Core-collapse Simulations Research Articles

Turbulence and the characteristics of circumstellar discs

ABSTRACT We investigate the properties of circumstellar discs (CDs) produced in hydrodynamical simulations of gravoturbulent core collapse considering Kolmogorov and Burger-type turbulence. We report that massive discs are more prevalent in the Kolmogorov regime than for Burger-type turbulence. A significant number of discs are formed with a radius of ∼15 au in both cases. However, the number of extended discs with radii >15 au is significantly larger in case of Kolmogorov turbulence. The two regimes of turbulence, in general, yield disc radii in the ranges of 7−30 au and 13−39 au, respectively. The corresponding ranges of the disc masses are 30.37 MJup−0.92 M⊙ and 2.09 MJup−0.13 M⊙, respectively. Moreover, the ratio Mdisc/Mstar is higher in models of Kolmogorov-type turbulence than in models of Burgers-type turbulence. We do not find any correlation between Rdisc and Mdisc over the explored range of initial temperatures (8−14 K) and the type of turbulence. Also, for these initial thermal variations, the turbulent CD structures do not exhibit signs of turbulent diffusion. Nonetheless, both sub- and supersonic velocity dispersions cause variations in the specific angular momentum of infalling gas, especially for CDs with radii ∼ 16−21 au. The radial profiles of CDs do not correlate with the initial conditions.

Three-dimensional GRMHD simulations of rapidly rotating stellar core collapse

ABSTRACT We present results from fully general relativistic (GR), three-dimensional (3D), neutrino-radiation magneto-hydrodynamic (MHD) simulations of stellar core collapse of a 20 M⊙ star with spectral neutrino transport. Our focus is to study the gravitational-wave (GW) signatures from the magnetorotationally (MR)-driven models. By parametrically changing the initial angular velocity and the strength of the magnetic fields in the core, we compute four models. Among our models, only those with cores having an initial magnetic field strength of 1012 G and rotation rates of 1 or 2 rad s−1 produce MHD jets. Seen from the direction perpendicular to the rotational axis, a characteristic waveform is obtained exhibiting a monotonic time increase in the wave amplitude. As previously identified, this stems from the propagating MHD outflows along the axis. We show that the GW amplitude from anisotropic neutrino emission becomes more than one order-of-magnitude bigger than that from the matter contribution, whereas seen from the rotational axis, both of the two components are in the same order-of-magnitudes. Due to the memory effect, the frequency of the neutrino GW from our full-fledged 3D-MHD models is in the range less than ∼10 Hz. Toward the future GW detection for a Galactic core-collapse supernova, if driven by the MR mechanism, the planned next-generation detector as DECIGO is urgently needed to catch the low-frequency signals.

Progenitor Stars Calculated with Small Reaction Networks should not be Used as Initial Conditions for Core Collapse

Core collapse initial conditions are a bottleneck in understanding the explosion mechanism(s) of massive stars. Stellar evolution codes struggle after carbon burning, and either stop or adopt numerical simplifications missing crucial physics. The use of small nuclear reaction networks (NRN) that account for energy production but bypass weak reactions is typical, but insufficient to study the dynamics of the collapse. We advise against the use of progenitors computed with small NRN in expensive multidimensional simulations of core collapse, bounce, (jet formation), and explosion.

Modeling the secular evolution of embedded protoplanetary disks

Context. Protoplanetary disks are known to form around nascent stars from their parent molecular cloud as a result of angular momentum conservation. As they progressively evolve and dissipate, they also form planets. While a lot of modeling efforts have been dedicated to their formation, the question of their secular evolution, from the so-called class 0 embedded phase to the class II phase where disks are believed to be isolated, remains poorly understood. Aims. We aim to explore the evolution between the embedded stages and the class II stage. We focus on the magnetic field evolution and the long-term interaction between the disk and the envelope. Methods. We used the GPU accelerated code IDEFIX to perform a 3D, barotropic, non ideal magnetohydrodynamic (MHD) secular core collapse simulation that covers the system evolution from the collapse of the pre-stellar core until 100 kyr after the first hydrostatic core formation and the disk settling while ensuring sufficient vertical and azimuthal resolutions (down to 10−2 au) to properly resolve the disk internal dynamics and non axisymmetric perturbations. Results. The disk evolution leads to a power-law gas surface density in Keplerian rotation that extends up to a few 10 au. The magnetic flux trapped in the disk during the initial collapse decreases from 100 mG at disk formation down to 1 mG by the end of the simulation. After the formation of the first hydrostatic core, the system evolves in three phases. A first phase with a small (∼10 au), unstable, strongly accreting (∼ 10−5 M⊙ yr−1) disk that loses magnetic flux over the first 15 kyr, a second phase where the magnetic flux is advected with a smooth, expanding disk fed by the angular momentum of the infalling material, and a final phase with a gravitationally regulated ∼60 au disk accreting at at few 10−7 M⊙ yr−1. The initial isotropic envelope eventually feeds large-scale vertically extended accretion streamers, with accretion rates similar to that onto the protostar (∼ 10−6 M⊙ yr−1). Some of the streamer material collides with the disk’s outer edge and produces accretion shocks, but a significant fraction of the material lands on the disk surface without producing any noticeable discontinuity. Conclusions. While the initial disk size and magnetization are set by magnetic braking, self-gravity eventually drives accretion, so that the disk ends up in a gravitationally regulated state. This evolution from magnetic braking to self-gravity is due to the weak coupling between the gas and the magnetic field once the disk has settled. The weak magnetic field at the end of the class I phase (Bz ∼ 1 mG) is a result of the magnetic flux dilution in the disk as it expands from its initial relatively small size. This expansion should not be interpreted as a viscous expansion, as it is driven by newly accreted material from large-scale streamers with large specific angular momentum.

Exploring the dust grain size and polarization mechanism in the hot and massive Class 0 disk IRAS 16293-2422 B

Context. Multiwavelength dust continuum and polarization observations arising from self-scattering have been used to investigate grain sizes in young disks. However, the likelihood of self-scattering being the polarization mechanism in embedded disks decreases for very highly optically thick disks and makes us reconsider some of the size constraints from polarization, particularly for younger and more massive disks. The 1.3 mm polarized emission detected toward the hot (≳400 K) Class 0 disk IRAS 16293-2422 B has been attributed to self-scattering, with predictions of bare grain sizes between 200 and 2000 µm. Aims. We aim to investigate the effects of changing the maximum grain sizes in the resultant continuum and continuum polarization fractions from self-scattering for a hot and massive Class 0 disk extracted from numerical simulations of prestellar core collapse and to compare them with IRAS 16293 B observations. Methods. We compared new and archival dust continuum and polarization observations at high resolution between 1.3 and 18 mm to a set of synthetic models. We developed a new publicly available tool to automate this process called Synthesizer. This tool is an easy-to-use program for generating synthetic observations from numerical simulations. Results. Optical depths are in the range of 130 to 2 from 1.3 to 18 mm, respectively. Predictions of significant grain growth populations, including amax = 1000 µm, are comparable to the observations from IRAS 16293 B at all observed wavelengths. The polarization fraction produced by self-scattering reaches a maximum of approximately 0.1% at 1.3 mm for a maximum grain size of 100 µm, which is an order of magnitude lower than the grain size observed toward IRAS 16293 B. Conclusions. From comparison of the Stokes I fluxes, we conclude that significant grain growth could be present in the young Class 0 disk IRAS 16293 B, particularly in the inner hot region (< 10 au, T > 300 K) where refractory organics evaporate. The polarization produced by self-scattering in our model is not high enough to explain the observations at 1.3 and 7 mm, and such effects as dichroic extinction and polarization reversal of elongated aligned grains remain other possible but untested scenarios.

Neutron Star Kicks and Implications for Their Rotation at Birth

Neutron stars are born out of core-collapse supernovae, and they are imparted natal kicks at birth as a consequence of asymmetric ejection of matter and possibly neutrinos. Unless the force resulting from the kicks is exerted exactly at their center, it will also cause the neutron star to rotate. In this paper, we discuss the possibility that neutron stars may receive off-center natal kicks at birth, which imprint a natal rotation. In this scenario, the observed pulsar spin and transverse velocity in the Galaxy are expected to correlate. We develop a model of the natal rotation imparted to neutron stars and constrain it by the observed population of pulsars in our Galaxy. When considering a single-kick position parameter, we find that the location of the off-center kick is km at 90% confidence, and is robust when considering pulsars with different observed periods, transverse velocities, and ages. Nonetheless, the model encounters challenges in effectively fitting the data, particularly at small transverse velocities, prompting the exploration of alternative models that include more complex physics. Our framework could be used as a guide for core-collapse simulations of massive stars.

Collapsar Black Holes Are Likely Born Slowly Spinning

Collapsing stars constitute the main black hole (BH) formation channel, and are occasionally associated with the launch of relativistic jets that power γ-ray bursts (GRBs). Thus, collapsars offer an opportunity to infer the natal (before spin-up/down by accretion) BH spin directly from observations. We show that once the BH saturates with a large-scale magnetic flux, the jet power is dictated by the BH spin and mass accretion rate. Core-collapse simulations by Halevi et al. and GRB observations favor stellar density profiles that yield an accretion rate of , weakly dependent on time. This leaves the spin as the main factor that governs the jet power. By comparing the jet power to characteristic GRB luminosities, we find that the majority of BHs associated with jets are likely born slowly spinning with a dimensionless spin of a ≃ 0.2, or a ≃ 0.5 for wobbling jets, with the main uncertainty originating in the unknown γ-ray radiative efficiency. This result could be applied to the entire core-collapse BH population, unless an anticorrelation between the stellar magnetic field and angular momentum is present. In a companion paper, Jacquemin-Ide et al., we show that regardless of the natal spin, the extraction of BH rotational energy leads to spin-down to a ≲ 0.2, consistent with gravitational-wave observations. We verify our results by performing the first 3D general-relativistic magnetohydrodynamic simulations of collapsar jets with characteristic GRB energies, powered by slowly spinning BHs. We find that jets of typical GRB power struggle to escape from the star, providing the first numerical indication that many jets fail to generate a GRB.

Aiolos – A multipurpose 1D hydrodynamics code for planetary atmospheres

ABSTRACT We present a new 1D multiphysics simulation code with use cases intended for, but not limited to, hydrodynamic escape problems of planetary atmospheres and planetary accretion models. Our formulation treats an arbitrary number of species as separated hydrodynamic fields, couples them via friction laws, allows for a multiband flux-limited radiation transport, and tracks ionization fronts in high-energy irradiation bands. Besides coupling various known numerical solution techniques together, we improve on the numerical stability of deep hydrostatic atmospheres by using a well-balanced scheme, hence preventing unphysical driving of atmospheric in- or outflow. We demonstrate the correct physical behaviour of the individual code modules and present a few simple, new applications, such as a proof-of-concept simulations of combined core-powered mass-loss and UV-driven atmospheric escape, along with a fully time-dependent core-collapse giant planet simulation. The multispecies nature of the code opens up the area of exploring simulations that are agnostic towards the dominant atmospheric species and can lead to implementations of advanced planetary evolution schemes.

Comparison of the Core-collapse Evolution of Two Nearly Equal-mass Progenitors

We compare the core-collapse evolution of a pair of 15.8 M ☉ stars with significantly different internal structures, a consequence of the bimodal variability exhibited by massive stars during their late evolutionary stages. The 15.78 and 15.79 M ☉ progenitors have core masses (masses interior to an entropy of 4 k B baryon−1) of 1.47 and 1.78 M ☉ and compactness parameters ξ 1.75 of 0.302 and 0.604, respectively. The core-collapse simulations are carried out in 2D to nearly 3 s postbounce and show substantial differences in the times of shock revival and explosion energies. The 15.78 M ☉ model begins exploding promptly at 120 ms postbounce when a strong density decrement at the Si–Si/O shell interface, not present in the 15.79 M ☉ progenitor, encounters the stalled shock. The 15.79 M ☉ model takes 100 ms longer to explode but ultimately produces a more powerful explosion. Both the larger mass accretion rate and the more massive core of the 15.79 M ☉ model during the first 0.8 s postbounce time result in larger ν e / luminosities and RMS energies along with a flatter and higher-density heating region. The more-energetic explosion of the 15.79 M ☉ model resulted in the ejection of twice as much 56Ni. Most of the ejecta in both models are moderately proton rich, though counterintuitively the highest electron fraction (Y e = 0.61) ejecta in either model are in the less-energetic 15.78 M ☉ model, while the lowest electron fraction (Y e = 0.45) ejecta in either model are in the 15.79 M ☉ model.

LSTM and CNN application for core-collapse supernova search in gravitational wave real data

Context. Core-collapse supernovae (CCSNe) are expected to emit gravitational wave signals that could be detected by current and future generation interferometers within the Milky Way and nearby galaxies. The stochastic nature of the signal arising from CCSNe requires alternative detection methods to matched filtering. Aims. We aim to show the potential of machine learning (ML) for multi-label classification of different CCSNe simulated signals and noise transients using real data. We compared the performance of 1D and 2D convolutional neural networks (CNNs) on single and multiple detector data. For the first time, we tested multi-label classification also with long short-term memory (LSTM) networks. Methods. We applied a search and classification procedure for CCSNe signals, using an event trigger generator, the Wavelet Detection Filter (WDF), coupled with ML. We used time series and time-frequency representations of the data as inputs to the ML models. To compute classification accuracies, we simultaneously injected, at detectable distance of 1 kpc, CCSN waveforms, obtained from recent hydrodynamical simulations of neutrino-driven core-collapse, onto interferometer noise from the O2 LIGO and Virgo science run. Results. We compared the performance of the three models on single detector data. We then merged the output of the models for single detector classification of noise and astrophysical transients, obtaining overall accuracies for LIGO (~99%) and (~80%) for Virgo. We extended our analysis to the multi-detector case using triggers coincident among the three ITFs and achieved an accuracy of ~98%.

A force explosion condition for spherically symmetric core-collapse supernovae

ABSTRACT Understanding which stars explode leaving behind neutron stars and which stars collapse forming black holes remains a fundamental astrophysical problem. We derive an analytic explosion condition for spherically symmetric core-collapse supernovae. The derivation starts with the exact governing equations, considers the balance of integral forces, includes the important dimensionless parameters, and includes an explicit set of self-consistent approximations. The force explosion condition is $\tilde{L}_\nu \tau _g - 0.06 \tilde{\kappa } \gt 0.38$, and only depends upon two dimensionless parameters. The first compares the neutrino power deposited in the gain region with the accretion power, $\tilde{L}_\nu \tau _g = L_{\nu } \tau _g R_{\rm NS}/ (G \dot{M} M_{\rm NS})$. The second, $\tilde{\kappa } = \kappa \dot{M} / \sqrt{G M_{\rm NS} R_{\rm NS}}$, parametrizes the neutrino optical depth in the accreted matter near the neutron star surface. Over the years, many have proposed approximate explosion conditions: the critical neutrino-luminosity, ante-sonic, and time-scale conditions. We are able to derive these other conditions from the force explosion condition, which unifies them all. Using numerical, steady-state and fully hydrodynamic solutions, we test the explosion condition. The success of these tests is promising in two ways. One, the force explosion condition helps to illuminate the underlying physics of explosions. Two, this condition may be a useful explosion diagnostic for more realistic, three-dimensional radiation hydrodynamic core-collapse simulations.

On the Origin of Pulsar and Magnetar Magnetic Fields

Abstract In order to address the generation of neutron star magnetic fields, with particular focus on the dichotomy between magnetars and radio pulsars, we consider the properties of dynamos as inferred from other astrophysical systems. With sufficiently low (modified) Rossby number, convective dynamos are known to produce dipole-dominated fields whose strength scales with convective flux, and we argue that these expectations should apply to the convective protoneutron stars (PNSs) at the centers of core-collapse supernovae. We analyze a suite of three-dimensional simulations of core collapse, featuring a realistic equation of state and full neutrino transport, in this context. All our progenitor models, ranging from 9 M ⊙ to 25 M ⊙, including one with initial rotation, have sufficiently vigorous PNS convection to generate dipole fields of order ∼1015 Gauss, if the modified Rossby number resides in the critical range. Thus, the magnetar/radio pulsar dichotomy may arise naturally in part from the distribution of core rotation rates in massive stars.

SNEWPY: A Data Pipeline from Supernova Simulations to Neutrino Signals

Abstract Current neutrino detectors will observe hundreds to thousands of neutrinos from Galactic supernovae, and future detectors will increase this yield by an order of magnitude or more. With such a data set comes the potential for a huge increase in our understanding of the explosions of massive stars, nuclear physics under extreme conditions, and the properties of the neutrino. However, there is currently a large gap between supernova simulations and the corresponding signals in neutrino detectors, which will make any comparison between theory and observation very difficult. SNEWPY is an open-source software package that bridges this gap. The SNEWPY code can interface with supernova simulation data to generate from the model either a time series of neutrino spectral fluences at Earth, or the total time-integrated spectral fluence. Data from several hundred simulations of core-collapse, thermonuclear, and pair-instability supernovae is included in the package. This output may then be used by an event generator such as sntools or an event rate calculator such as the SuperNova Observatories with General Long Baseline Experiment Simulator (SNOwGLoBES). Additional routines in the SNEWPY package automate the processing of the generated data through the SNOwGLoBES software and collate its output into the observable channels of each detector. In this paper we describe the contents of the package, the physics behind SNEWPY, the organization of the code, and provide examples of how to make use of its capabilities.

The Character of Three-Dimensional Core-Collapse Simulation Results

We here in graphical form highlight some of the early phases in the explosion of core-collapse supernovae and the birth of neutron stars. The graphics presented summarizes the results of the almost fifty three-dimensional radiation/hydrodynamic simulations the Princeton group has simulated during the last three years using its state-of-the-art (though still evolving and improving) code Fornax.

Core-collapse simulation of SN 1987A binary progenitor and its multimessenger signals

We perform a three-dimensional self-consistent core-collapse supernova simulation using a binary evolution progenitor model of SN 1987A. This progenitor model is based on a slow-merger of 14 M∹ and 9 M∹ stars and it satisfies most of the observational constraints such as red-to-blue evolution, lifetime, total mass and position in the Hertzsprung-Russell diagram at collapse, and chemical anomalies. We find that this progenitor model successfully present explosion and leave a 1.53 M∹ neutron star with a kick velocity of 70kms−1 and a spin period of 0.1 s. Assuming a detector sensitivity of Kamiokande-II and the distance to the supernova of 51 kpc, we obtain 16 neutrino detection events in one second. Some characteristic modes in its gravitational wave signal are also discussed in this article.

Revisiting the explodability of single massive star progenitors of stripped-envelope supernovae

Stripped-envelope supernovae (Types IIb, Ib, and Ic) that show little or no hydrogen comprise roughly one-third of the observed explosions of massive stars. Their origin and the evolution of their progenitors are not yet fully understood. Very massive single stars stripped by their own winds (≳25−30 M⊙ at solar metallicity) are considered viable progenitors of these events. However, recent 1D core-collapse simulations show that some massive stars may collapse directly into black holes after a failed explosion, with a weak or no visible transient. In this Letter, we estimate the effect of direct collapse into a black hole on the rates of stripped-envelope supernovae that arise from single stars. For this, we compute single-star MESA models at solar metallicity and map their final state to their core-collapse outcome following prescriptions commonly used in population synthesis. According to our models, no single stars that have lost their entire hydrogen-rich envelope are able to explode, and only a fraction of progenitors left with a thin hydrogen envelope do (IIb progenitor candidates), unless we use a prescription that takes the effect of turbulence into account or invoke increased wind mass-loss rates. This result increases the existing tension between the single-star paradigm to explain most stripped-envelope supernovae and their observed rates and properties. At face value, our results point toward an even higher contribution of binary progenitors to stripped-envelope supernovae. Alternatively, they may suggest inconsistencies in the common practice of mapping different stellar models to core-collapse outcomes and/or higher overall mass loss in massive stars.

SNEWPY: A Data Pipeline from Supernova Simulations to Neutrino Signals

Current neutrino detectors will observe hundreds to thousands of neutrinos from a Galactic supernovae, and future detectors will increase this yield by an order of magnitude or more. With such a data set comes the potential for a huge increase in our understanding of the explosions of massive stars, nuclear physics under extreme conditions, and the properties of the neutrino. However, there is currently a large gap between supernova simulations and the corresponding signals in neutrino detectors, which will make any comparison between theory and observation very difficult. SNEWPY is an open-source software package which bridges this gap. The SNEWPY code can interface with supernova simulation data to generate from the model either a time series of neutrino spectral fluences at Earth, or the total time-integrated spectral fluence. Data from several hundred simulations of core-collapse, thermonuclear, and pair-instability supernovae is included in the package. This output may then be used by an event generator such as sntools or an event rate calculator such as SNOwGLoBES. Additional routines in the SNEWPY package automate the processing of the generated data through the SNOwGLoBES software and collate its output into the observable channels of each detector. In this paper we describe the contents of the package, the physics behind SNEWPY, the organization of the code, and provide examples of how to make use of its capabilities.

Neutrinos from the cosmic noon: a probe of the cosmic star formation history

Multiple astrophysical probes of the cosmic star formation history (CSFH) indicate a period of peak star formation known as the cosmic noon between 1.5 ≲ z ≲ 3.5.In this work, we explore the potential of future measurements of the diffuse supernova neutrino background (DSNB) at the Hyper-Kamiokande (HK) experiment to be sensitive to variations in the expected star formation rate at cosmic noon. Motivated by a statistically mild discrepancy between Hα and UV/IR probes of the CSFH, which indicate a factor of ∼ 3 difference in the CSFH near the cosmic noon, we construct two benchmark hypotheses based on these different data sets. Since the cosmic noon neutrinos are redshifted to low energies, a low threshold sensitivity of any detector will be critical for discriminating between these benchmark hypotheses. We explore whether HK loaded with Gadolinium, with a positron threshold energy of 10 MeV, would be sensitive to the difference between these two CSFH hypotheses. Assuming that the supernova (SN) neutrino spectrum can be well determined by calibrating core-collapse SNe simulations to observations of nearby SNe, we find that only for very high core-collapse SN temperatures, HK would be sensitive to the difference in the benchmark CSFHs, and even then, over a 10–30 year time scale. Future experiments must lower their thresholds to be more sensitive to cosmic noon neutrinos.

Determining the Structure of Rotating Massive Stellar Cores with Gravitational Waves

Abstract 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 neutrino-radiation-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.

The Role of Core-collapse Physics in the Observability of Black Hole Neutron Star Mergers as Multimessenger Sources

Abstract Recent 1D core-collapse simulations indicate a nonmonotonicity of the explodability of massive stars with respect to their precollapse core masses, which is in contrast to commonly used prescriptions. In this work, we explore the implications of these results on the formation of coalescing black hole (BH)–neutron star (NS) binaries. Furthermore, we investigate the effects of natal kicks and the NS’s radius on the synthesis of such systems and potential electromagnetic counterparts (EMCs) linked to them. Models based on 1D core-collapse simulations result in a BH–NS merger detection rate ( ∼ 2.3 yr−1), 5–10 times larger than the predictions of “standard” prescriptions. This is primarily due to the formation of low-mass BHs via direct collapse, and hence no natal kicks, favored by the 1D simulations. The fraction of observed systems that will produce an EMC, with the supernova engine from 1D simulations, ranges from 2% to 25%, depending on the NS equation of state. Notably, in most merging systems with EMCs, the NS is the first-born compact object, as long as the NS’s radius is ≲ 12 km. Furthermore, models with negligible kicks for low-mass BHs increase the detection rate of GW190426_152155-like events to ∼ 0.6 yr−1, with an associated probability of EMC ≤10% for all supernova engines. Finally, models based on 1D core-collapse simulations predict a ratio of BH–NSs to binary BHs’ merger rate density that is at least twice as high as other prescriptions, but at the same time overpredicting the measured local merger density rate of binary black holes.