Stars with ∼ 8 − 10 M ⊙ evolve to form a strongly degenerate ONeMg core. When the core mass becomes close to the Chandrasekhar mass, the core undergoes electron captures on 24 Mg and 20 Ne, which induce the electron-capture supernova (ECSN). In order to clarify whether the ECSN leads to a collapse or thermonuclear explosion, we calculate the evolution of an 8.4 M ⊙ star from the main sequence until the oxygen ignition in the ONeMg core. We apply the latest electron-capture rate on 20 Ne including the second forbidden transition, and investigate how the location of the oxygen ignition (center or off-center) and the Y e distribution depend on the input physics and the treatment of the semiconvection and convection. The central density when the oxygen deflagration is initiated, ρ c,def , can be significantly higher than that of the oxygen ignition thanks to the convection, and we estimate log 10 (ρ c,def /g cm −3 ) > 10.10. We perform two-dimensional simulations of the flame propagation to examine how the final fate of the ONeMg core depends on the Y e distribution and ρ c,def . We find that the deflagration starting from log 10 (ρ c,def /g cm −3 ) > 10.01(< 10.01) leads to a collapse (thermonuclear explosion). Since our estimate of ρ c,def exceeds this critical value, the ONeMg core is likely to collapse, although further studies of the convection and semiconvection before the deflagration are important.

High-resolution spectroscopy of the core of the Perseus Cluster of galaxies, using the Hitomi satellite above 2 keV and the XMM-Newton Reflection Grating Spectrometer at lower energies, provides reliable constraints on the abundances of O, Ne, Mg, Si, S, Ar, Ca, Cr, Mn, Fe, and Ni. Accounting for all known systematic uncertainties, the Ar/Fe, Ca/Fe, and Ni/Fe ratios are determined with a remarkable precision of less than 10%, while the constraints on Si/Fe, S/Fe, and Cr/Fe are at the 15% level, and Mn/Fe is measured with a 20% uncertainty. The average biases in determining the chemical composition using archival CCD spectra from XMM-Newton and Suzaku range typically from 15-40%. A simple model in which the enrichment pattern in the Perseus Cluster core and the proto-solar nebula are identical gives a surprisingly good description of the high-resolution X-ray spectroscopy results, with χ 2 = 10.7 for 10 d.o.f. However, this pattern is challenging to reproduce with linear combinations of existing supernova nucleosynthesis calculations, particularly given the precise measurements of intermediate α-elements enabled by Hitomi. We discuss in detail the degeneracies between various supernova progenitor models and explosion mechanisms, and the remaining uncertainties in these theoretical models. We suggest that including neutrino physics in the core-collapse supernova yield calculations may improve the agreement with the observed pattern of α-elements in the Perseus Cluster core. Our results provide a complementary benchmark for testing future nucleosynthesis calculations required to understand the origin of chemical elements.

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Electron capture on 20 Ne is critically important for the final stage of evolution of stars with the initial masses of 8 -10 M ⊙ . In the present paper, we evaluate electron capture rates for a forbidden transition 20 Ne (0 + g.s. ) → 20 F (2 + g.s. ) in stellar environments by the multipole expansion method with the use of shell-model Hamiltonians. These rates have not been accurately determined in theory as well as in experiments. Our newly evaluated rates are compared with those obtained by a prescription that treats the transition as an allowed Gamow-Teller (GT) transition with the strength determined from a recent β-decay experiment for 20 F (2 + g.s. ) → 20 Ne (0 + g.s. ) (Kirsebom et al. 2018). We find that different electron energy dependence of the transition strengths between the two methods leads to sizable differences in the weak rates of the two methods. We also find that the Coulomb effects, that is, the effects of screening on ions and electrons are non-negligible. We apply our e-capture rates on 20 Ne to the calculation of the evolution of high-density O-Ne-Mg cores of 8 -10 M ⊙ stars. We find that our new rates affect the abundance distribution and the central density at the final stage of evolution.

We present axisymmetric hydrodynamical simulations of accretion-induced collapse (AIC) of dark matter (DM) admixed rotating white dwarfs (WD) and their burst gravitational-wave (GW) signals. For initial WD models with the same central baryon density, the admixed DM is found to delay the plunge and bounce phases of AIC, and decrease the central density and mass of the proto-neutron star (PNS) produced. The bounce time, central density and PNS mass generally depend on two parameters, the admixed DM mass M DM and the ratio between the rotational kinetic and gravitational energies of the inner core at bounce β ic,b . The emitted GWs have generic waveform shapes and the variation of their amplitudes h + show a degeneracy on β ic,b and M DM . We found that the ratios between the GW amplitude peaks around bounce allow breaking the degeneracy and extraction of both β ic,b and M DM . Even within the uncertainties of nuclear matter equation of state, a DM core can be inferred if its mass is greater than 0.03 M ⊙ . We also discuss possible DM effects on the GW signals emitted by PNS g-mode oscillations. GW may boost the possibility for the detection of AIC, as well as open a new window in the indirect detection of DM.

Recently observed pulsars with masses ∼ 1.1 M ⊙ challenge the conventional neutron star (NS) formation path by core-collapse supernova (CCSN). Using spherically symmetric hydrodynamics simulations, we follow the collapse of a massive white dwarf (WD) core triggered by electron capture, until the formation of a proto-NS (PNS). For initial WD models with the same central density, we study the effects of a static, compact dark matter (DM) admixed core on the collapse and bounce dynamics and mass of the PNS, with DM mass ∼ 0.01 M ⊙ . We show that increasing the admixed DM mass generally leads to slower collapse and smaller PNS mass, down to about 1.0 M ⊙ . Our results suggest that the accretion-induced collapse of dark matter admixed white dwarfs can produce low-mass neutron stars, such as the observed low-mass pulsar J0453+1559, which cannot be obtained by conventional NS formation path by CCSN.

We study the consequences of a hadron-quark phase transition (PT) in failing core-collapse supernovae (CCSNe) that give birth to stellar-mass black holes (BH). We perform a suite of neutrino-transport general-relativistic hydrodynamic simulations in spherical symmetry with 21 progenitor models and a hybrid equation of state (EoS) including hadrons and quarks. We find that the effect of the PT on the CCSN postbounce dynamics is a function of the bounce compactness parameter . For , the PT leads to a second dynamical collapse of the protocompact star (PCS). While BH formation starts immediately after this second collapse for models with , the PCS experiences a second bounce and oscillations for models with . These models emit potent oscillatory neutrino signals with a period of ∼1 ms for tens of milliseconds after the second bounce, which can be a strong indicator of the PT in failing CCSNe if detected in the future. However, no shock revival occurs and BH formation inevitably takes place in our spherically symmetric simulations. Furthermore, via a diagram of mass-specific entropy evolution of the PCS, the progenitor dependence can be understood through the appearance of a third family of compact stars emerging at large entropy induced by the PT.

Gravitational waves (GWs) provide unobscured insight into the birthplace of neutron stars and black holes in core-collapse supernovae (CCSNe). The nuclear equation of state (EOS) describing these dense environments is yet uncertain, and variations in its prescription affect the proto−neutron star (PNS) and the post-bounce dynamics in CCSN simulations, subsequently impacting the GW emission. We perform axisymmetric simulations of CCSNe with Skyrme-type EOSs to study how the GW signal and PNS convection zone are impacted by two experimentally accessible EOS parameters, (1) the effective mass of nucleons, m ⋆, which is crucial in setting the thermal dependence of the EOS, and (2) the isoscalar incompressibility modulus, K sat. While K sat shows little impact, the peak frequency of the GWs has a strong effective mass dependence due to faster contraction of the PNS for higher values of m ⋆ owing to a decreased thermal pressure. These more compact PNSs also exhibit more neutrino heating, which drives earlier explosions and correlates with the GW amplitude via accretion plumes striking the PNS, exciting the oscillations. We investigate the spatial origin of the GWs and show the agreement between a frequency-radial distribution of the GW emission and a perturbation analysis. We do not rule out overshoot from below via PNS convection as another moderately strong excitation mechanism in our simulations. We also study the combined effect of effective mass and rotation. In all our simulations we find evidence for a power gap near ∼1250 Hz; we investigate its origin and report its EOS dependence.