Published papers

Magnetars, neutron stars thought to be with ultrastrong magnetic fields of 1014-15 G, are observed to be much hotter than ordinary pulsars with ∼1012 G, and additional heating sources are required. One possibility is heating by the ambipolar diffusion in the stellar core. This scenario is examined by calculating the models using the relativistic thermal evolutionary code without making the isothermal approximation. The results show that this scenario can be consistent with most of the observed magnetar temperature data.

Axion-like particles (ALPs) are a class of hypothetical pseudo-scalar particles and can be created in hot astrophysical plasma through the interaction between photons. I calculate the production of ALPs in type Ia supernovae. It is found that many ALPs lighter than a few MeV can be produced in type Ia supernovae. Once produced, heavy ALPs decay into photons during propagation in the interstellar space. I calculate the flux of the decay photons and find that it may be detected by future MeV γ-ray telescopes if a type Ia supernova explodes near the Solar System.

Light sterile neutrinos, νs, are often introduced to explain an anomalous deficit in the electron antineutrino flux from nuclear reactors. If they exist, sterile neutrinos would also be produced in collapsing massive stars through the active-sterile neutrino oscillation. In order to investigate the impacts of sterile neutrinos on supernova dynamics, we perform two-dimensional neutrino-radiation hydrodynamic simulations of stellar core-collapse coupled with the active-sterile oscillation through the Mikheyev-Smirnov-Wolfenstein effect. As the initial condition of our simulations, we adopt a blue supergiant model that is tuned to reproduce observational features of the SN 1987A progenitor to compare our models with observations of the event. It is found that the active-sterile oscillation reduces the νe and ν¯e fluxes and decreases the explosion energy. We also find that, if the mixing angle θ and the mass difference δms2 between νe and νs are large enough, the star fails to explode. This suggests that these mixing parameters relevant to sterile neutrinos could be constrained by supernova explodability, though other uncertainties in supernova theory need to be addressed to refine them. In addition, we predict neutrino signals from a nearby supernova event and find that the neutrino event number can significantly decrease because the νe and ν¯e fluxes are reduced. In particular, DUNE observations of νe will be useful to search for a signature of sterile neutrinos with a tiny mixing angle because a smaller mixing angle leads to a larger effect on the νe flux.

A recent spectroscopic survey has shown that the lithium abundances in low-mass red-clump stars are higher than theoretical prediction. Motivated by their finding, we implemented effects of additional energy losses induced by neutrino magnetic moments on stellar evolution models. It is found that the lithium abundances in red giants increase because the finite magnetic moments of neutrinos enhance the efficiency of deep mixing and 7Be is conveyed from the hydrogen burning shell to the convective envelope. The results motivate further studies on the effects of non-standard energy losses on the surface compositions of low-mass red giants and red-clump stars.

Since 7Li is easily destroyed in low temperatures, the surface lithium abundance decreases as stars evolve. This is supported by the lithium depletion observed in the atmosphere of most red giants. However, recent studies show that almost all of red clump stars have high lithium abundances A(Li) > −0.9, which are not predicted by the standard theory of the low-mass stellar evolution. In order to reconcile the discrepancy between the observations and the model, we consider additional energy loss channels that may come from physics beyond the Standard Model. A(Li) slightly increases near the tip of the red giant branch even in the standard model with thermohaline mixing because of the 7Be production by the Cameron–Fowler mechanism, but the resultant 7Li abundance is much lower than the observed values. We find that the production of 7Be becomes more active if there are additional energy loss channels, because themohaline mixing becomes more efficient and a heavier helium core is formed.

We report on the effects of strong magnetic fields on neutrino emission in the modified Urca process. We show that the effect of Landau levels on the various Urca pairs affects the neutrino emission spectrum and leads to an angular asymmetry in the neutrino emission. For low magnetic fields, the Landau levels have almost no effect on the cooling. However, as the field strength increases, the electron chemical potential increases resulting in a lower density at which Urca pairs can exist. For intermediate field strength, there is an interesting interference between the Landau level distribution and the Fermi distribution. For high enough field strength, the entire electron energy spectrum is eventually confined to a single Landau level producing dramatic spikes in the emission spectrum.

Axion-like particles (ALPs) are hypothetical bosons which may couple with photons. Since many ALPs can be emitted from hot and dense astrophysical plasma, nearby supernovae (SNe) are a possible probe into their properties, including the ALP mass ma and the coupling constant gaγ between ALPs and photons. I calculate the ALP emission from a type Ia SN (SN Ia) model with near-Chandrasekhar mass, finding that ALP luminosities from SNe Ia reach ∼1043(gaγ /10-10 GeV-1)2 erg s-1 if ma ≲ 1 MeV. Heavy ALPs emitted from SNe are unstable and decay into photons. I predict the time delay and the flux of decay photons that reach Earth from a nearby SN Ia, finding that the decay photons may provide a constraint on gaγ which is as stringent as the SN 1987A limit if an SN Ia is located 1 kpc away or closer and next-generation MeV γ-ray satellites observe it ∼1–10 yr after the explosion.

Core-collapse supernovae provide a unique opportunity to probe axions because they can be a copious source of the particles. It has recently been proposed that axion helioscopes can be used for the direct search for supernova axions if a supernova event appears within a few hundred parsecs. However, the event number of supernova axions has been estimated only within the post-process framework. In this study, we perform long-term supernova simulations for a 9.6M⊙star coupled with the axion emission to reevaluate the event number of axions detected by the helioscopes. We find that the additional cooling induced by the axion emission can significantly decrease the temperature in the proto-neutron star. As a result, the axion luminosity and hence the axion event number are reduced, compared with the result obtained through post-processing. Our result indicates that the nonlinear feedback of the axion emission is an essential factor to predict the axion detectability, and underscores the need for systematic simulation studies across various progenitor models.

It was recently proposed that exotic particles can trigger a new stellar instability that is analogous to the e − e + pair instability if they are produced and reach equilibrium in the stellar plasma. In this study, we construct axion instability supernova (AISN) models caused by the new instability to predict their observational signatures. We focus on heavy axion-like particles (ALPs) with masses of ∼400 keV-2 MeV and coupling with photons of g a γ ∼ 10−5 GeV−1. It is found that the 56Ni mass and the explosion energy are significantly increased by ALPs for a fixed stellar mass. As a result, the peak times of the light curves of AISNe occur earlier than those of standard pair-instability supernovae by 10-20 days when the ALP mass is equal to the electron mass. Also, the event rate of AISNe is 1.7-2.6 times higher than that of pair-instability supernovae, depending on the high mass cutoff of the initial mass function.

Core-collapse supernovae are a useful laboratory to probe the nature of exotic particles. If axionlike particles (ALPs) are produced in supernovae, they can affect the transfer of energy and leave traces in observational signatures. In this work, we present results from two-dimensional supernova models including the effects of the production and the absorption of ALPs that couple with photons. It is found that the additional heating induced by ALPs can enhance the diagnostic energy of explosion, Ediag. For example, for moderate ALP-photon coupling, we find explosion energies ∼0.6×1051 erg compared to our reference model without ALPs of ∼0.4×1051 erg in the first ∼0.5 s postbounce explored in this work. Our findings indicate that when the coupling constant is sufficiently high, the neutrino luminosities and mean energies are decreased because of the additional cooling of the proto-neutron star via ALPs. The gravitational wave amplitude is also reduced because the mass accretion on the proto-neutron star is suppressed. Although the ALP-photon coupling can foster explodability, including enhancing the explosion energy closer to recent observations, more long-term simulations in three spatial dimensions are needed to draw robust conclusions.

We calculate the production of ultralight axionlike particles (ALPs) in a nearby supernova progenitor. Once produced, ALPs escape from the star and a part of them is converted into photons during propagation in the Galactic magnetic field. It is found that the MeV photon flux that reaches Earth may be detectable by γ-ray telescopes for ALPs lighter than ∼1 neV when Betelgeuse undergoes oxygen and silicon burning. The dependence of the γ-ray flux on the stellar mass is much smaller than the uncertainty that originates from the Galactic magnetic field. If ALPs are lighter than ∼0.1 neV and the supernova progenitor is close enough to the Solar System, the γ-ray flux is insensitive to the distance d because the ALP-photon conversion probability is proportional to d2. (Non)detection of γ rays from a supernova progenitor with next-generation γ-ray telescopes just after presupernova neutrino alerts would lead to an independent constraint on ALP parameters as stringent as a SN 1987A limit.

We perform spherically symmetric simulations of core-collapse supernovae with the aid of heavy axionlike particles (ALPs) which interact with photons and redistribute energy within supernova matter. We explore a wide ALP parameter space that includes MeV-scale ALP mass ma and the ALP-photon coupling constant gaγ∼10-10 GeV-1, employing three progenitor models with zero-age main-sequence mass of 11.2M⊙, 20.0M⊙, and 25.0M⊙. We find a general trend that, given ma 300 MeV, heavier ALPs are favorable for the shock wave to be successfully revived, aiding the onset of the neutrino-driven explosion. However, if ALPs are heavier than ∼400 MeV, the explosion is failed or weaker than that for the models with smaller ma, because of an insufficient temperature inside the supernova core to produce heavy ALPs. The maximum temperature in the core depends on the initial progenitor structure. Our simulations indicate that the high-temperature environment in the collapsing core of massive progenitors leads to a significant impact of ALPs on the explodability.

Context. Pulsational pair-instability supernovae (PPISNe) and pair instability supernovae (PISNe) are the result of a thermonuclear runaway in the presence of a background electron-positron pair plasma. As such, their evolution and resultant black hole masses could possibly be affected by screening corrections due to the electron pair plasma. Aims. The sensitivity of PISNe and PPISNe to relativistic weak screening has been explored. Methods. In this paper a weak screening model that includes effects from relativistic pair production has been developed and applied at temperatures approaching and exceeding the threshold for pair production. This screening model replaces screening commonly used in astrophysics. Modifications to the weak screening electron Debye length were incorporated in a computationally tractable analytic form. Results. In PPISNe the BH masses were found to increase somewhat at high temperatures, though this increase is small. The BH collapse is also found to occur at earlier times, and the pulsational morphology also changes. In addition to the resultant BH mass, the sensitivity to the screening model of the pulsational period, the pulse structure, the PPISN-to-PISN transition, and the shift in the BH mass gap has been analyzed. The dependence of the composition of the ejected mass was also examined.

Nucleosynthesis of iron-group elements in Type Ia supernovae is studied for single-degenerate models with the use of electron-capture rates updated with the new shell-model Hamiltonian in pf-shell. An over-production problem of neutron-rich iron-group isotopes compared with the solar abundances is now found to be suppressed within a factor of about twice for the updated weak rates. Effects of screening on nucleosynthesis are investigated for explosion models of fast deflagration and slow deflagration with delayed detonation. The e-capture rates are reduced by the screening, especially by the screening effects on the ions. The production yields of most neutron-rich isotopes such as 50Ti, 54Cr and 58Fe are found to be suppressed most by the screening. The inclusion of the screening is desirable for precise evaluation of abundances of neutron-rich nuclides.

Axionlike particles (ALPs) are a class of hypothetical pseudoscalar particles which feebly interact with ordinary matter. The hot plasma of core-collapse supernovae is a possible laboratory to explore physics beyond the standard model including ALPs. Once produced, some of the ALPs can be absorbed by the supernova matter and affect energy transfer. In this study, we calculate the ALP emission in core-collapse supernovae and the backreaction on supernova dynamics consistently. It is found that the stalled bounce shock can be revived if the coupling between ALPs and photons is as high as ga?~10-9 GeV-1 and the ALP mass is 40-400 MeV. Most of the models result in more energetic explosions than the average observed supernova. While this can be used to place constraints on those ALPs, long-term simulations across multiple progenitors need to be further investigated to place robust limits.

We perform three-dimensional supernova simulations with a phenomenological treatment of neutrino flavor conversions. We show that the explosion energy can increase to as high as ∼ 1051 erg depending on the critical density for the onset of flavor conversions, due to a significant enhancement of the mean energy of electron antineutrinos. Our results confirm previous studies showing such energetic explosions, but for the first time in three-dimensional configurations. In addition, we predict neutrino and gravitational wave (GW) signals from a nearby supernova explosion aided by flavor conversions. We find that the neutrino event number decreases because of the reduced flux of heavy-lepton neutrinos. In order to detect GWs, next-generation GW telescopes such as Cosmic Explorer and the Einstein Telescope are needed even if the supernova event is located at the Galactic Center. These findings show that the neutrino flavor conversions can significantly change supernova dynamics and highlight the importance of further studies on the quantum kinetic equations to determine the conditions of the conversions and their asymptotic states.

Core-collapse supernovae can be a copious source of sterile neutrinos, hypothetical particles that mix with active neutrinos. We develop two-dimensional stellar core-collapse models that incorporate the mixing between tau neutrinos and heavy sterile neutrinos - those with the mass of 150-200 MeV - to investigate signatures of sterile neutrinos in supernova observables. We find that the decay channel of a sterile neutrino into a pion and a tau neutrino can enhance the explosion energy and the synthesized nickel mass. Although the inclusion of sterile neutrinos considered in this study slightly reduce the neutrino and gravitational-wave signals, we find that they are still detectable for a Galactic event. Furthermore, we point out that if sterile neutrinos are as massive as ∼200 MeV, they produce high-energy tau antineutrinos with energies of ∼80 MeV, the detection of which can be a smoking signature of the sterile neutrinos and where Hyper-Kamiokande should play a pivotal role.