60Fe in core-collapse supernovae and prospects for X-ray and gamma-ray detection in supernova remnants

Samuel W Jones, Heiko Möller, Chris L Fryer, Christopher J Fontes, Reto Trappitsch, Wesley P Even, Aaron Couture, Matthew R Mumpower, Samar Safi-Harb

Monthly Notices of the Royal Astronomical Society 485, 3, 4287 (2019)

We investigate 60Fe in massive stars and core-collapse supernovae focussing on uncertainties that influence its production in 15, 20, and 25 M⊙ stars at solar metallicity. We find that the 60Fe yield is a monotonic increasing function of the uncertain 59Fe(n, γ)60Fe cross-section and that a factor of 10 reduction in the reaction rate results in a factor of 8–10 reduction in the 60Fe yield, while a factor of 10 increase in the rate increases the yield by a factor of 4–7. We find that none of the 189 simulations we have performed are consistent with a core-collapse supernova triggering the formation of the Solar system, and that only models using 59Fe(n, γ)60Fe cross-section that is less than or equal to that from NON-SMOKER can reproduce the observed 60Fe/26Al line flux ratio in the diffuse interstellar medium. We examine the prospects of detecting old core-collapse supernova remnants (SNRs) in the Milky Way from their gamma-ray emission from the decay of 60Fe, finding that the next generation of gamma-ray missions could be able to discover up to ∼100 such old SNRs as well as measure the 60Fe yields of a handful of known Galactic SNRs. We also predict the X-ray spectrum that is produced by atomic transitions in 60Co following its ionization by internal conversion and give theoretical X-ray line fluxes as a function of remnant age as well as the Doppler and fine-structure line broadening effects. The X-ray emission presents an interesting prospect for addressing the missing SNR problem with future X-ray missions.

Discovery of an Exceptionally Strong -Decay Transition of and Implications for the Fate of Intermediate-Mass Stars

OS Kirsebom, S Jones, DF Strömberg, G Martínez-Pinedo, K Langanke, FK Röpke, BA Brown, T Eronen, HOU Fynbo, M Hukkanen, A Idini, A Jokinen, A Kankainen, J Kostensalo, I Moore, H Möller, ST Ohlmann, H Penttilä, K Riisager, S Rinta-Antila, PC Srivastava, J Suhonen, WH Trzaska, J Äystö

Physical Review Letters 123, 26 (2019)

A significant fraction of stars between 7 and 11 solar masses are thought to become supernovae, but the explosion mechanism is unclear. The answer depends critically on the rate of electron capture on 20Ne in the degenerate oxygen-neon stellar core. However, because of the unknown strength of the transition between the ground states of 20Ne and 20F, it has not previously been possible to fully constrain the rate. By measuring the transition, we establish that its strength is exceptionally large and that it enhances the capture rate by several orders of magnitude. This has a decisive impact on the evolution of the core, increasing the likelihood that the star is (partially) disrupted by a thermonuclear explosion rather than collapsing to form a neutron star. Importantly, our measurement resolves the last remaining nuclear physics uncertainty in the final evolution of degenerate oxygen-neon stellar cores, allowing future studies to address the critical role of convection, which at present is poorly understood.

Remnants and ejecta of thermonuclear electron-capture supernovae-Constraining oxygen-neon deflagrations in high-density white dwarfs

S Jones, FK Röpke, C Fryer, AJ Ruiter, IR Seitenzahl, LR Nittler, ST Ohlmann, R Reifarth, M Pignatari, K Belczynski

Astronomy & Astrophysics 622, A74 (2019)

The explosion mechanism of electron-capture supernovae (ECSNe) remains equivocal: it is not completely clear whether these events are implosions in which neutron stars are formed, or incomplete thermonuclear explosions that leave behind bound ONeFe white dwarf remnants. Furthermore, the frequency of occurrence of ECSNe is not known, though it has been estimated to be of the order of a few per cent of all core-collapse supernovae. 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 the 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 48Ca, 50Ti, 54Cr , 60Fe 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 54Cr/52Cr and 50Ti/48Ti isotopic ratios in the ejecta are a near-perfect match with recent measurements of extreme pre-solar meteoritc oxide grains, and 53Cr/52Cr can also be matched if the ejecta condenses before mixing with the interstellar medium. The composition of the ejecta of our simulations implies that ECSNe, including accretion-induced collapse of oxygen-neon white dwarfs, could actually be partial thermonuclear explosions and not implosions that form neutron stars. There is still much work to do to improve the hydrodynamic simulations of such phenomena, but it is encouraging that our results are consistent with the predictions from stellar evolution modelling and population synthesis simulations, and can explain several key isotopic ratios in a sub-set of pre-solar oxide meteoritic grains. Theoretical mass-radius relations for the bound ONeFe WD remnants of these explosions are apparently consistent with several observational WD candidates. The composition of the remnants in our simulations can reproduce several, but not all, of the spectroscopically-determined elemental abundances from one such candidate WD.

Idealised hydrodynamic simulations of turbulent oxygen-burning shell convection in 4π geometry

Samuel Jones, Robert Andrassy, Stou Sandalski, Austin Davis, Paul Woodward, Falk Herwig

Monthly Notices of the Royal Astronomical Society, 465, 3, 2991 (2017)

This work investigates the properties of convection in stars with particular emphasis on entrainment across the upper convective boundary (CB). Idealized simulations of turbulent convection in the O-burning shell of a massive star are performed in 4π geometry on 7683 and 15363 grids, driven by a representative heating rate. A heating series is also performed on the 7683 grid. The 15363 simulation exhibits an entrainment rate at the upper CB of 1.33 × 10-6 M⊙ s-1. The 7683 simulation with the same heating rate agrees within 17 per cent. The entrainment rate at the upper CB is found to scale linearly with the driving luminosity and with the cube of the shear velocity at the upper boundary, while the radial rms fluid velocity scales with the cube root of the driving luminosity, as expected. The mixing is analysed in a 1D diffusion framework, resulting in a simple model for CB mixing. The analysis confirms that limiting the MLT mixing length to the distance to the CB in 1D simulations better represents the spherically averaged radial velocity profiles from the 3D simulations and provide an improved determination of the reference diffusion coefficient D0 for the exponential diffusion CB mixing model in 1D. From the 3D simulation data, we adopt as the CB the location of the maximum gradient in the horizontal velocity component which has 2σ spatial fluctuations of ≈0.17HP. The exponentially decaying diffusion CB mixing model with f = 0.03 reproduces the spherically averaged 3D abundance profiles.

Do electron-capture supernovae make neutron stars?-First multidimensional hydrodynamic simulations of the oxygen deflagration

Samuel Jones, Friedrich K Röpke, Ruediger Pakmor, Ivo R Seitenzahl, Sebastian T Ohlmann, Philipp VF Edelmann

Astronomy & Astrophysics 593, A72 (2016)

Context. In the classical picture, electron-capture supernovae and the accretion-induced collapse of oxygen-neon white dwarfs undergo an oxygen deflagration phase before gravitational collapse produces a neutron star. These types of core collapse events are postulated to explain several astronomical phenomena. In this work, the oxygen deflagration phase is simulated for the first time using multidimensional hydrodynamics.
Aims: By simulating the oxygen deflagration with multidimensional hydrodynamics and a level-set-based flame approach, new insights can be gained into the explosive deaths of 8-10 M⊙ stars and oxygen-neon white dwarfs that accrete material from a binary companion star. The main aim is to determine whether these events are thermonuclear or core-collapse supernova explosions, and hence whether neutron stars are formed by such phenomena.
Methods: The oxygen deflagration is simulated in oxygen-neon cores with three different central ignition densities. The intermediate density case is perhaps the most realistic, being based on recent nuclear physics calculations and 1D stellar models. The 3D hydrodynamic simulations presented in this work begin from a centrally confined flame structure using a level-set-based flame approach and are performed in 2563 and 5123 numerical resolutions.
Results: In the simulations with intermediate and low ignition density, the cores do not appear to collapse into neutron stars. Instead, almost a solar mass of material becomes unbound from the cores, leaving bound remnants. These simulations represent the case in which semiconvective mixing during the electron-capture phase preceding the deflagration is inefficient. The masses of the bound remnants double when Coulomb corrections are included in the equation of state, however they still do not exceed the effective Chandrasekhar mass and, hence, would not collapse into neutron stars. The simulations with the highest ignition density (log 10ρc = 10.3), representing the case where semiconvective mixing is very efficient, show clear signs that the core will collapse into a neutron star.

Advanced burning stages and fate of 8–10 M☉ stars

Samuel Jones, Raphael Hirschi, Ken'ichi Nomoto, Tobias Fischer, Frank X Timmes, Falk Herwig, Bill Paxton, Hiroshi Toki, Toshio Suzuki, Gabriel Martinez-Pinedo, Yi Hua Lam, Michael G Bertolli

The Astrophysical Journal 772, 2, 150 (2013)

The stellar mass range 8 <~ M/M ⊙ <~ 12 corresponds to the most massive asymptotic giant branch (AGB) stars and the most numerous massive stars. It is host to a variety of supernova (SN) progenitors and is therefore very important for galactic chemical evolution and stellar population studies. In this paper, we study the transition from super-AGB (SAGB) star to massive star and find that a propagating neon-oxygen-burning shell is common to both the most massive electron capture supernova (EC-SN) progenitors and the lowest mass iron-core-collapse supernova (FeCCSN) progenitors. Of the models that ignite neon-burning off-center, the 9.5 M ⊙ star would evolve to an FeCCSN after the neon-burning shell propagates to the center, as in previous studies. The neon-burning shell in the 8.8 M ⊙ model, however, fails to reach the center as the URCA process and an extended (0.6 M ⊙) region of low Y e (0.48) in the outer part of the core begin to dominate the late evolution; the model evolves to an EC-SN. This is the first study to follow the most massive EC-SN progenitors to collapse, representing an evolutionary path to EC-SN in addition to that from SAGB stars undergoing thermal pulses (TPs). We also present models of an 8.75 M ⊙ SAGB star through its entire TP phase until electron captures on 20Ne begin at its center and of a 12 M ⊙ star up to the iron core collapse. We discuss key uncertainties and how the different pathways to collapse affect the pre-SN structure. Finally, we compare our results to the observed neutron star mass distribution.