We propose a temperature-resolved Monte Carlo (MC) approach to identify the temperature regimes in which low-energy helium-burning reaction rates most strongly affect nucleosynthesis in very massive stars that undergo pair-instability supernovae (PISNe). By performing MC simulations of PISNe, we quantify how temperature-dependent variations in key helium-burning reaction rates, i.e., the triple-α and ^12 C(α,γ)^16 O rates, influence ^56 Ni synthesis. Thousands of stellar evolution calculations using reveal that both the ^12 C(α,γ)^16 O and triple-α reactions exhibit their strongest sensitivity at T ≃ 2.5 × 10^8 K, but with opposite correlation signs. We show that this temperature corresponds to the regime in which the ratio of the sampled rate multipliers is most clearly imprinted on the pre-carbon-burning C/O composition. This demonstrates that PISN nucleosynthesis can probe helium-burning reaction rates in specific low-temperature regimes.

We propose a novel method to constrain low-energy helium-burning reaction rates using very massive stars. By performing Monte Carlo simulations of pair-instability supernovae (PISNe), we identify the temperature regimes where these reaction rates have the most significant impact on the synthesized amount of 56Ni. Thousands of stellar evolution calculations using MESA reveal that the 12 C(α, γ) 16 O reaction and triple-α reaction are most influential at T 10 8.4 K. This work offers a potential observational pathway to constrain reaction rates that are otherwise difficult to measure experimentally.

A neutron star is born as a hot, lepton-rich protoneutron star (PNS) and cools via neutrino emission, eventually allowing heavy ions in the outer layers to crystallize into a solid crust. We develop a simple analytic estimate for the onset time of this crust formation during the late, post-convective PNS cooling phase. Using a diffusion-based neutrino luminosity and the resulting entropy evolution together with an approximately isentropic interior structure, we obtain the time-dependent density and temperature at the neutrinosphere. We then impose the Coulomb crystallization condition for heavy nuclei, expressed through the Coulomb coupling parameter, and determine when the neutrinosphere temperature first falls below the crystallization threshold evaluated at the neutrinosphere density. This procedure yields closed expressions for the entropy at crystallization and the corresponding crust-formation time, with explicit dependence on the PNS mass and radius, an effective diffusion/cooling normalization, and composition parameters such as the ionic charge Z and heavy-nuclei mass fraction. For canonical microphysics, we find that the first solid phase typically appears at t(crust) similar to 100-500 s. These closed-form scalings provide a useful late-time analytic benchmark for the onset of crust formation and clarify its dependence on PNS and composition parameters.

Abstract Fallback in core-collapse supernovae plays a central role in setting compact-remnant masses and may produce late-time emission. In hydrogen rich progenitors, the reverse shock arising at the hydrogen-helium interface can substantially enhance fallback, yet its overall impact across a broad explosion-energy range has not been systematically quantified. Using one-dimensional hydrodynamic simulations for metal-poor progenitors with M ZAMS = 18 – 28 M ⊙ and models with and without hydrogen envelopes, we explore fallback over explosion energies of 10 48 – 10 52 erg . We find a clear mass-transition behavior in this model suite: when the explosion energy reaches only 2 – 3 times the binding energy of the hydrogen envelope, the reverse shock returns to the center and sharply increases the remnant mass by > 2 M ⊙ . Above this threshold, the reverse shock escapes and hydrogen-rich and stripped-envelope progenitors yield nearly identical remnant masses. By normalizing the results with the envelope binding energy, we show that the progenitor models follow an approximately common fallback relation. We further provide a simple analytic prescription that connects explosion energy, hydrogen-envelope binding energy, and final compact-remnant mass. This relation provides an important link between progenitor properties and compact-remnant masses, and is useful for population-synthesis and galactic chemical-evolution studies.

HiZ-GUNDAM is a candidate for JAXA's competitive medium-class mission program, with its concept approved by ISAS/JAXA in 2018. This proposed satellite aims to play a leading role in time-domain astronomy in the 2030s by pursuing two primary scientific goals: (1) probing the early universe through the detection of high-redshift gamma-ray bursts (GRBs) and (2) enabling the rapid identification of X-ray and optical-near-infrared counterparts of multimessenger sources. To achieve these objectives, HiZ-GUNDAM is equipped with two key instruments. A wide-field X-ray monitor, EAGLE, utilizes a micropore optics array and a focal plane imaging sensor to observe transients across similar to 0.5 sr in the 0.4 to 4 keV energy range. To follow up on this observation, an optical-near-infrared telescope, MONSTER, features a 30 cm aperture and conducts simultaneous five-band photometry over the 0.5 to 2.5 mu m wavelength range. It employs a K & ouml;sters-type prism for multi-band photometry to follow up on transients detected by the EAGLE. A sun-synchronous dawn-dusk orbit has been selected to ensure thermal stability for the MONSTER. We present a comprehensive overview of the HiZ-GUNDAM mission concept. The mission is expected to make a significant contribution to our understanding of cosmic evolution through observations of high-redshift GRBs, as well as to the identification of the multiwavelength properties of multimessenger sources by enhancing the observational capabilities for transient searches. The specifications and concepts discussed herein are subject to refinement as the mission progresses.

Fallback in core-collapse supernovae plays a central role in setting compact-remnant masses and may produce late-time emission. In hydrogen rich progenitors, the reverse shock arising at the hydrogen-helium interface has the potential to dramatically enhance fallback, yet its overall impact across a broad explosion-energy range has not been systematically quantified. Using one-dimensional hydrodynamic simulations for metal-poor progenitors with M_ ZAMS=18-28 M_⊙ and models with and without hydrogen envelopes, we explore fallback over explosion energies of 10^48-10^52 erg. We find a robust and universal mass-transition behaviour: when the explosion energy reaches only 2-3 times the binding energy of the hydrogen envelope, the reverse shock returns to the centre and sharply increases the remnant mass by ≳ 2 M_⊙. Above this threshold, the reverse shock escapes and hydrogen-rich and stripped-envelope progenitors yield nearly identical remnant masses. By normalizing the results with the envelope binding energy, we show that all progenitor models converge to a common fallback relation. We further provide a simple analytic prescription that connects explosion energy, hydrogen-envelope binding energy, and final compact-remnant mass. This relation provides an important link between progenitor properties and compact-remnant masses, and is useful for population-synthesis and galactic chemical-evolution studies.

Neutrinos from supernovae, especially those emitted during the late phase of core collapse, are essential for understanding the final stages of massive star evolution. We have been dedicated to developing methods for the analysis of neutrinos emitted during the late phase and observed at Super-Kamiokande (SK). Our previous studies have successfully demonstrated the potential of various analysis methods in extracting essential physical properties; however, the lack of background consideration has limited their practical application. In this study, we address this issue by incorporating a realistic treatment of the experimental signal and background events with the on-going SK experiment. We therefore optimize our analysis framework to reflect realistic observational conditions, including both signal and background events. Using this framework we study several long-time supernova models, simulating the late phase neutrino observation in SK and focusing in particular on the identification of the last observed event. We discuss the possibility of model discrimination methods using timing information from this last observed event.

Simulations of presupernova evolution suggest that intense O-shell burning can be so active that, in extreme cases, it can merge with the outer C-burning shell, changing the initial conditions for the supernova explosion. However, such violent activity in the interior of stars has been difficult to confirm from observations of stars. Here we propose that the elemental composition of O-rich ejecta in supernova remnants (SNRs) can be a tool to test for this kind of intense shell burning activity in the final stages of progenitor evolution. As an example, we discuss the origin of “Mg-rich” ejecta in the SNR N49B. A high Mg/Ne mass ratio ≳ 1 suggests that the Ne- or O-burning shell has broken into or merged with the outer shell before the collapse. Such Mg-rich (or Ne-poor) ejecta has been identified in some other SNRs, supporting the idea that some destratification process, such as a shell merger, does indeed occur in the interiors of some massive stars, although they may not be the majority. Our results suggest that X-ray observations of O-rich ejecta in core-collapse SNRs will be a unique tool to probe the shell burning activity in the stellar interior during the final weeks to days of a massive star’s evolution.

A key question in astronomy is how ubiquitous Earth-like rocky planets are. The formation of terrestrial planets in our Solar System was strongly influenced by the radioactive decay heat of short-lived radionuclides (SLRs), particularly 26 Al (aluminum-26), likely delivered from nearby supernovae. However, current models struggle to reproduce the abundance of SLRs inferred from meteorite analysis without destroying the protosolar disk. We propose the “immersion” mechanism, where cosmic-ray nucleosynthesis in a supernova shockwave reproduces estimated SLR abundances at a supernova distance (~1 parsec), preserving the disk. We estimate that solar mass stars in star clusters typically experience at least one such supernova within 1 parsec, supporting the feasibility of this scenario. This suggests that Solar System–like SLR abundances and terrestrial planet formation are more common than previously thought.

Neutrinos are pivotal signals in multimessenger observations of supernovae (SNe). Recent advancements in the analysis method of supernova (SN) neutrinos, especially in quantitative analysis, have significantly broadened scientific possibilities. This study demonstrates the feasibility of estimating distances to SNe using neutrinos. This estimation utilizes the direct relationship between the radius of a neutron star (NS) and the distance to the supernova, which is analogous to main-sequence fitting. The radius of an NS is determined with an approximate uncertainty of 10% through observations such as X-rays and gravitational waves. By integrating this information, the distance to the supernova can be estimated with an uncertainty of within 15% at a 95% confidence level. It has been established that neutrinos can pinpoint the direction of supernovae, and when combined with distance estimates, three-dimensional localization becomes achievable. This capability is vital for follow-up observations using multimessenger approaches. Moreover, more precise distance determinations to SNe through follow-up observations, such as optical observations, allow for accurate measurements of neutron-star radii. This data, via the neutron-star mass–radius relationship, could provide various insights into nuclear physics.

We investigate neutrino signals associated with black hole formation resulting from the gravitational collapse of massive stars, motivated by the candidate failed supernova M31-2014-DS1 in the Andromeda Galaxy (M31). By compiling numerical simulation results for stellar collapse, we predict the expected neutrino emission and compare these predictions with observational limits from Super-Kamiokande (SK). The simulations reveal a characteristic precursor signal consisting of a short, intense burst whose average neutrino energy rises rapidly and then ceases abruptly once the black hole forms. We examine several nuclear equations of state, specifically the Lattimer & Swesty, Shen, Togashi, and SFHo models, to evaluate how the emission depends on neutron-star properties and nuclear-physics uncertainties. Comparison of the predicted event counts with SK's non-detection of neutrinos coincident with M31-2014-DS1 already rules out part of the model space and highlights the sensitivity of current neutrino detectors to both progenitor mass and the EOS. These findings demonstrate the capability of neutrino astronomy to probe core collapse and black hole formation in failed supernova scenarios.

HiZ-GUNDAM is a future satellite mission whose mission concept was approved by ISAS/JAXA, and it is one of the future satellite candidates of JAXA's competitive medium-class mission. HiZ-GUNDAM will lead time-domain astronomy in 2030s, and its key sciences are (1) exploration of the early universe with high-redshift gamma-ray bursts, and (2) contribution to the multi-messenger astronomy. Two mission payloads are aboard HiZ-GUNDAM to realize these two scientific issues. The wide field X-ray monitors which consist of Lobster Eye optics array and focal imaging sensor, monitor similar to 0.5 steradian field of view in 0.5-4 keV energy range. The near infrared telescope with an aperture size of 30 cm in diameter performs simultaneous 5-band photometric observation in 0.5-2.5 mu m wavelength with Koester's prism for X-ray transients discovered by Wide Field X-ray Monitor. In this paper, we introduce the mission overview of HiZ-GUNDAM while the information contained herein may change in future studies.

The early solar system contained a short-lived radionuclide, Al-26 (its half-life time t(1/2) = 0.7 Myr). The decay energy of Al-26 is thought to have controlled the thermal evolution of planetesimals and, possibly, the water contents of planets. Many hypotheses have been proposed for the origin of Al-26 in the solar system. One of the possible hypotheses is the "disk injection scenario": when the protoplanetary disk of the solar system had already formed, a nearby (<1 pc) supernova injected radioactive material directly into the disk. Such a Al-26 injection hypothesis has been tested so far with limited setups for disk structure and supernova distance, which have treated disk disruption and Al-26 injection separately. Here, we revisit this problem, to investigate whether there are self-consistent conditions under which the surviving disk radius can receive enough Al-26 to account for the abundance in the early solar system. We also consider a range of disk masses and structures, Al-26 yields from supernova, and a large dust mass fraction eta d. We find that Al-26 yields of supernova are required as greater than or similar to 2.1x10(-3)M(circle dot)(eta d/0.2)(-1) , which are challenging to achieve with the known possible Al-26 ejection and dust mass fraction ranges. Furthermore, we find that even if the above conditions are met, the supernova flow changes the disk temperature, which may not be consistent with the solar system record. Our results place a strong constraint on the disk injection scenario. Rather, we suggest that the fresh Al-26 of the early solar system must have been synthesized/injected in other ways.

This paper provides collapses of massive, fully convective, and non-rotating white dwarfs (WDs) formed by accretion-induced collapse or merger-induced collapse and the subsequent explosions with the general relativistic neutrino-radiation hydrodynamics simulations. We produce initial WDs in hydrostatic equilibrium, which have super-Chandrasekhar mass and are about to collapse. The WDs have masses of 1.6$M_\odot$ with different initial central densities specifically at $10^{10}$, $10^{9.6}$, $10^{9.3}$ and $10^{9.0}\,{\rm g\,cm^{-3}}$. First, we check whether initial WDs are stable without weak interactions. Second, we calculate the collapse of WDs with weak interactions. We employ hydrodynamics simulations with Newtonian gravity in the first and second steps. Third, we calculate the formation of neutron stars and accompanying explosions with general relativistic simulations. As a result, WDs with the highest density of $10^{10}\,{\rm g\,cm^{-3}}$ collapse not by weak interactions but by the photodissociation of the iron, and three WDs with low central densities collapse by the electron capture as expected at the second step and succeed in the explosion with a small explosion energy of $\sim 10^{48}$ erg at the third step. By changing the surrounding environment of WDs, we find that there is a minimum value of ejecta masses being $\sim 10^{-5}M_{\odot}$. With the most elaborate simulations of this kind so far, the value is one to two orders of magnitude smaller than previously reported values and is compatible with the estimated ejecta mass from FRB~121102.

A new analysis method for supernova model identification using supernova neutrino observations in Super-Kamiokande (SK) is developed. This new method uses information on late-phase neutrinos observed in SK, such as the duration of supernova neutrino and average neutrino energy. In this paper, we report the evaluation results of the supernova identification performance, demonstrating 90% or more in identification performance.

ABSTRACT Nuclear reactions are key to our understanding of stellar evolution, particularly the $^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$ rate, which is known to significantly influence the lower and upper ends of the black hole (BH) mass distribution due to pair-instability supernovae (PISNe). However, these reaction rates have not been sufficiently determined. We use the mesa stellar evolution code to explore the impact of uncertainty in the $^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$ rate on PISN explosions, focusing on nucleosynthesis and explosion energy by considering the high resolution of the initial mass. Our findings show that the mass of synthesized radioactive nickel (56Ni) and the explosion energy increase with $^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$ rate for the same initial mass, except in the high-mass edge region. With a high (about twice the starlib standard value) rate, the maximum amount of nickel produced falls below 70 M⊙, while with a low rate (about half of the standard value) it increases up to 83.9 M⊙. These results highlight that carbon ‘preheating’ plays a crucial role in PISNe by determining core concentration when a star initiates expansion. Our results also suggest that the onset of the expansion, which means the end of compression, competes with collapse caused by helium photodisintegration, and the maximum mass that can lead to an explosion depends on the $^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$ reaction rate.

Details of the core-collapse supernova (CCSN) explosion mechanism still need to be fully understood. There is an increasing number of successful examples of reproducing explosions in multidimensional hydrodynamic simulations, but subsequent studies pointed out that the growth rates of the explosion energy $\dot{E}_\mathrm{expl}$ of these simulations are insufficient to produce enough $^{56}$Ni to match observations. This issue is known as the `$^{56}$Ni problem' in CCSNe. Recently, however, some studies have suggested that this $^{56}$Ni problem is derived from the simplicity of the explosion model. In response, we investigate the effect of the explosion energy growth rate $\dot{E}_\mathrm{expl}$ on the behavior of nucleosynthesis in CCSNe in a more realistic model. We employ the 1D Lagrangian hydrodynamic code, in which we take neutrino heating and cooling terms into account with the light-bulb approximation. We reiterate that, consistent with previous rebuttal studies, there is the $^{56}$Ni problem: Although $^{56}$Ni is synthesized to almost the same mass coordinate independent of $\dot{E}_\mathrm{expl}$, some of the innermost material in the low-$\dot{E}_\mathrm{expl}$ model failed to escape, leading to a shift in the innermost mass coordinate of the ejecta to the outer positions. Comparing our results with observations, we find that while modern slow explosions can, in principle, reproduce observations of standard Type II SNe, this is not possible with stripped-envelope SNe. Our finding places a strong constraint on the explosion mechanism. There are significant differences in the progenitor structures and the explosion mechanism between Type II and stripped-envelope SNe.

Details of the core-collapse supernova (CCSN) explosion mechanism still need to be fully understood. There is an increasing number of successful examples of reproducing explosions in multidimensional hydrodynamic simulations, but subsequent studies pointed out that the growth rates of the explosion energy Ė_expl of these simulations are insufficient to produce enough ^56Ni to match observations. This issue is known as the `^56Ni problem' in CCSNe. Recently, however, some studies have suggested that this ^56Ni problem is derived from the simplicity of the explosion model. In response, we investigate the effect of the explosion energy growth rate Ė_expl on the behavior of nucleosynthesis in CCSNe in a more realistic model. We employ the 1D Lagrangian hydrodynamic code, in which we take neutrino heating and cooling terms into account with the light-bulb approximation. We reiterate that, consistent with previous rebuttal studies, there is the ^56Ni problem: Although ^56Ni is synthesized to almost the same mass coordinate independent of Ė_expl, some of the innermost material in the low-Ė_expl model failed to escape, leading to a shift in the innermost mass coordinate of the ejecta to the outer positions. Comparing our results with observations, we find that while modern slow explosions can, in principle, reproduce observations of standard Type II SNe, this is not possible with stripped-envelope SNe. Our finding places a strong constraint on the explosion mechanism. There are significant differences in the progenitor structures and the explosion mechanism between Type II and stripped-envelope SNe.