediate mass black holes

AbstractWe calculate evolution, collapse, explosion, and nucleosynthesis of Population III very massive stars with 500 M⊙ and 1000 M⊙. It was found that both 500 M⊙ and 1000 M⊙ models enter the region of pair-instability but continue to undergo core collapse to black holes. For moderately aspherical explosions, the patterns of nucleosynthesis match the observational data of intergalactic and intercluster medium and hot gases in M82, better than models involving hypernovae and pair instability supernovae.Our results suggest that explosions of Population III core-collapse very massive stars contribute significantly to the chemical evolution of gases in clusters of galaxies. The final black hole masses are about 500 M⊙ for our most massive 1000 M⊙ models. This result may support the view that Population III very massive stars are responsible for the origin of intermediate mass black holes which were recently reported to be discovered.

Core‐Collapse Very Massive Stars: Evolution, Explosion, and Nucleosynthesis of Population III 500–1000M⊙Stars

We calculate evolution, collapse, explosion, and nucleosynthesis of Population III very massive stars with 500 and 1000 M☉. Presupernova evolution is calculated in spherical symmetry. Collapse and explosion are calculated by a two-dimensional code, based on the bipolar jet models. We compare the results of nucleosynthesis with the abundance patterns of intracluster matter, hot gases in M82, and extremely metal-poor stars in the Galactic halo. It was found that both 500 and 1000 M☉ models enter the region of pair instability but continue to undergo core collapse. In the presupernova stage, silicon-burning regions occupy a large fraction, more than 20% of the total mass. For moderately aspherical explosions, the patterns of nucleosynthesis match the observational data of both the intracluster medium and M82. Our results suggest that explosions of Population III core-collapse very massive stars contribute significantly to the chemical evolution of gases in clusters of galaxies. For Galactic halo stars our [O/Fe] ratios are smaller than the observational abundances. However, our proposed scenario is naturally consistent with this outcome. The final black hole masses are ~230 and ~500 M☉ for the 500 and 1000 M☉ models, respectively. This result may support the view that Population III very massive stars are responsible for the origin of intermediate-mass black holes, which were recently reported to be discovered.

Explosive Nucleosynthesis in Aspherical Hypernova Explosions and Late‐Time Spectra of SN 1998bw

Aspherical explosion models for the hypernova (hyperenergetic supernova) SN 1998bw are presented. Nucleosynthesis in aspherical explosions is examined with a two-dimensional hydrodynamical code and a detailed nuclear reaction network. Aspherical explosions lead to a strong α-rich freezeout, thus enhancing the abundance ratios [44Ca, 48Ti, 64Zn/Fe] in the ejecta. The nebular line profiles of the Fe-dominated blend near 5200 Å and of [O I] 6300, 6363 Å are calculated and compared with the observed late-time spectra of SN 1998bw. Compared with the spherical model, the unusual features of the observed nebular spectra can be better explained if SN 1998bw is a strongly aspherical explosion with a kinetic energy of ~1052 ergs viewed from near the jet direction.

Aspherical Explosion Models for SN 1998bw/GRB 980425

The recent discovery of the unusual supernova SN 1998bw and its apparent correlation with the γ-ray burst GRB 980425 has raised new issues concerning both γ-ray bursts and supernovae. Although the spectra of SN 1998bw resemble those of Type Ic supernovae (SNe Ic), there are distinct differences. At early times the expansion velocities inferred by the Doppler shift of (unidentified) absorption features were very high, and SN 1998bw was unusually bright and red at maximum light (Galama et al.). These distinctions make SN 1998bw a candidate of a hypernova, with explosion energies exceeding supernovae by a factor in excess of 10. We present an alternative picture that allows SN 1998bw to have an explosion energy and ejecta mass consistent with core-collapse supernovae, although at the bright end of the typical range. We specifically propose that all SNe Ic are significantly asymmetric and that SN 1998bw is a SN Ic that is distinguished principally by being viewed close to the symmetry axis. We investigate the hypothesis that SNe Ic and SN 1998bw are the results of aspherical explosions along the rotational axis of basically spherical, nondegenerate C/O cores of massive stars. Light curves for aspherical explosions are computed assuming an ellipsoidal geometry for the ejecta. Guided by the polarization observations of normal SN Ic and related events, we assume an axis ratio of 2 near maximum light. The evolution of the isodensity contours with time is discussed. We show that the light curve of SN 1998bw may be understood with an explosion energy of 2 × 1051 ergs, a total ejecta mass of 2 M☉, and a mass of 56Ni of 0.2 M☉ if it is observed at a large angle (≥60°) with respect to the plane of symmetry. In this picture, the high expansion velocities are a direct consequence of an aspherical explosion mechanism which, in turn, produces oblate isodensity contours. Prolate isodensity contours are ruled out. This interpretation suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.