A Subgrid-scale Model for Deflagration-to-Detonation Transitions in Type Ia Supernova Explosion Simulations - Numerical implementation

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.Comment: accepted for publication in Astronomy and Astrophysic

The light curve of SN 1987A revisited: constraining production masses of radioactive nuclides

We revisit the evidence for the contribution of the long-lived radioactive nuclides 44Ti, 55Fe, 56Co, 57Co, and 60Co to the UVOIR light curve of SN 1987A. We show that the V-band luminosity constitutes a roughly constant fraction of the bolometric luminosity between 900 and 1900 days, and we obtain an approximate bolometric light curve out to 4334 days by scaling the late time V-band data by a constant factor where no bolometric light curve data is available. Considering the five most relevant decay chains starting at 44Ti, 55Co, 56Ni, 57Ni, and 60Co, we perform a least squares fit to the constructed composite bolometric light curve. For the nickel isotopes, we obtain best fit values of M(56Ni) = (7.1 +- 0.3) x 10^{-2} Msun and M(57Ni) = (4.1 +- 1.8) x 10^{-3} Msun. Our best fit 44Ti mass is M(44Ti) = (0.55 +- 0.17) x 10^{-4} Msun, which is in disagreement with the much higher (3.1 +- 0.8) x 10^{-4} Msun recently derived from INTEGRAL observations. The associated uncertainties far exceed the best fit values for 55Co and 60Co and, as a result, we only give upper limits on the production masses of M(55Co) < 7.2 x 10^{-3} Msun and M(60Co) < 1.7 x 10^{-4} Msun. Furthermore, we find that the leptonic channels in the decay of 57Co (internal conversion and Auger electrons) are a significant contribution and constitute up to 15.5% of the total luminosity. Consideration of the kinetic energy of these electrons is essential in lowering our best fit nickel isotope production ratio to [57Ni/56Ni]=2.5+-1.1, which is still somewhat high but is in agreement with gamma-ray observations and model predictions.Comment: 7 pages, 6 pages, 2 table

The Effects of Variations in Nuclear Interactions on Nucleosynthesis in Thermonuclear Supernovae

The impact of nuclear physics uncertainties on nucleosynthesis in thermonuclear supernovae has not been fully explored using comprehensive and systematic studies with multiple models. To better constrain predictions of yields from these phenomena, we have performed a sensitivity study by post-processing thermodynamic histories from two different hydrodynamic, Chandrasekhar-mass explosion models. We have individually varied all input reaction and, for the first time, weak interaction rates by a factor of ten and compared the yields in each case to yields using standard rates. Of the 2305 nuclear reactions in our network, we find that the rates of only 53 reactions affect the yield of any species with an abundance of at least 10^-8 M_sun by at least a factor of two, in either model. The rates of the 12C(a,g), 12C+12C, 20Ne(a,p), 20Ne(a,g) and 30Si(p,g) reactions are among those that modify the most yields when varied by a factor of ten. From the individual variation of 658 weak interaction rates in our network by a factor of ten, only the stellar 28Si(b+)28Al, 32S(b+)32P and 36Ar(b+)36Cl rates significantly affect the yields of species in a model. Additional tests reveal that reaction rate changes over temperatures T > 1.5 GK have the greatest impact, and that ratios of radionuclides that may be used as explosion diagnostics change by a factor of less than two from the variation of individual rates by a factor of 10. Nucleosynthesis in the two adopted models is relatively robust to variations in individual nuclear reaction and weak interaction rates. Laboratory measurements of a limited number of reactions would help to further constrain predictions. As well, we confirm the need for a consistent treatment for relevant stellar weak interaction rates since simultaneous variation of these rates (as opposed to individual variation) has a significant effect on yields in our models.Comment: accepted by A&A, 14 pages, 5 figures, 2 table

Asymmetry and the Nucleosynthetic Signature of Nearly Edge-Lit Detonation in White Dwarf Cores

Most of the leading explosion scenarios for Type Ia supernovae involve the nuclear incineration of a white dwarf star through a detonation wave. Several scenarios have been proposed as to how this detonation may actually occur, but the exact mechanism and environment in which it takes place remain unknown. We explore the effects of an off-center initiated detonation on the spatial distribution of the nucleosynthetic yield products in a toy model -- a pre-expanded near Chandrasekhar-mass white dwarf. We find that a single-point near edge-lit detonation results in asymmetries in the density and thermal profiles, notably the expansion timescale, throughout the supernova ejecta. We demonstrate that this asymmetry of the thermodynamic trajectories should be common to off-center detonations where a small amount of the star is burned prior to detonation. The sensitivity of the yields on the expansion timescale results in an asymmetric distribution of the elements synthesized as reaction products. We tabulate the shift in the center of mass of the various elements produced in our model supernova and find an odd-even pattern for elements past silicon. Our calculations show that off-center single-point detonations in carbon-oxygen white dwarfs are marked by significant composition asymmetries in their remnants which bear potentially observable signatures in both velocity and coordinate space, including an elemental nickel mass fraction which varies by a factor of two to three from one side of the remnant to the other.Comment: 7 pages, 7 figures, accepted for publication in the Astrophysical Journa

Proton-Rich Nuclear Statistical Equilibrium

Proton-rich material in a state of nuclear statistical equilibrium (NSE) is one of the least studied regimes of nucleosynthesis. One reason for this is that after hydrogen burning, stellar evolution proceeds at conditions of equal number of neutrons and protons or at a slight degree of neutron-richness. Proton-rich nucleosynthesis in stars tends to occur only when hydrogen-rich material that accretes onto a white dwarf or neutron star explodes, or when neutrino interactions in the winds from a nascent proto-neutron star or collapsar-disk drive the matter proton-rich prior to or during the nucleosynthesis. In this paper we solve the NSE equations for a range of proton-rich thermodynamic conditions. We show that cold proton-rich NSE is qualitatively different from neutron-rich NSE. Instead of being dominated by the Fe-peak nuclei with the largest binding energy per nucleon that have a proton to nucleon ratio close to the prescribed electron fraction, NSE for proton-rich material near freeze-out temperature is mainly composed of Ni56 and free protons. Previous results of nuclear reaction network calculations rely on this non-intuitive high proton abundance, which this paper will explain. We show how the differences and especially the large fraction of free protons arises from the minimization of the free energy as a result of a delicate competition between the entropy and the nuclear binding energy.Comment: 4 pages, 7 figure

Nucleosynthesis in thermonuclear supernovae with tracers: convergence and variable mass particles

Nucleosynthetic yield predictions for multi-dimensional simulations of thermonuclear supernovae generally rely on the tracer particle method to obtain isotopic information of the ejected material for a given supernova simulation. We investigate how many tracer particles are required to determine converged integrated total nucleosynthetic yields. For this purpose, we conduct a resolution study in the number of tracer particles for different hydrodynamical explosion models at fixed spatial resolution. We perform hydrodynamic simulations on a co-expanding Eulerian grid in two dimensions assuming rotational symmetry for both pure deflagration and delayed detonation Type Ia supernova explosions. Within a given explosion model, we vary the number of tracer particles to determine the minimum needed for the method to give a robust prediction of the integrated yields of the most abundant nuclides. For the first time, we relax the usual assumption of constant tracer particle mass and introduce a radially vary- ing distribution of tracer particle masses. We find that the nucleosynthetic yields of the most abundant species (mass fraction > 10E-5) are reasonably well predicted for a tracer number as small as 32 per axis and direction - more or less independent of the explosion model. We conclude that the number of tracer particles that were used in extant published works appear to have been sufficient as far as integrated yields are concerned for the most copiously produced nuclides. Additionally we find that a suitably chosen tracer mass distribution can improve convergence for nuclei produced in the outer layer of the supernova where the constant tracer mass prescription suffers from poor spatial resolution.Comment: 9 pages, 5 figures, accepted for publication in MNRA

Neutrinos from type Ia supernovae: the deflagration-to-detonation transition scenario

It has long been recognized that the neutrinos detected from the next core-collapse supernova in the Galaxy have the potential to reveal important information about the dynamics of the explosion and the nucleosynthesis conditions as well as allowing us to probe the properties of the neutrino itself. The neutrinos emitted from thermonuclear - type Ia - supernovae also possess the same potential, although these supernovae are dimmer neutrino sources. For the first time, we calculate the time, energy, line of sight, and neutrino-flavor-dependent features of the neutrino signal expected from a three-dimensional delayed-detonation explosion simulation, where a deflagration-to-detonation transition triggers the complete disruption of a near-Chandrasekhar mass carbon-oxygen white dwarf. We also calculate the neutrino flavor evolution along eight lines of sight through the simulation as a function of time and energy using an exact three-flavor transformation code. We identify a characteristic spectral peak at $\sim 10$ MeV as a signature of electron captures on copper. This peak is a potentially distinguishing feature of explosion models since it reflects the nucleosynthesis conditions early in the explosion. We simulate the event rates in the Super-K, Hyper-K, JUNO, and DUNE neutrino detectors with the SNOwGLoBES event rate calculation software and also compute the IceCube signal. Hyper-K will be able to detect neutrinos from our model out to a distance of $\sim 10$ kpc. At 1 kpc, JUNO, Super-K, and DUNE would register a few events while IceCube and Hyper-K would register several tens of events.Comment: 44 pages, 29 figures & 2 tables. Updated to match Phys. Rev. D version, including a new event channel discussion and improved IceCube result

[Fe XIV] and [Fe XI] reveal the forward shock in SNR 1E0102.2-7219

Aims. We study the forward shock in the oxygen-rich young supernova remnant (SNR) 1E0102.2-7219 (1E0102 in short) via optical coronal emission from [Fe XIV] and [Fe XI]: emission lines which offer an alternative method to X-rays to do so. Methods. We have used the Multi-Unit Spectroscopic Explorer (MUSE) optical integral field spectrograph at the Very Large Telescope (VLT) on Cerro Paranal to obtain deep observations of SNR 1E0102 in the Small Magellanic Cloud. Our observations cover the entire extent of the remnant with a seeing limited spatial resolution of 0.7" = 0.2 pc at the distance of 1E 0102. Results. Our MUSE observations unambiguously reveal the presence of [Fe XIV] and [Fe XI] emission in 1E0102. The emission largely arises from a thin, partial ring of filaments surrounding the fast moving O-rich ejecta in the system. The brightest [Fe XIV] and [Fe XI] emission is found along the eastern and north-western sides of 1E0102, where shocks are driven into denser ISM material, while fainter emission along the northern edge reveals the location of the forward shock in lower density gas, possibly the relic stellar wind cavity. Modeling of the eastern shocks and the photoionization precursor surrounding 1E0102, we derive a pre-shock density $n_H$ = (7.4 +-1.5) cm$^{-3}$, and a shock velocity 330 km/s < $v_s$ < 350 km/s.Comment: 4 pages, 4 figures, accepted for publications in A&A as a Letter to the Edito

SN1991bg-like supernovae are a compelling source of most Galactic antimatter

The Milky Way Galaxy glows with the soft gamma ray emission resulting from the annihilation of $\sim 5 \times 10^{43}$ electron-positron pairs every second. The origin of this vast quantity of antimatter and the peculiar morphology of the 511keV gamma ray line resulting from this annihilation have been the subject of debate for almost half a century. Most obvious positron sources are associated with star forming regions and cannot explain the rate of positron annihilation in the Galactic bulge, which last saw star formation some $10\,\mathrm{Gyr}$ ago, or else violate stringent constraints on the positron injection energy. Radioactive decay of elements formed in core collapse supernovae (CCSNe) and normal Type Ia supernovae (SNe Ia) could supply positrons matching the injection energy constraints but the distribution of such potential sources does not replicate the required morphology. We show that a single class of peculiar thermonuclear supernova - SN1991bg-like supernovae (SNe 91bg) - can supply the number and distribution of positrons we see annihilating in the Galaxy through the decay of $^{44}$Ti synthesised in these events. Such $^{44}$Ti production simultaneously addresses the observed abundance of $^{44}$Ca, the $^{44}$Ti decay product, in solar system material.Comment: Accepted for publication in Proceedings of IAU Symposium 322: The Multimessenger Astrophysics of the Galactic Center 4 page