arXiv subjects

Sean M. Couch

Publications and source records attributed to Sean M. Couch.

At least 19 recordsLinked to original sources

arXiv2026

Three-dimensional Core-Collapse Supernova Simulations: From shell burning to shock revival

The outcome of core-collapse supernova simulations depends sensitively on the multi-dimensional structure of the progenitor star at the onset of collapse. We perform three-dimensional simulations of the final ~10-15 minutes of stellar evolution for five non-rotating solar-metallicity progenitors with zero-age main-sequence masses of 20, 21.5, 24.5, 26, and 29 solar masses, mapped from one-dimensional MESA models into the FLASH hydrodynamics code. Convection develops in the oxygen-rich layers of all five models, with convective velocities reaching several hundred km/s, and in some models strong convection also develops in the inner silicon- and oxygen-burning shells. For the 24.5 solar mass progenitor, we carry out three core-collapse simulations: one initialised from the fully three-dimensional model, one from its angle-averaged counterpart, and one from the original one-dimensional MESA progenitor. We find that the multi-dimensional progenitor leads to 35 to 50% higher non-radial kinetic energy in the post-shock region and an average shock radius 5 to 10% larger than in the angle-averaged model, and shows the earliest shock revival of the three. The gravitational-wave emission is similar in all three models and strengthens after shock revival, driven by a change in the downflows reaching the protoneutron star rather than by progenitor asymmetries.

astro-ph.HE↗

arXiv2026

A Three-Dimensional Exploration of Magnetic Fields, Rotation, and Shock Revival in a $39 M_\odot$ Core-Collapse Supernova Progenitor

We present three-dimensional hydrodynamic and magnetohydrodynamic core-collapse supernova simulations of a rapidly rotating, high-compactness $39 M_\odot$ progenitor to investigate the roles of rotation and magnetic fields in shock revival and outflow morphology. This study is designed to separate neutrino-driven expansion, rotation-induced deformation, and magnetically aided polar outflow within the same progenitor. We evolve three models: a non-rotating hydrodynamic baseline, a rotating hydrodynamic model, and a rotating magnetized model. All three models reach runaway shock expansion within the simulated interval, but with markedly different morphologies and timescales. The magnetized model revives first and develops the clearest bipolar outflow. The rotating non-magnetized model undergoes the latest shock revival and remains comparatively compact at the end of the simulation. The non-rotating model also undergoes shock revival, but subsequently collapses to a black hole about one second after core bounce. In the magnetized model, Maxwell stresses redistribute angular momentum and extract energy from the differential rotation of the protoneutron star, reducing the inner-core spin and helping channel rotational free energy into the emerging polar outflow. Neutrino emission provides an additional, though smaller, angular-momentum sink in both rotating models. We find that rapid rotation and strong magnetic fields can launch an early magnetically aided polar outflow in 3D, while the resulting dynamics remain intrinsically non-axisymmetric. In this extreme progenitor, rotation also provides significant support against prompt black-hole formation, although the longer-term remnant stability remains uncertain beyond the simulated interval.

astro-ph.HE↗

arXiv2026

Black Hole Supernovae Outcomes Across a Wide Progenitor Range

Black hole supernovae (BHSNe), the term we use for core-collapse events in which black hole (BH) formation occurs after shock revival but before the explosion is complete, have emerged as a natural outcome of multidimensional simulations as these calculations have been extended to seconds after bounce. Yet they remain one of the least studied outcomes of core collapse. Here, we assess whether they are confined to the most compact and massive progenitors, whose birth rates are low, or whether they arise systematically across a wider range of progenitor structures. We perform 23 long-term axisymmetric core-collapse simulations of progenitors spanning 19.51-60$\,M_\odot$ and compactnesses $0.31 \lesssim \xi_{2.5} \lesssim 0.63$. We find 18 BHSN outcomes across nearly the full ZAMS mass range considered, corresponding to progenitors with $0.40 \lesssim \xi_{2.5} \lesssim 0.63$. BH formation occurs between $\sim0.7$ s and $\sim4.4$ s after bounce. After BH formation, we continue the evolution with an excision treatment to at least 5000 s. The final explosion energies span $\sim2\times10^{49}$-$3\times10^{51}$ erg, while the final BH gravitational masses span $\sim3$-$26\,M_\odot$. We find a clear remnant-mass trend with CO-core mass, but show that the CO core alone is not an adequate proxy for the final BH mass, especially for progenitors at the low- and high-mass ends of the CO-core distribution. Except for the highest CO-core mass models, no single spherical mass coordinate cleanly separates ejecta from remnant material. Finally, a 2D axisymmetric and a 3D model are compared as we discuss differences between the two geometries.

astro-ph.HE↗

arXiv2026

The Gravitational-Wave Power Gap in Core-Collapse Supernovae: Insights from 60 Axisymmetric Simulations

We analyse the gravitational-wave emission from 60 two-dimensional core-collapse supernova simulations. The models cover a range of progenitors and equations of state. We focus on the narrow frequency interval in the gravitational-wave spectrum where the emitted power is strongly suppressed (the power gap) and how its central frequency relates to the physical properties of the simulations. We find that the power-gap frequency exhibits strong and systematic correlations with the properties of the inner core of the forming neutron star, for example the sound speed, suggesting that the gap encodes information about the behaviour of matter at extreme densities. We further examine how well several mechanisms proposed in the literature account for the presence and evolution of the gap in our simulations. Finally, we explore a scenario in which the gap arises from destructive interference between a narrow oscillation mode and a broadband background signal, demonstrating that such an interaction can produce a sharp minimum in the emitted gravitational-wave power.

astro-ph.HE↗

arXiv2026

thornado+Flash-X: A Hybrid DG-IMEX and Finite-Volume Framework for Neutrino-Radiation Hydrodynamics in Core-Collapse Supernovae

We present neutrino-transport algorithms implemented in the toolkit for high-order neutrino-radiation hydrodynamics (thornado) and their coupling to self-gravitating hydrodynamics within the adaptive mesh refinement (AMR)-based multiphysics simulation framework Flash-X. thornado, developed primarily for simulations of core-collapse supernovae (CCSNe), employs a spectral, six-species two-moment formulation with algebraic closure and special-relativistic observer corrections accurate to $O(v/c)$, and uses discontinuous Galerkin (DG) methods for phase-space discretization combined with implicit-explicit time stepping. A key development is a nonlinear neutrino-matter coupling algorithm based on nested fixed-point iteration with Anderson acceleration, enabling fully implicit treatment of collisional processes, including energy-coupling interactions such as neutrino-electron scattering and pair production. Coupling to finite-volume (FV) hydrodynamics is achieved with a hybrid DG-FV representation of the fluid variables and operator-split evolution in Flash-X. The implementation is verified using basic transport tests with idealized opacities and relaxation and deleptonization problems with tabulated microphysics. Spherically symmetric CCSN simulations demonstrate accuracy and robustness of the coupled scheme, including close agreement with the CCSN simulation code Chimera. An axisymmetric CCSN simulation further demonstrates the viability of DG-based neutrino transport for multidimensional supernova modeling within Flash-X. thornado's neutrino-transport solver is GPU-enabled using OpenMP offloading or OpenACC, and all CCSN applications included in this work use the GPU implementation. Together, these results establish a foundation for future enhancements in physics fidelity, numerical algorithms, and computational performance, for increasingly realistic large-scale CCSN simulations.

astro-ph.IM↗

arXiv2024

Physics-driven Explosions of Stripped High-Mass Stars: Synthetic Light Curves and Spectra of Stripped-Envelope Supernovae with Broad Lightcurves

Stripped-envelope supernovae (SESNe) represent a significant fraction of core-collapse supernovae, arising from massive stars that have shed their hydrogen and, in some cases, helium envelopes. The origins and explosion mechanisms of SESNe remain a topic of active investigation. In this work, we employ radiative-transfer simulations to model the light curves and spectra of a set of explosions of single, solar-metallicity, massive Wolf-Rayet (WR) stars with ejecta masses ranging from 4 to 11 Msun, that were computed from a turbulence-aided and neutrino-driven explosion mechanism. We analyze these synthetic observables to explore the impact of varying ejecta mass and helium content on observable features. We find that the light curve shape of these progenitors with high ejecta masses is consistent with observed SESNe with broad light curves but not the peak luminosities. The commonly used analytic formula based on rising bolometric light curves overestimates the ejecta mass of these high-initial-mass progenitor explosions by a factor up to 2.6. In contrast, the calibrated method by Haynie et al., which relies on late-time decay tails, reduces uncertainties to an average of 20% within the calibrated ejecta mass range.Spectroscopically, the He I 1.083 um line remains prominent even in models with as little as 0.02 Msun of helium. However, the strength of the optical He I lines is not directly proportional to the helium mass but instead depends on a complex interplay of factors such as 56Ni distribution, composition, and radiation field. Thus, producing realistic helium features requires detailed radiative transfer simulations for each new hydrodynamic model.

astro-ph.HE↗

arXiv2024

Black Hole Supernovae, their Equation of State Dependence and Ejecta Composition

Recent literature on core-collapse supernovae suggests that a black hole (BH) can form within $\sim 1$ s of shock revival, while still culminating in a successful supernova. We refer to these as black hole supernovae, as they are distinct from other BH formation channels in both timescale and impact on the explosion. We simulate these events self-consistently from core-collapse until $20\text{-}50$ days after collapse using three axisymmetric models of a $60$ M$_\odot$ zero-age main sequence progenitor star and investigate how the composition of the ejecta is impacted by the BH formation. We employ Skyrme-type equations of state (EOSs) and vary the uncertain nucleonic effective mass, which affects the pressure inside the proto-neutron star through the thermal part of the EOS. This results in different BH formation times and explosion energies at BH formation, yielding final explosion energies between $0.06\text{-}0.72\times 10^{51}$ erg with $21.8\text{-}23.3$ M$_\odot$ of ejecta, of which $0\text{-}0.018$ M$_\odot$ is $^{56}$Ni. Compared to expectations from 1D simulations, we find a more nuanced EOS dependence of the explosion dynamics, the mass of the BH remnant, and the elemental composition of the ejecta. We investigate why the explosions survive despite the massive overburden and link the shape of the diagnostic energy curve and character of the ejecta evolution to the progenitor structure.

astro-ph.HE↗

arXiv2024

Gray Two-moment Neutrino Transport: Comprehensive Tests and Improvements for Supernova Simulations

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. 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. 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.

astro-ph.HE↗

arXiv2023

Neutrino Fast Flavor Instability in three dimensions for a Neutron Star Merger

The flavor evolution of neutrinos in core collapse supernovae and neutron star mergers is a critically important unsolved problem in astrophysics. Following the electron flavor evolution of the neutrino system is essential for calculating the thermodynamics of compact objects as well as the chemical elements they produce. Accurately accounting for flavor transformation in these environments is challenging for a number of reasons, including the large number of neutrinos involved, the small spatial scale of the oscillation, and the nonlinearity of the system. We take a step in addressing these issues by presenting a method which describes the neutrino fields in terms of angular moments. We apply our moment method to neutron star merger conditions and show it simulates fast flavor neutrino transformation in a region where this phenomenon is expected to occur. By comparing with particle-in-cell calculations we show that the moment method is able to capture the three phases of growth, saturation, and decoherence, and correctly predicts the lengthscale of the fastest growing fluctuations in the neutrino field.

hep-ph↗

arXiv2023

Two-Moment Neutrino Flavor Transformation with applications to the Fast Flavor Instability in Neutron Star Mergers

Multi-messenger astrophysics has produced a wealth of data with much more to come in the future. This enormous data set will reveal new insights into the physics of core collapse supernovae, neutron star mergers, and many other objects where it is actually possible, if not probable, that new physics is in operation. To tease out different possibilities, we will need to analyze signals from photons, neutrinos, gravitational waves, and chemical elements. This task is made all the more difficult when it is necessary to evolve the neutrino component of the radiation field and associated quantum-mechanical property of flavor in order to model the astrophysical system of interest -- a numerical challenge that has not been addressed to this day. In this work, we take a step in this direction by adopting the technique of angular-integrated moments with a truncated tower of dynamical equations and a closure, convolving the flavor-transformation with spatial transport to evolve the neutrino radiation quantum field. We show that moments capture the dynamical features of fast flavor instabilities in a variety of systems, although our technique is by no means a universal blueprint for solving fast flavor transformation. To evaluate the effectiveness of our moment results, we compare to a more precise particle-in-cell method. Based on our results, we propose areas for improvement and application to complementary techniques in the future.

astro-ph.HE↗

arXiv2023

Characterizing the Directionality of Gravitational Wave Emission from Matter Motions within Core-collapse Supernovae

We analyze the directional dependence of the gravitational wave (GW) emission from 15 3D neutrino radiation hydrodynamic simulations of core-collapse supernovae. Using spin weighted spherical harmonics, we develop a new analytic technique to quantify the evolution of the distribution of GW emission over all angles. We construct a physics-informed toy model that can be used to approximate GW distributions for general ellipsoid-like systems, and use it to provide closed form expressions for the distribution of GWs for different CCSN phases. Using these toy models, we approximate the PNS dynamics during multiple CCSN stages and obtain similar GW distributions to simulation outputs. When considering all viewing angles, we apply this new technique to quantify the evolution of preferred directions of GW emission. For nonrotating cases, this dominant viewing angle drifts isotropically throughout the supernova, set by the dynamical timescale of the protoneutron star. For rotating cases, during core bounce and the following tens of ms, the strongest GW signal is observed along the equator. During the accretion phase, comparable -- if not stronger -- GW amplitudes are generated along the axis of rotation, which can be enhanced by the low T/|W| instability. We show two dominant factors influencing the directionality of GW emission are the degree of initial rotation and explosion morphology. Lastly, looking forward, we note the sensitive interplay between GW detector site and supernova orientation, along with its effect on detecting individual polarization modes.

astro-ph.HE↗

arXiv2023

Inferring Type II-P Supernova Progenitor Masses from Plateau Luminosities

Connecting observations of core-collapse supernova explosions to the properties of their massive star progenitors is a long-sought, and challenging, goal of supernova science. Recently, Barker et al. (2022) presented bolometric light curves for a landscape of progenitors from spherically symmetric neutrino-driven core-collapse supernova (CCSN) simulations using an effective model. They find a tight relationship between the plateau luminosity of the Type II-P CCSN light curve and the terminal iron core mass of the progenitor. Remarkably, this allows us to constrain progenitor properties with photometry alone. We analyze a large observational sample of Type II-P CCSN light curves and estimate a distribution of iron core masses using the relationship of Barker et al 2022. The inferred distribution matches extremely well with the distribution of iron core masses from stellar evolutionary models, and namely, contains high-mass iron cores that suggest contributions from very massive progenitors in the observational data. We use this distribution of iron core masses to infer minimum and maximum mass of progenitors in the observational data. Using Bayesian inference methods to locate optimal initial mass function parameters, we find M$_{\mathrm{min}}=9.8^{+0.37}_{-0.27}$ and M$_{\mathrm{max}}=24.0^{+3.9}_{-1.9}$ solar masses for the observational data.

astro-ph.HE↗

arXiv2022

Comparison of Electron Capture Rates in the N=50 Region using 1D Simulations of Core-collapse Supernovae

Recent studies have highlighted the sensitivity of core-collapse supernovae (CCSNe) models to electron-capture (EC) rates on neutron-rich nuclei near the N=50 closed-shell region. In this work, we perform a large suite of one-dimensional CCSN simulations for 200 stellar progenitors using recently updated EC rates in this region. For comparison, we repeat the simulations using two previous implementations of EC rates: a microphysical library with parameterized N=50 rates (LMP), and an older independent-particle approximation (IPA). We follow the simulations through shock revival up to several seconds post-bounce, and show that the EC rates produce a consistent imprint on CCSN properties, often surpassing the role of the progenitor itself. Notable impacts include the timescale of core collapse, the electron fraction and mass of the inner core at bounce, the accretion rate through the shock, the success or failure of revival, and the properties of the central compact remnant. We also compare the observable neutrino signal of the neutronization burst in a DUNE-like detector, and find consistent impacts on the counts and mean energies. Overall, the updated rates result in properties that are intermediate between LMP and IPA, and yet slightly more favorable to explosion than both.

astro-ph.HE↗

arXiv2022

Hydrodynamic Simulations of Electron-capture Supernovae: Progenitor and Dimension Dependence

We present neutrino-transport hydrodynamic simulations of electron-capture supernovae (ECSNe) in \texttt{FLASH} with new two-dimensional (2D) collapsing progenitor models. These progenitor models feature the 2D modelling of oxygen-flame propagation until the onset of core collapse. We perform axisymmetric simulations with 6 progenitor models that, at the time of collapse, span a range of propagating flame front radii. For comparison, we also perform a simulation with the same setup using the canonical, spherically-symmetrical progenitor model n8.8. We found that the variations in the progenitor models inherited from simulations of stellar evolution and flame propagation do not significantly alter the global properties of the neutrino-driven ECSN explosion, such as the explosion energy ($\sim1.36$-$1.48\times10^{50}$ erg) and the mass ($\sim0.017$-$0.018M_\odot$) and composition of the ejecta. Due to aspherical perturbations induced by the 2D flame, the ejecta contains a small amount ($\lesssim1.8\times10^{-3}~M_\odot$) of low-$Y_e$ ($0.35<Y_e<0.4$) component. The baryonic mass of the protoneutron star is $\sim1.34~M_\odot$ ($\sim1.357~M_\odot$) with the new (n8.8) progenitor models when simulations end at $\sim400$ ms and the discrepancy is due to updated weak-interaction rates in the progenitor evolutionary simulations. Our results reflect the nature of ECSN progenitors containing a strongly degenerate ONeMg core and suggest a standardized ECSN explosion initialized by ONeMg core collapse. Moreover, we carry out a rudimentary three-dimensional simulation and find that the explosion properties are fairly compatible with the 2D counterpart. Our paper facilitates a more thorough understanding of ECSN explosions following the ONeMg core collapse, though more three-dimensional simulations are still needed.

astro-ph.HE↗

arXiv2022

Connecting the Light Curves of Type IIP Supernovae to the Properties of their Progenitors

Observations of core-collapse supernovae (CCSNe) reveal a wealth of information about the dynamics of the supernova ejecta and its composition but very little direct information about the progenitor. Constraining properties of the progenitor and the explosion requires coupling the observations with a theoretical model of the explosion. Here, we begin with the CCSN simulations of Couch et al 2020 ApJ 890 127, which use a non-parametric treatment of the neutrino transport while also accounting for turbulence and convection. In this work we use the SuperNova Explosion Code to evolve the CCSN hydrodynamics to later times and compute bolometric light curves. Focusing on SNe IIP, we then (1) directly compare the theoretical STIR explosions to observations and (2) assess how properties of the progenitor's core can be estimated from optical photometry in the plateau phase alone. First, the distribution of plateau luminosities (L$_{50}$) and ejecta velocities achieved by our simulations is similar to the observed distributions. Second, we fit our models to the light curves and velocity evolution of some well-observed SNe. Third, we recover well-known correlations, as well as the difficulty of connecting any one SN property to zero-age main sequence mass. Finally, we show that there is a usable, linear correlation between iron core mass and L$_{50}$ such that optical photometry alone of SNe IIP can give us insights into the cores of massive stars. Illustrating this by application to a few SNe, we find iron core masses of 1.3-1.5 solar masses with typical errors of ~0.05 solar masses. Data are publicly available online (\url{https://doi.org/10.5281/zenodo.6631964}).

astro-ph.HE↗

arXiv2021

Gravitational-wave Signature of a First-order Quantum Chromodynamics Phase Transition in Core-Collapse Supernovae

A first-order quantum chromodynamics (QCD) phase transition (PT) may take place in the protocompact star (PCS) produced by a core-collapse supernova (CCSN). In this work, we study the consequences of such a PT in a non-rotating CCSN with axisymmetric hydrodynamic simulations. We find that the PT leads to the collapse of the PCS and results in a loud burst of gravitational waves (GWs). The amplitude of this GW burst is $\sim30$ times larger than the post-bounce GW signal normally found for non-rotating CCSN. It shows a broad peak at high frequencies ($\sim2500-4000$ Hz) in the spectrum, has a duration of $\lesssim5 {\rm ms}$, and carries $\sim3$ orders of magnitude more energy than the other episodes. Also, the peak frequency of the PCS oscillation increases dramatically after the PT-induced collapse. In addition to a second neutrino burst, the GW signal, if detected by the ground-based GW detectors, is decisive evidence of the first-order QCD PT inside CCSNe and provides key information about the structure and dynamics of the PCS.

astro-ph.HE↗

arXiv2021

Determining the Structure of Rotating Massive Stellar Cores with Gravitational Waves

The gravitational wave (GW) signal resulting from stellar core collapse encodes a wealth of information about the physical parameters of the progenitor star and the resulting core-collapse supernova (CCSN). We present a novel approach to constrain CCSN progenitor properties at collapse using two of the most detectable parts of the GW signal: the core-bounce signal and evolution of the dominant frequency mode from the protoneutron star. We focus on the period after core bounce but before explosion and investigate the predictive power of GWs from rotating CCSNe to constrain properties of the progenitor star. We analyze 34 2D and four 3D neutrinoradiation-hydrodynamic simulations of stellar core collapse in progenitors of varied initial mass and rotation rate. Extending previous work, we verify the compactness of the progenitor at collapse to correlate with the early ramp-up slope, and in rotating cases, also with the core angular momentum. Combining this information with the bounce signal, we present a new analysis method to constrain the pre-collapse core compactness of the progenitor. Because these GW features occur less than a second after core bounce, this analysis could allow astronomers to predict electromagnetic properties of a resulting CCSN even before shock breakout.

astro-ph.HE↗

arXiv2021

Equation of State Dependence of Gravitational Waves in Core-Collapse Supernovae

Gravitational waves (GWs) provide unobscured insight into the birthplace of neutron stars (NSs) 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 CCSNe 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, $m^\star$, which is crucial in setting the thermal dependence of the EOS, and (2) the isoscalar incompressibility modulus, $K_{\rm{sat}}$. While $K_{\rm{sat}}$ 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 $m^\star$ 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 $\sim$1250 Hz, we investigate its origin and report its EOS dependence.

astro-ph.HE↗