arXiv subjects

Shing-Chi Leung

Publications and source records attributed to Shing-Chi Leung.

42 records · Page 3Linked to original sources

arXiv2019

Evolution of ONeMg Core in Super-AGB Stars towards Electron-Capture Supernovae: Effects of Updated Electron-Capture Rate

Stars with $\sim 8-10~{M}_{\odot}$ 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 $8.4~M_\odot$ 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, $ρ_{\rm c,def}$, can be significantly higher than that of the oxygen ignition thanks to the convection, and we estimate $\log_{10}(ρ_{\rm c,def}/\mathrm{g~cm^{-3}})>10.10$. 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 $ρ_{\rm c,def}$. We find that the deflagration starting from $\log_{10}(ρ_{\rm c,def}/\mathrm{g~cm^{-3}})>10.01 (< 10.01)$ leads to a collapse (thermonuclear explosion). Since our estimate of $ρ_{\rm c,def}$ exceeds this critical value, the ONeMg core is likely to collapse, although further studies of the convection and semiconvection before the deflagration are important.

astro-ph.HE↗

arXiv2019

Electron-capture Rates in $^{20}$Ne for a Forbidden Transition to the Ground State of $^{20}$F Relevant to Final Evolution of High-density O-Ne-Mg Cores

Electron capture on $^{20}$Ne is critically important for the final stage of evolution of stars with the initial masses of 8 - 10 $M_{\odot}$. In the present paper, we evaluate electron capture rates for a forbidden transition $^{20}$Ne (0$_{g.s.}^{+}$) $\rightarrow$ $^{20}$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$_{g.s.}^{+}$) $\rightarrow$ $^{20}$Ne (0$_{g.s.}^{+}$) \citep{Kirsebom}. 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 $M_{\odot}$ stars. We find that our new rates affect the abundance distribution and the central density at the final stage of evolution.

astro-ph.SR↗

arXiv2019

Accretion-Induced Collapse of Dark Matter Admixed White Dwarfs -- II: Rotation and Gravitational-wave Signals

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 $M_\mathrm{DM}$ and the ratio between the rotational kinetic and gravitational energies of the inner core at bounce $β_\mathrm{ic,b}$. The emitted GWs have generic waveform shapes and the variation of their amplitudes $h_+$ show a degeneracy on $β_\mathrm{ic,b}$ and $M_\mathrm{DM}$. We found that the ratios between the GW amplitude peaks around bounce allow breaking the degeneracy and extraction of both $β_\mathrm{ic,b}$ and $M_\mathrm{DM}$. Even within the uncertainties of nuclear matter equation of state, a DM core can be inferred if its mass is greater than 0.03 $M_{\odot}$. 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.

astro-ph.HE↗

arXiv2019

Single Degenerate Models for Type Ia Supernovae Progenitor's Evolution and Nucleosythesis Yields

(${Abridged~version}$) We review how the single degenerate models for Type Ia supernovae (SNe Ia) works. In the binary star system of a white dwarf (WD) and its non-degenerate companion star, the WD accretes either H-rich matter or He and undergoes H and He shell-burning. We summarize how such shell-burning depend on the accretion rate and the WD mass and how the WD blows strong wind. We identify the following evolutionary routes for the accreting WD to trigger a thermonuclear explosion. With a decreasing rate with time, 5 phases are observed: 1. The WD increases its mass by stable H burning and blows a strong wind, which strips a part of the companion star's envelope to control the accretion rate and forms circumstellar matter (CSM). The WD may explode within CSM as an "SN Ia-CSM". 2. The wind stops and an SN Ia is triggered under steady-stable H shell-burning, which is observed as a super-soft X-ray source: "SN Ia-SSXS". 3. H shell-burning becomes unstable and many flashes recur. The WD undergoes recurrent nova (RN) whose mass ejection is smaller than the accreted matter, evolving to an "SN Ia-RN". 4. If the companion is a He star or WD, the accretion of He can trigger He and C double detonations at the sub-Chandrasekhar (sub-Chand) mass or the WD grows to the Chand mass while producing and He-wind: "SN Ia-He CSM". 5. If the accreting WD rotates rapidly, delayed trigger of an SN Ia beyond standard Chand mass can happen. The (super-Chand) WD contracts to become a delayed SN Ia after losing angular momentum. The companion star has become a He WD and CSM has disappeared: "SN Ia-He WD". We update nucleosynthesis yields of the carbon deflagration model W7, delayed detonation model WDD2, and the sub-Chand mass model to constrain the yields (such as Mn) by comparing the observations. We note the important metallicity effects on $^{58}$Ni and $^{55}$Mn.

astro-ph.HE↗

arXiv2019

Explosive Nucleosynthesis in Near-Chandrasekhar Mass White Dwarf Models for Type Ia Supernovae: Dependence on Model Parameters

We present two-dimensional hydrodynamics simulations of near-Chandrasekhar mass white dwarf (WD) models for Type Ia supernovae (SNe Ia) using the turbulent deflagration model with deflagration-detonation transition (DDT). We perform a parameter survey for 41 models to study the effects of the initial central density (i.e., WD mass), metallicity, flame shape, DDT criteria, and turbulent flame formula for a much wider parameter space than earlier studies. The final isotopic abundances of $^{11}$C to $^{91}$Tc in these simulations are obtained by post-process nucleosynthesis calculations. The survey includes SNe Ia models with the central density from $5 \times 10^8$ g cm$^{-3}$ to $5 \times 10^9$ g cm$^{-3}$ (WD masses of 1.30 - 1.38 $M_\odot$), metallicity from 0 to 5 $Z_{\odot}$, C/O mass ratio from 0.3 - 1.0 and ignition kernels including centered and off-centered ignition kernels. We present the yield tables of stable isotopes from $^{12}$C to $^{70}$Zn as well as the major radioactive isotopes for 33 models. Observational abundances of $^{55}$Mn, $^{56}$Fe, $^{57}$Fe and $^{58}$Ni obtained from the solar composition, well-observed SNe Ia and SN Ia remnants are used to constrain the explosion models and the supernova progenitor. The connection between the pure turbulent deflagration model and the subluminous SNe Iax is discussed. We find that dependencies of the nucleosynthesis yields on the metallicity and the central density (WD mass) are large. To fit these observational abundances and also for the application of galactic chemical evolution modeling, these dependencies on the metallicity and WD mass should be taken into account.

astro-ph.SR↗

arXiv2018

Nucleosynthesis Constraints on the Explosion Mechanism for Type Ia Supernovae

Observations of type Ia supernovae include information about the characteristic nucleosynthesis associated with these thermonuclear explosions. We consider observational constraints from iron-group elemental and isotopic ratios, to compare with various models obtained with the most-realistic recent treatment of electron captures. The nucleosynthesis is sensitive to the highest white-dwarf central densities. Hence, nucleosynthesis yields can distinguish high-density Chandrasekhar-mass models from lower-density burning models such as white-dwarf mergers. We discuss new results of post-processing nucleosynthesis for two spherical models (deflagration and/or delayed detonation models) based upon new electron capture rates. We also consider cylindrical and 3D explosion models (including deflagration, delayed-detonation, or a violent merger model). Although there are uncertainties in the observational constraints, we identify some trends in observations and the models. We make a new comparison of the models with elemental and isotopic ratios from five observed supernovae and three supernova remnants. We find that the models and data tend to fall into two groups. In one group low-density cores such as in a 3D merger model are slightly more consistent with the nucleosynthesis data, while the other group is slightly better identified with higher-density cores such as in single-degenerate 1D-3D deflagration models. Hence, we postulate that both types of environments appear to contribute nearly equally to observed SNIa. We also note that observational constraints on the yields of $^{54}$Cr and $^{54}$Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SNIa explosions.

astro-ph.HE↗