Neutrino Oscillations in Core-Collapse Supernovae and Neutron Star Mergers

Fast-pairwise neutrino oscillations potentially affect many aspects of core-collapse supernova (CCSN): the explosion mechanism, neutrino signals, and nucleosynthesis in the ejecta. This particular mode of collective neutrino oscillations has a deep connection to the angular structure of neutrinos in momentum space; for instance, the appearance of electron neutrinos lepton number (ELN) angular crossings in momentum space is a good indicator of occurrences of the flavor conversions. However, many multi-dimensional (multi-D) CCSN simulations are carried out with approximate neutrino transport (such as two-moment methods), which limits the access to the angular distributions of neutrinos, i.e., inhibits ELN-crossing searches. In this paper, we develop a new method of ELN-crossing search in these CCSN simulations. The required data is the zero-th and first angular moments of neutrinos and matter profile, all of which are available in CCSN models with two-moment method. One of the novelties of our new method is to use a ray-tracing neutrino transport to determine ELNs in the direction of the stellar center. It is designed to compensate for shortcomings of the crossing searches only with the two angular moments. We assess the capability of the method by carrying out a detailed comparison to results of full Boltzmann neutrino transport in 1D and 2D CCSN models. We find that the ray-tracing neutrino transport improves the accuracy of crossing searches; indeed, the appearance/disappearance of the crossings is accurately detected even in the region of forward-peaked angular distributions. The new method is computationally cheap and has a benefit of efficient parallelization; hence, it will be useful for ELN-crossing searches in any CCSN models employed two-moment neutrino transport.

Core-collapse supernovae (CCSN) mark the end of the life of massive stars and are cosmic laboratories for physics at the extremes. Numerical simulations of these explosions are essential to understanding the complex mechanisms that are involved. All four fundamental interactions have to be taken into account, which requires the combined knowledge of astrophysics, nuclear physics, particle physics, and observations. A key ingredient in simulations is the equation of state (EOS), which determines the contraction behavior of the proto-neutron star (PNS), and thus impacts neutrino energies and explosion dynamics. However, the EOS for hot and dense matter is still not fully understood and CCSN simulations rely on phenomenological EOS models that differ in their underlying theory as well as nuclear physics input. In this thesis, we investigate the impact of uncertainties in the EOS in CCSN simulations. Further, we present an extension of the high-density EOS models to lower densities and temperatures, which enables us to perform long-time simulations of CCSN, following the shock evolution up to several seconds after bounce. In the first part of this thesis, we present the first systematic study on the effect of different nuclear matter properties of the EOS in CCSN simulations. We investigate the impact of varying the nucleon effective mass, incompressibility, symmetry energy, and nuclear saturation point on the PNS contraction and its implication on the shock evolution. This allows us to examine possible reasons for differences in simulations with commonly used EOS models. We find that the contraction behavior of the PNS is mainly governed by the effective mass, which determines the thermal nucleonic contributions to the EOS. Larger effective masses result in smaller pressures at nuclear densities and a lower thermal index. This modifies the density, and thus the PNS contraction behavior, and consequently the shock propagation. We observe that variations in the symmetry energy impact the electron fraction, entropy, and temperature in the PNS interior. Our results suggest that differences among CCSN EOS mainly originate from their different nuclear matter properties. We verify that our models give reasonable modifications to the mass-radius relation of cold neutron stars and further investigate details of the explosion dynamics. Moreover, our EOS models are tested in different CCSN simulation codes, which yield similar results. Finally, we show that the choice of neutrino treatment impacts the PNS interior. In the second part, we perform long-time CCSN simulations that follow the shock evolution several seconds after bounce, which requires a large simulation domain. To this end, we present a formalism for a high-density EOS transition to lower densities and temperatures. This formalism is tested for various EOS models and different progenitors in spherical symmetry. Additionally, we verify its functionality in cylindrical symmetry and for several neutrino transport schemes. With the transition, we perform the first long-time CCSN simulations in FLASH for exploding models, following the shock expansion up to five seconds after bounce. Different CCSN scenarios are investigated, varying the rotational profile and the explosion energetics by enhancing the neutrino energy deposition in the neutrino leakage scheme. We find that additional rotation and heating favors neutrino-driven winds, which impacts the diagnostic energy. Our results indicate that rotation decreases the mass accretion and reduces neutrino luminosities, as suggested in previous studies. Moreover, the results are compared to simulations performed with an M1 neutrino transport scheme. This allows us to analyze differences in the electron fraction, which need to be considered for future nucleosynthesis studies.

In the most extreme astrophysical environments, such as core-collapse supernovae (CCSNe) and neutron star mergers (NSMs), neutrinos can undergo fast flavor conversions (FFCs) on exceedingly short scales. Intensive simulations have demonstrated that FFCs can attain equilibrium states in certain models. In this study, we utilize physics-informed neural networks (PINNs) to predict the asymptotic outcomes of FFCs, by specifically targeting the first two moments of neutrino angular distributions. This makes our approach suitable for state-of-the-art CCSN and NSM simulations. Through effective feature engineering and the incorporation of customized loss functions that penalize discrepancies in the predicted total number of νe and ν¯e, our PINNs demonstrate remarkable accuracies, with an error margin of ≲3%. Our study represents a substantial leap forward in the potential incorporation of FFCs into simulations of CCSNe and NSMs, thereby enhancing our understanding of these extraordinary astrophysical events. Published by the American Physical Society 2024

This is an exciting time for the study of r-process nucleosynthesis. Recently, a neutron star merger GW170817 was observed in extraordinary detail with gravitational waves and electromagnetic radiation from radio to γ rays. The very red color of the associated kilonova suggests that neutron star mergers are an important r-process site. Astrophysical simulations of neutron star mergers and core collapse supernovae are making rapid progress. Detection of both electron neutrinos and antineutrinos from the next galactic supernova will constrain the composition of neutrino-driven winds and provide unique nucleosynthesis information. Finally, FRIB and other rare-isotope beam facilities will soon have dramatic new capabilities to synthesize many neutron-rich nuclei that are involved in the r-process. The new capabilities can significantly improve our understanding of the r-process and likely resolve one of the main outstanding problems in classical nuclear astrophysics. However, to make best use of the new experimental capabilities and to fully interpret the results, a great deal of infrastructure is needed in many related areas of astronomy, astrophysics, and nuclear theory. We place these experiments in context by discussing astrophysical simulations and observations of r-process sites, observations of stellar abundances, galactic chemical evolution, and nuclear theory for the structure and reactions of very neutron-rich nuclei. This review paper was initiated at a three-week International Collaborations in Nuclear Theory program in June 2016, where we explored promising r-process experiments and discussed their likely impact, and their astronomical, astrophysical, and nuclear theory context.

I present a stochastic chemical evolution model to investigate the enrichment of the interstellar medium (ISM) during Galaxy formation. Contrary to classical chemical evolution models, it is able to resolve local chemical inhomogeneities in the ISM caused by single core-collapse supernovae. These inhomogeneities lead to different element abundance patterns in very metal-poor stars which can be seen as scatter in the abundances of halo stars with metallicities [Fe/H] . The early chemical evolution of the halo proceeds in different enrichment phases: At [Fe/H] , the halo ISM is unmixed and dominated by local inhomogeneities caused by individual core-collapse supernova events. For metallicities [Fe/H] the halo ISM is well mixed, showing an element abundance pattern integrated over the initial mass function. In the range [Fe/H] a continuous transition from the unmixed to the well mixed ISM occurs. For some elements (Si, Ca), the scatter in element-to-iron ratios of metal-poor halo stars can be reproduced. Stellar yields of other elements, however, predict a scatter which, compared to observations, is too large (O, Mg) or too small (Ni). This shows, that inhomogeneous chemical evolution models are heavily dependent on theoretical nucleosynthesis yields of core-collapse supernovae. Hence inhomogeneous chemical evolution models present themselves as a test for stellar nucleosynthesis calculations. One problem revealed by the model is the predicted scatter in [O/Fe] and [Mg/Fe] which is too large compared to the one observed in metal-poor halo stars. This can be either due to the oxygen or magnesium yields or due to the iron yields (or both). However, oxygen and magnesium are -elements that are produced mainly during hydrostatic burning and thus are not affected by the theoretical uncertainties afflicting the collapse and explosion of a massive star. Stellar iron yields, on the other hand, depend heavily on the choice of the mass-cut between ejecta and proto-neutron star and are therefore very uncertain. In this work, iron yield distributions as function of progenitor mass are derived which are consistent with the abundance distribution of metal-poor halo stars and are in agreement with observed yields of core-collapse supernovae with known progenitor masses. The iron yields of lower-mass Type II supernovae (in ) are well constrained by these observations. Present observations, however, do not allow the range us to determine a unique solution for higher-mass Type II supernovae. Nevertheless, the main dependence of the stellar iron yields as function of progenitor mass can be derived and may be used as a constraint for future core-collapse supernova/hypernova models. A prediction of hypernova models which can be tested by future observations is the existence of ultra -element enhanced stars at metallicities [Fe/H] . The results are of importance for the earliest stages of galaxy formation when the ISM is dominated by local chemical inhomogeneities and the instantaneous mixing approximation is not valid. The astrophysical nature of r-process sites is a long standing mystery and many probable sources were suggested in the past, among them lower-mass core-collapse supernovae (in the range ), higher-mass core-collapse supernovae (with masses ) and neutron star mergers. In this work, I present a detailed inhomogeneous chemical evolution study that considers for the first time neutron star mergers as major rprocess sources, and compare this scenario to the ones in which core-collapse supernovae act as dominant r-process sites. Furthermore, the enrichment of the interstellar medium with neutron-capture elements during Galaxy formation by r- and s-process sources is investigated. I conclude that, due to the lack of reliable iron and r-process yields as function of progenitor mass, it is not possible to date to distinguish between the lower-mass and higher-mass supernovae scenario within the framework of inhomogeneous chemical evolution. However, neutron-star mergers seem to be ruled out as dominant r-process source, since their low coalescence rates are not consistent with observations of r-process elements at very low metallicities. Furthermore, the considerable injection of r-process material by a single neutron-star merger leads to a scatter in r-process abundances at later times which is much too large compared to observations. Finally, a low star-formation efficiency is required during halo formation to be consistent with the appearance of s-process elements at very low metallicities.

Aims. In this work we extended an energy-integrated neutrino transport method to facilitate efficient, yet precise, modeling of compact astrophysical objects. We particularly focus on core-collapse supernovae. Methods. We implemented a gray neutrino-transport framework from the literature into FLASH and performed a detailed evaluation of its accuracy in core-collapse supernova simulations. Based on comparisons with results from simulations using energy-dependent neutrino transport, we incorporated several improvements to the original scheme. Results. Our analysis shows that our gray neutrino transport method successfully reproduces key aspects from more complex energy-dependent transport across a variety of progenitors and equations of state. We find both qualitative and reasonable quantitative agreement with multi-group M1 transport simulations. However, the gray scheme tends to slightly favor shock revival. In terms of gravitational wave and neutrino signals, there is a good alignment with the energy-dependent transport, although we find 15–30% discrepancies in the average energy and luminosity of heavy-lepton neutrinos. Simulations using the gray transport are around four times faster than those using energy-dependent transport.

Neutrino transport in core collapse supernovae

Magnetohydrodynamic (MHD) turbulence in neutron star (NS) merger remnants can impact their evolution and multi-messenger signatures, complicating the interpretation of present and future observations. Due to the high Reynolds numbers and the large computational costs of numerical relativity simulations, resolving all the relevant scales of the turbulence will be impossible for the foreseeable future. Here, we adopt a method to include subgrid-scale turbulence in moderate resolution simulations by extending the large-eddy simulation (LES) method to general relativity (GR). We calibrate our subgrid turbulence model with results from very-high-resolution GRMHD simulations, and we use it to perform NS merger simulations and study the impact of turbulence. We find that turbulence has a quantitative, but not qualitative, impact on the evolution of NS merger remnants, on their gravitational wave signatures, and on the outflows generated in binary NS mergers. Our approach provides a viable path to quantify uncertainties due to turbulence in NS mergers.

We present a linear stability analysis of the fast-pairwise neutrino flavor conversion based on a result of our latest axisymmetric core-collapse supernova (CCSN) simulation with full Boltzmann neutrino transport. In the CCSN simulation, coherent asymmetric neutrino emissions of electron-type neutrinos (ν e) and their antiparticles ( ), in which the asymmetries of ν e and are anticorrelated with each other, occur at almost the same time as the onset of aspherical shock expansion. We find that the asymmetric neutrino emissions play a crucial role on occurrences of fast flavor conversions. The linear analysis shows that unstable modes appear in both pre- and post-shock flows; for the latter, they appear only in the hemisphere of higher emissions (the same hemisphere with stronger shock expansion). We analyze the characteristics of electron–lepton number (ELN) crossing in depth by closely inspecting the angular distributions of neutrinos in momentum space. The ELN crossing happens in various ways, and the property depends on the radius: in the vicinity of neutron star, (ν e) dominates over ν e ( ) in the forward (backward) direction; at the larger radius, the ELN crossing occurs in the opposite way. We also find that the non-radial ELN crossing occurs at the boundary between no ELN crossing and the radial one, which is an effect of genuine multi-dimensional transport. Our findings indicate that the collective neutrino oscillation may occur more commonly in CCSNe and suggest that the CCSN community needs to accommodate these oscillations self-consistently in the modeling of CCSNe.

We provide yields from 189 neutrino-driven core-collapse supernova (CCSN) simulations covering zero-age main sequence masses between 11 and $75\ \mathrm{M}_\odot$ and three different metallicities. Our CCSN simulations have two main advantages compared to previous methods used for applications in Galactic chemical evolution (GCE). First, the mass cut between remnant and ejecta evolves naturally. Secondly, the neutrino luminosities and thus the electron fraction are not modified. Both are key to obtain an accurate nucleosynthesis. We follow the composition with an in situ nuclear reaction network including the 16 most abundant isotopes and use the yields as input in a GCE model of the Milky Way. We adopt a GCE that takes into account infall of gas as well as nucleosynthesis from a large variety of stellar sources. The GCE model is calibrated to reproduce the main features of the solar vicinity. For the CCSN models, we use different calibrations and propagate the uncertainty. We find a big impact of the CCSN yields on our GCE predictions. We compare the abundance ratios of C, O, Ne, Mg, Si, S, Ar, Ca, Ti, and Cr with respect to Fe to an observational data set as homogeneous as possible. From this, we conclude that at least half of the massive stars have to explode to match the observed abundance ratios. If the explosions are too energetic, the high amount of iron will suppress the abundance ratios. With this, we demonstrate how GCE models can be used to constrain the evolution and death of massive stars.

The astrophysical nature of r-process sites is a long-standing mystery and many probable sources have been suggested, among them lower-mass core-collapse supernovae (in the range 8-10 M ○. ), higher-mass core-collapse supernovae (with masses >20 M ○. ) and neutron star mergers. In this work, we present a detailed inhomogeneous chemical evolution study that considers for the first time neutron star mergers as major r-process sources, and compare this scenario to the ones in which core-collapse supernovae act as dominant r-process sites. We conclude that, due to the lack of reliable iron and r-process yields as a function of progenitor mass, it is not possible at present to distinguish between the lower-mass and higher-mass supernovae scenarios within the framework of inhomogeneous chemical evolution. However, neutron-star mergers seem to be ruled out as the dominant r-process source, since their low rates of occurrence would lead to r-process enrichment that is not consistent with observations at very low metallicities. Additionally, the considerable injection of r-process material by a single neutron-star merger leads to a scatter in [r-process/Fe] ratios at later times which is much too large compared to observations.

The cosmic evolution of the neutron star merger (NSM) rate can be deduced from the observed cosmic star formation rate. This allows to estimate the rate expected in the horizon of the gravitational wave detectors advanced Virgo and ad LIGO and to compare those rates with independent predictions. In this context, the rapid neutron capture process, or r process, can be used as a constraint assuming NSM is the main astrophysical site for this nucleosynthetic process. We compute the early cosmic evolution of a typical r process element, Europium. Eu yields from NSM are taken from recent nucleosynthesis calculations. The same approach allows to compute the cosmic rate of Core Collapse SuperNovae (CCSN) and the associated evolution of Eu. We find that the bulk of Eu observations at high iron abundance can be rather well fitted by either CCSN or NSM scenarios. However, at lower metallicity, the early Eu cosmic evolution favors NSM as the main astrophysical site for the r process. A comparison between our calculations and spectroscopic observations at very low metallicities allows to constrain the coalescence timescale in the NSM scenario to about 0.1 to 0.2 Gyr. These values are in agreement with the coalescence timescales of some observed binary pulsars. Finally, the cosmic evolution of Eu is used to put constraints on the NSM rate, the merger rate in the horizon of the gravitational wave detectors advanced Virgo/ad LIGO, as well as the expected rate of electromagnetic counterparts to mergers (kilonovae) in large near-infrared surveys.

In dense neutrino gases, the neutrino-neutrino coherent forward scattering gives rise to a complex flavor oscillation phenomenon not fully incorporated in simulations of neutron star mergers (NSM) and core collapse supernovae (CCSNe). Moreover, it has been proposed to be chaotic, potentially limiting our ability to predict neutrino flavor transformations in simulations. To address this issue, we explore how small flavor perturbations evolve in the nonlinear regime of the neutrino quantum kinetic equation within a narrow centimeter-scale region inside a NSM and a toy neutrino distribution. Our findings reveal that paths in the flavor state space of solutions with similar initial conditions diverge exponentially, exhibiting chaos. This inherent chaos makes the microscopic scales of neutrino flavor transformations unpredictable. However, the domain-averaged neutrino density matrix remains relatively stable, with chaos minimally affecting it. This particular property suggests that domain-averaged quantities remain reliable despite the exponential amplification of errors. Published by the American Physical Society 2024

A comparative study of the $\\Lambda$ hyperon equations of state of Banik,\nHempel and Banyopadhyay (BHB) \\citep{bhb} and \\citet{shen11} (denoted as HShen\n$\\Lambda$) for core collapse supernova (CCSN) simulations is carried out in\nthis work. The dynamical evolution of a protoneutron star (PNS) into a black\nhole is investigated in core collapse supernova simulations in the general\nrelativistic one dimensional code using the BHB$\\Lambda \\phi$ and HShen\n$\\Lambda$ equation of state (EoS) tables and different progenitor models from\nWoosley and Heger \\citep{Woos}. Radial profiles of the mass fractions of\nbaryons, the density as well as the temperature in the PNS at different moments\nin time, are compared for both EoS tables. The behaviour of the central density\nof the PNS with time is demonstrated for those two $\\Lambda$ hyperon EoS tables\nand compared with their corresponding nuclear EoS tables. It is observed that\nthe black hole formation time is higher in the BHB$\\Lambda \\phi$ case than in\nthe HShen $\\Lambda$ EoS for the entire set of progenitor models adopted here,\nbecause the repulsive $\\Lambda$-$\\Lambda$ interaction makes the BHB$\\Lambda\n\\phi$ EoS stiffer. Neutrino emission with the $\\Lambda$ hyperon EoS ceases\nearlier than that of its nuclear counterpart. The long duration evolution of\nthe shock radius and gravitational mass of the PNS after a successful supernova\nexplosion with enhanced neutrino heating are studied with the BHB$\\Lambda \\phi$\nEoS and $s$20WH07 progenitor model. The PNS is found to remain stable for 4 s\nand might evolve into a cold neutron star.\n

ABSTRACTIn this paper, we present a novel method to estimate the time evolution of the proto-neutron star (PNS) structure from the neutrino signal in a core-collapse supernova (CCSN). Employing recent results from multidimensional CCSN simulations, we delve into a relation between the total emitted neutrino energy (TONE) and PNS mass/radius, and we find that they are strongly correlated with each other. We fit the relation by simple polynomial functions connecting the TONE to the mass and radius of the PNS as a function of time. By combining another fitting function representing the correlation between the TONE and the cumulative number of events at each neutrino observatory, the PNS mass and radius can be retrieved from purely observed neutrino data. We demonstrate retrievals of PNS mass and radius from mock data of the neutrino signal, and we assess the capability of our proposed method. While underlining the limitations of the method, we also discuss the importance of the joint analysis with the gravitational wave signal. This would reduce uncertainties of parameter estimations in our method, and may narrow down the possible neutrino oscillation model. The proposed method is a very easy and inexpensive computation, which will be useful in real data analysis of the CCSN neutrino signal.