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Register a new userMerging White Dwarfs and Thermonuclear Supernovae
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Added by
Marten van Kerkwijk
Publication date
2012
fields
Physics
and research's language is
English
Authors
Marten H. van Kerkwijk
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Thermonuclear supernovae result when interaction with a companion reignites nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway, a catastrophic gain in pressure, and the disintegration of the whole white dwarf. It is usually thought that fusion is reignited in near-pycnonuclear conditions when the white dwarf approaches the Chandrasekhar mass. I briefly describe two long-standing problems faced by this scenario, and our suggestion that these supernovae instead result from mergers of carbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar mass remnants. I then turn to possible observational tests, in particular those that test the absence or presence of electron captures during the burning.
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Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). While it is not entirely clear if and when an explosion is triggered in such systems, numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such peri-merger detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase. Synthetic light curves and spectra are generated for comparison with observations. Three models are considered with primary masses 0.96 Msun, 1.06 Msun, and 1.20 Msun. Of these, the 0.96 Msun dwarf merging with an 0.81 Msun companion, with a Ni56 yield of 0.58 Msun, is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to super-Chandrasekhar mass SNe Ia. While the synthetic light curves and spectra of some of the models resemble observed SNe Ia, the significant asymmetry of the ejecta leads to large orientation effects. The peak bolometric luminosity varies by more than a factor of 2 with the viewing angle, and the velocities of the spectral absorption features are lower when observed from angles where the light curve is brightest. The largest orientation effects are seen in the ultraviolet, where the flux varies by more than an order of magnitude. Despite the large variation with viewing angle, the set of three models roughly obeys a width-luminosity relation, with the brighter light curves declining more slowly in the B-band. Spectral features due to unburned carbon from the secondary star are also seen in some cases.
Merging carbon-oxygen (CO) white dwarfs are a promising progenitor system for Type Ia supernovae (SN Ia), but the underlying physics and timing of the detonation are still debated. If an explosion occurs after the secondary star is fully disrupted, the exploding primary will expand into a dense CO medium that may still have a disk-like structure. This interaction will decelerate and distort the ejecta. Here we carry out multi-dimensional simulations of ``tamped SN Ia models, using both particle and grid-based codes to study the merger and explosion dynamics, and a radiative transfer code to calculate synthetic spectra and light curves. We find that post-merger explosions exhibit an hourglass-shaped asymmetry, leading to strong variations in the light curves with viewing angle. The two most important factors affecting the outcome are the scale-height of the disk, which depends sensitively on the binary mass ratio, and the total ${}^{56}$Ni yield, which is governed by the central density of the remnant core. The synthetic broadband light curves rise and decline very slowly, and the spectra generally look peculiar, with weak features from intermediate mass elements but relatively strong carbon absorption. We also consider the effects of the viscous evolution of the remnant, and show that a longer time delay between merger and explosion probably leads to larger ${}^{56}$Ni yields and more symmetrical remnants. We discuss the relevance of this class of aspherical ``tamped SN Ia for explaining the class of ``super-Chandrasekhar SN Ia.
Suggestive evidence has accumulated that intermediate mass black holes (IMBH) exist in some globular clusters. As stars diffuse in the cluster, some will inevitable wander sufficiently close to the hole that they suffer tidal disruption. An attractive feature of the IMBH hypothesis is its potential to disrupt not only solar-type stars but also compact white dwarf stars. Attention is given to the fate of white dwarfs that approach the hole close enough to be disrupted and compressed to such extent that explosive nuclear burning may be triggered. Precise modeling of the dynamics of the encounter coupled with a nuclear network allow for a realistic determination of the explosive energy release, and it is argued that ignition is a natural outcome for white dwarfs of all varieties passing well within the tidal radius. Although event rates are estimated to be significantly less than the rate of normal Type Ia supernovae, such encounters may be frequent enough in globular clusters harboring an IMBH to warrant a search for this new class of supernova.
Type Ia supernovae (SNe) are thought to originate from the thermonuclear explosions of carbon-oxygen (CO) white dwarfs (WDs). The proposed progenitors of standard type Ia SNe have been studied for decades and can be, generally, divided into explosions of CO WDs accreting material from stellar non-degenerate companions (single-degenerate; SD models), and those arising from the explosive interaction of two CO WDs (double-degenerate; DD models). However, current models for the progenitors of such SNe fail to reproduce the diverse properties of the observed explosions, nor do they explain the inferred rates and the characteristics of the observed populations of type Ia SNe and their expected progenitors. Here we show that the little-studied mergers of CO-WDs with hybrid Helium-CO (He-CO) WDs can provide for a significant fraction of the normal type Ia SNe. Here we use detailed thermonuclear-hydrodynamical and radiative-transfer models to show that a wide range of mergers of CO WDs with hybrid He-CO WDs can give rise to normal type Ia SNe. We find that such He-enriched mergers give rise to explosions for which the synthetic light-curves and spectra resemble those of observed type Ia SNe, and in particular, they can produce a wide range of peak-luminosities, MB(MR)~ 18.4 to 19.2 (~ 18.5 to 19:45), consistent with those observed for normal type Ia SNe. Moreover, our population synthesis models show that, together with the contribution from mergers of massive double CO-WDs (producing the more luminous SNe), they can potentially reproduce the full range of type Ia SNe, their rate and delay-time distribution.
(Abridged) The explosion mechanism of electron-capture supernovae (ECSNe) remains equivocal. We attempt to constrain the explosion mechanism (neutron-star-forming implosion or thermonuclear explosion) and the frequency of occurrence of ECSNe using nucleosynthesis simulations of the latter scenario, population synthesis, the solar abundance distribution, pre-solar meteoritic oxide grain isotopic ratio measurements and the white dwarf mass-radius relation. Tracer particles from 3d hydrodynamic simulations were post-processed with a large nuclear reaction network in order to determine the complete compositional state of the bound ONeFe remnant and the ejecta, and population synthesis simulations were performed in order to estimate the ECSN rate with respect to the CCSN rate. The 3d deflagration simulations drastically overproduce the neutron-rich isotopes $^{48}$Ca, $^{50}$Ti, $^{54}$Cr, $^{60}$Fe and several of the Zn isotopes relative to their solar abundances. Using the solar abundance distribution as our constraint, we place an upper limit on the frequency of thermonuclear ECSNe as 1$-$3~% the frequency at which core-collapse supernovae (FeCCSNe) occur. This is on par with or 1~dex lower than the estimates for ECSNe from single stars. The upper limit from the yields is also in relatively good agreement with the predictions from our population synthesis simulations. The $^{54}$Cr/$^{52}$Cr and $^{50}$Ti/$^{48}$Ti isotopic ratios in the ejecta are a near-perfect match with recent measurements of extreme pre-solar meteoritc oxide grains, and $^{53}$Cr/$^{52}$Cr can also be matched if the ejecta condenses before mixing with the interstellar medium. Theoretical mass-radius relations for the bound ONeFe WD remnants of these explosions are apparently consistent with several observational WD candidates.
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