arXiv subjects

Bernhard Müller

Publications and source records attributed to Bernhard Müller.

At least 37 records · Page 2Linked to original sources

arXiv2024

Universality in supernova gravitational waves with proto-neutron star properties

Gravitational wave signals from core-collapse supernovae are one of the important observables for extracting the information of dense matter. To extract the properties of proto-neutron stars produced via core-collapse supernovae by asteroseismology, we perform a linear perturbation analysis using data obtained from two-dimensional numerical simulations. We employ 12 and 20 solar-mass progenitors and compare two different treatments of gravity. One is a general relativistic one with a conformal flatness condition and the other is an effective gravitational potential mimicking the Tolman-Oppenheimer-Volkoff solution. We discuss how the frequencies of the proto-neutron star oscillations corresponding to the gravitational wave signals in the simulations depend on the proto-neutron star properties. In our models, we find that the gravitational wave frequencies of the proto-neutron stars determined with the Cowling approximation can be expressed to very good approximation as a function of the proto-neutron star average density almost independently of the progenitor mass, treatment of gravity in the simulations, and the interpolations in the simulations. On the other hand, if one considers the gravitational wave frequencies as a function of the surface gravity of proto-neutron stars, such a relation appears sensitive to the treatment of gravity and other numerical details in the simulations. Thus, the average density of proto-neutron stars seems more suitable for universally expressing the supernova gravitational wave frequencies, instead of the surface gravity.

astro-ph.HE↗

arXiv2024

Nucleosynthesis in the Innermost Ejecta of Magnetorotational Supernova Explosions in 3-dimensions

Core-collapse supernova (CCSN) explosions powered by rotation and magnetic fields present an interesting astrophysical site for nucleosynthesis that potentially contributes to the production of $r$-process elements. Here we present yields of the innermost ejecta in 3D magnetorotational CCSN models simulated using the CoCoNuT-FMT code. Strong magnetic fields tap the rotational energy of the proto-neutron star and lead to earlier and more energetic ($\sim 3\times 10^{51}$ erg) explosions than typical neutrino-driven CCSNe. Compared to a reference non-magnetic model, the ejecta in the magnetorotational models have much more neutron-rich components with Ye down to $\sim$0.25. Our post-processing calculations with the reaction network SkyNet show significant production of weak $r$-process elements up to mass number $\sim$130. We find negligible differences in the synthesis of heavy elements between two magnetorotational models with different initial field strength of 10$^{10}$ and 10$^{12}$ G, in accord with their similar explosion dynamics. The magnetorotational models produce about $\sim$0.19 and 0.14 Msun of radioactive $^{56}$Ni, on the low end of inferred hypernova nickel masses. The yields are publicly available at Zenodo: doi:10.5281/zenodo.10578981 for comparison with stellar abundance patterns, inclusion in modelling galactic chemical evolution, and comparison with other yield calculations. Our results add to the yet restricted corpus of nucleosynthesis yields from 3D magnetorotational supernova simulations and will help quantify yield uncertainties.

astro-ph.HE↗

arXiv2024

Supernova Simulations

Magnetohydrodynamic simulations of core-collapse supernovae have become increasingly mature and important in recent years. Magnetic fields take center stage in scenarios for explaining hypernova explosions, but are now also considered in supernova theory more broadly as an important factor even in neutrino-driven explosions, especially in the context of neutron star birth properties. Here we present an overview of simulation approaches currently used for magnetohydrodynamic supernova simulations and sketch essential physical concepts for understanding the role of magnetic fields in supernovae of slowly or rapidly rotating massive stars. We review progress on simulations of neutrino-driven supernovae, magnetorotational supernovae, and the relevant field amplification processes. Recent results on the nucleosynthesis and gravitational wave emission from magnetorotational supernovae are also discussed. We highlight efforts to provide better initial conditions for magnetohydrodynamic supernova models by simulating short phases of the progenitor evolution in 3D to address uncertainties in the treatment of rotation and magnetic fields in current stellar evolution models.

astro-ph.HE↗

arXiv2024

Determining the core-collapse supernova explosion mechanism with current and future gravitational-wave observatories

Gravitational waves are emitted from deep within a core-collapse supernova, which may enable us to determine the mechanism of the explosion from a gravitational-wave detection. Previous studies suggested that it is possible to determine if the explosion mechanism is neutrino-driven or magneto-rotationally powered from the gravitational-wave signal. However, long duration magneto-rotational waveforms, that cover the full explosion phase, were not available during the time of previous studies, and explosions were just assumed to be magneto-rotationally driven if the model was rapidly rotating. Therefore, we perform an updated study using new 3D long-duration magneto-rotational core-collapse supernova waveforms that cover the full explosion phase, injected into noise for the Advanced LIGO, Einstein Telescope and NEMO gravitational-wave detectors. We also include a category for failed explosions in our signal classification results. We then determine the explosion mechanism of the signals using three different methods: Bayesian model selection, dictionary learning, and convolutional neural networks. The three different methods are able to distinguish between neutrino-driven explosions and magneto-rotational explosions, even if the neutrino-driven explosion model is rapidly rotating. However they can only distinguish between the non-exploding and neutrino-driven explosions for signals with a high signal to noise ratio.

astro-ph.HE↗

arXiv2023

Gravitational Waves from a Core g-Mode in Supernovae as Probes of the High-Density Equation of State

Using relativistic supernova simulations of massive progenitor stars with a quark-hadron equation of state (EoS) and a purely hadronic EoS, we identify a distinctive feature in the gravitational-wave signal that originates from a buoyancy-driven mode (g-mode) below the proto-neutron star convection zone. The mode frequency lies in the range $200\lesssim f\lesssim 800\,\text{Hz}$ and decreases with time. As the mode lives in the core of the proto-neutron star, its frequency and power are highly sensitive to the EoS, in particular the sound speed around twice saturation density.

astro-ph.HE↗

arXiv2023

Synthetic Light Curves and Spectra from a Self-Consistent 2D Simulation of an Ultra-strippped Supernova

Spectroscopy is an important tool for providing insights into the structure of core-collapse supernova explosions. We use the Monte Carlo radiative transfer code ARTIS to compute synthetic spectra and light curves based on a two-dimensional explosion model of an ultra-stripped supernova. These calculations are designed both to identify observable fingerprints of ultra-stripped supernovae and as a proof-of-principle for using synthetic spectroscopy to constrain the nature of stripped-envelope supernovae more broadly. We predict very characteristic spectral and photometric features for our ultra-stripped explosion model, but find that these do not match observed ultra-stripped supernova candidates like SN 2005ek. With a peak bolometric luminosity of $6.8\times10^{41}\,\mathrm{erg}\,\mathrm{s}^{-1}$, a peak magnitude of $-15.9\,\mathrm{mag}$ in R-band, and $Δm_{15,\mathrm{R}}=3.50$, the model is even fainter and evolves even faster than SN 2005ek as the closest possible analogue in photometric properties. The predicted spectra are extremely unusual. The most prominent features are Mg II lines at 2,800 Angstrom and 4,500 Angstrom and the infrared Ca triplet at late times. The Mg lines are sensitive to the multi-dimensional structure of the model and are viewing-angle dependent. They disappear due to line blanketing by Fe group elements in a spherically averaged model with additional microscopic mixing. In future studies, multi-D radiative transfer calculations need to be applied to a broader range of models to elucidate the nature of observed Type Ib/c supernovae.

astro-ph.SR↗

arXiv2023

Light Curves of Type IIP Supernovae from Neutrino-driven Explosions of Red Supergiants Obtained by a Semi-analytic Approach

Type IIP supernovae (SNe IIP) mark the explosive death of red supergiants (RSGs), evolved massive stars with an extended hydrogen envelope. They are the most common supernova type and allow for benchmarking of supernova explosion models by statistical comparison to observed population properties rather than comparing individual models and events. We construct a large synthetic set of SNe IIP light curves (LCs) using the radiation hydrodynamics code \texttt{SNEC} and explosion energies and nickel masses obtained from an efficient semi-analytic model for two different sets of stellar progenitor models. By direct comparison we demonstrate that the semi-analytic model yields very similar predictions as alternative phenomenological explosion models based on one-dimensional simulations. We find systematic differences of a factor of $\mathord{\sim}2$ in plateau luminosities between the two progenitor sets due to different stellar radii, which highlights the importance of the RSG envelope structure as a major uncertainty in interpreting LCs of SNe IIP. A comparison to a volume-limited sample of observed SNe IIP shows decent agreement in plateau luminosity, plateau duration and nickel mass for at least one of the synthetic LC sets. The models, however, do not produce sufficient events with very small nickel mass $M_\mathrm{Ni}<0.01\,M_\odot$ and predict an anticorrelation between plateau luminosity and plateau duration that is not present in the observed sample, a result that warrants further study. Our results suggest that a better understanding of RSG stellar structure is no less important for reliably explaining the light curves of SNe IIP than the explosion physics.

astro-ph.HE↗

arXiv2023

Black holes as the end state of stellar evolution: Theory and simulations

The collapse of massive stars is one of the most-studied paths to black hole formation. In this chapter, we review black hole formation during the collapse of massive stars in the broader context of single and binary stellar evolution and the theory of supernova explosions. We provide a concise overview of the evolutionary channels that may lead to black hole formation -- the classical route of iron core collapse, collapse due to pair instability in very massive stars, and the hypothetical scenario of supermassive star collapse. We then review the current understanding of the parameter space for black hole formation and black hole birth properties that has emerged from theoretical and computational modelling of supernova explosions and transient observations. Finally, we discuss what the intricate interplay between stellar evolution, stellar explosions, and binary interactions implies for the formation of stellar-mass black holes.

astro-ph.HE↗

arXiv2023

Calibration of neutron star natal kick velocities to isolated pulsar observations

Current prescriptions for supernova natal kicks in rapid binary population synthesis simulations are based on fits of simple functions to single pulsar velocity data. We explore a new parameterization of natal kicks received by neutron stars in isolated and binary systems developed by Mandel & Müller, which is based on 1D and 3D supernova simulations and accounts for the physical correlations between progenitor properties, remnant mass, and the kick velocity. We constrain two free parameters in this model using very long baseline interferometry velocity measurements of Galactic single pulsars. We find that the inferred values of natal kick parameters do not differ significantly between single and binary evolution scenarios. The best-fit values of these parameters are $v_{\rm ns} = 520$ km s$^{-1}$ for the scaling pre-factor for neutron star kicks, and $σ_{\rm ns}=0.3$ for the fractional stochastic scatter in the kick velocities.

astro-ph.HE↗

arXiv2022

Differential Rotation in a 3D Simulation of Oxygen Shell Burning

We study differential rotation in late-stage shell convection in a 3D hydrodynamic simulation of a rapidly rotating $16M_\odot$ helium star with a particular focus on the convective oxygen shell. We find that the oxygen shell develops a quasi-stationary pattern of differential rotation that is neither described by uniform angular velocity as assumed in current stellar evolution models of supernova progenitors, nor by uniform specific angular momentum. Instead, the oxygen shell develops a positive angular velocity gradient with faster rotation at the equator than at the pole by tens of percent. We show that the angular momentum transport inside the convection zone is not adequately captured by a diffusive mixing-length flux proportional to the angular velocity or angular momentum gradient. Zonal flow averages reveal stable large-scale meridional flow and an entropy deficit near the equator that mirrors the patterns in the angular velocity. The structure of the flow is reminiscent of simulations of stellar surface convection zones and the differential rotation of the Sun, suggesting that similar effects are involved; future simulations will need to address in more detail how the interplay of buoyancy, inertial forces, and turbulent stresses shapes differential rotation during late-stage convection in massive stars. If convective regions develop positive angular velocity gradients, angular momentum could be shuffled out of the core region more efficiently, potentially making the formation of millisecond magnetars more difficult. Our findings have implications for neutron star birth spin periods and supernova explosion scenarios that involve rapid core rotation.

astro-ph.SR↗

arXiv2022

Stripped-envelope stars in different metallicity environments. II. Type I supernovae and compact remnants

Stripped-envelope stars can be observed as Wolf-Rayet (WR) stars, or as less luminous hydrogen-poor stars with low mass loss rates and transparent winds. Both types are potential progenitors of Type I core-collapse supernovae (SNe). We use grids of core-collapse models obtained from helium stars at different metallicities to study the effects of metallicity on the transients and remnants these stars produce. We characterise the surface and core properties of our core collapse models, and investigate their explodability employing three criteria. In cases where explosions are predicted, we estimate the ejecta mass, explosion energy, nickel mass and neutron star (NS) mass. Otherwise, we predict the mass of the resulting black hole (BH). We construct a simplified population model, and find that the properties SNe and compact objects depend strongly on metallicity. Ejecta masses and explosion energies for Type Ic SNe are best reproduced by models with Z=0.04 which exhibit strong winds during core helium burning. This implies that either their mass loss rates are underestimated, or that Type Ic SN progenitors experience mass loss through other mechanisms before exploding. The distributions of ejecta masses, explosion energies and nickel mass for Type Ib SNe are not well reproduced by progenitor models with WR mass loss, but are better reproduced if we assume no mass loss in progenitors with luminosities below the minimum WR star luminosity. We find that Type Ic SNe become more common as metallicity increases, and that the vast majority of progenitors of Type Ib SNe must be transparent-wind stripped-envelope stars. We find several models with pre-collapse CO-masses of up to $\sim 30 M_{\odot}$ may form $\sim 3 M_{\odot}$ BHs in fallback SNe. This may carry important consequences for our understanding of SNe, binary BH and NS systems, X-ray binary systems and gravitational wave transients.

astro-ph.SR↗

arXiv2022

Inferring Astrophysical Parameters of Core-Collapse Supernovae from their Gravitational-Wave Emission

Nearby core-collapse supernovae (CCSNe) are powerful multi-messenger sources for gravitational-wave, neutrino and electromagnetic telescopes as they emit gravitational waves in the ideal frequency band for ground based detectors. Once a CCSN gravitational-wave signal is detected, we will need to determine the parameters of the signal, and understand how those parameters relate to the source's explosion, progenitor and remnant properties. This is a challenge due to the stochastic nature of CCSN explosions, which is imprinted on their time series gravitational waveforms. In this paper, we perform Bayesian parameter estimation of CCSN signals using an asymmetric chirplet signal model to represent the dominant high-frequency mode observed in spectrograms of CCSN gravitational-wave signals. We use design sensitivity Advanced LIGO noise and CCSN waveforms from four different hydrodynamical supernova simulations with a range of different progenitor stars. We determine how well our model can reconstruct time-frequency images of the emission modes, and show how well we can determine parameters of the signal such as the frequency, amplitude, and duration. We show how the parameters of our signal model may allow us to place constraints on the proto-neutron star mass and radius, the turbulent kinetic energy onto the proto-neutron star, and the time of shock revival.

astro-ph.HE↗

arXiv2021

Gravitational Wave Signals from Two-Dimensional Core-Collapse Supernova Models with Rotation and Magnetic Fields

We investigate the impact of rotation and magnetic fields on the dynamics and gravitational wave emission in 2D core-collapse supernova simulations with neutrino transport. We simulate 17 different models of $15\,M_\odot$ and $39\,M_\odot$ progenitor stars with various initial rotation profiles and initial magnetic fields strengths up to $10^{12}\, \mathrm{G}$, assuming a dipolar field geometry in the progenitor. Strong magnetic fields generally prove conducive to shock revival, though this trend is not without exceptions. The impact of rotation on the post-bounce dynamics is more variegated, in line with previous studies. A significant impact on the time-frequency structure of the gravitational wave signal is found only for rapid rotation or strong initial fields. For rapid rotation, the angular momentum gradient at the proto-neutron star surface can appreciably affect the frequency of the dominant mode, so that known analytic relations for the high-frequency emission band no longer hold. In case of two magnetorotational explosion models, the deviation from these analytic relations is even more pronounced. One of the magnetorotational explosions has been evolved to more than half a second after the onset of the explosion and shows a subsidence of high-frequency emission at late times. Its most conspicuous gravitational wave signature is a high-amplitude tail signal. We also estimate the maximum detection distances for our waveforms. The magnetorotational models do not stick out for higher detectability during the post-bounce and explosion phase.

astro-ph.HE↗

arXiv2021

Explodability fluctuations of massive stellar cores enable asymmetric compact object mergers such as GW190814

The first three observing runs with Advanced LIGO and Virgo have resulted in the detection of binary black hole mergers (BBH) with highly unequal mass components, which are difficult to reconcile with standard formation paradigms. The most representative of these is GW190814, a highly asymmetric merger between a 23 M$_{\odot}$ black hole and a 2.6 M$_{\odot}$ compact object. Here, we explore recent results suggesting that a sizeable fraction of stars with pre-collapse carbon-oxygen core masses above 10 M$_{\odot}$, and extending up to at least 30 M$_{\odot}$, may produce objects inside the so-called lower mass gap that bridges the division between massive pulsars and BHs in Galactic X-ray binaries. We demonstrate that such an explosion landscape would naturally cause a fraction of massive binaries to produce GW190814-like systems instead of symmetric-mass BBHs. We present examples of specific evolutionary channels leading to the formation of GW190814 and GW200210, a 24+2.8 M$_{\odot}$ merger discovered during the O3b observing run. We estimate the merger-rate density of these events in our scenario to be $\mathcal{O}$(5%) of the total BBH merger rate. Finally, we discuss the broader implications of this formation channel for compact object populations, and its possible relevance to less asymmetric merger events such as GW200105 and GW200115

astro-ph.HE↗

arXiv2021

Universal relations for gravitational-wave asteroseismology of proto-neutron stars

State-of-the-art numerical simulations of core-collapse supernovae reveal that the main source of gravitational waves is the excitation of proto-neutron star modes during post-bounce evolution. In this work we derive universal relations that relate the frequencies of the most common oscillation modes observed, i.e. g-modes, p-modes and the f-mode, with fundamental properties of the system, such as the surface gravity of the proto-neutron star or the mean density in the region enclosed by the shock. These relations are independent of the equation of state, the neutrino treatment, and the progenitor mass and hence can be used to build methods to infer proto-neutron star properties from gravitational-wave observations alone. We outline how these measurements could be done and the constraints that could be placed on the proto-neutron star properties.

gr-qc↗

arXiv2021

3D Simulations of Oxygen Shell Burning with and without Magnetic Fields

We present a first 3D magnetohydrodynamic (MHD) simulation of convective oxygen and neon shell burning in a non-rotating $18\, M_\odot$ star shortly before core collapse to study the generation of magnetic fields in supernova progenitors. We also run a purely hydrodynamic control simulation to gauge the impact of the magnetic fields on the convective flow and on convective boundary mixing. After about 17 convective turnover times, the magnetic field is approaching saturation levels in the oxygen shell with an average field strength of $\mathord{\sim}10^{10}\, \mathrm{G}$, and does not reach kinetic equipartition. The field remains dominated by small to medium scales, and the dipole field strength at the base of the oxygen shell is only $10^{9}\, \mathrm{G}$. The angle-averaged diagonal components of the Maxwell stress tensor mirror those of the Reynolds stress tensor, but are about one order of magnitude smaller. The shear flow at the oxygen-neon shell interface creates relatively strong fields parallel to the convective boundary, which noticeably inhibit the turbulent entrainment of neon into the oxygen shell. The reduced ingestion of neon lowers the nuclear energy generation rate in the oxygen shell and thereby slightly slows down the convective flow. Aside from this indirect effect, we find that magnetic fields do not appreciably alter the flow inside the oxygen shell. We discuss the implications of our results for the subsequent core-collapse supernova and stress the need for longer simulations, resolution studies, and an investigation of non-ideal effects for a better understanding of magnetic fields in supernova progenitors.

astro-ph.SR↗

arXiv2021

The final core collapse of pulsational pair instability supernovae

We present 3D core-collapse supernova simulations of massive Pop-III progenitor stars at the transition to the pulsational pair instability regime. We simulate two progenitor models with initial masses of $85\,\mathrm{M}_{\odot}$ and $100\,\mathrm{M}_\odot$ with the LS220, SFHo, and SFHx equations of state. The $85\,\mathrm{M}_{\odot}$ progenitor experiences a pair instability pulse coincident with core collapse, whereas the $100\,\mathrm{M}_{\odot}$ progenitor has already gone through a sequence of four pulses $1\mathord,500$ years before collapse in which it ejected its H and He envelope. The $85\,\mathrm{M}_{\odot}$ models experience shock revival and then delayed collapse to a black hole (BH) due to ongoing accretion within hundreds of milliseconds. The diagnostic energy of the incipient explosion reaches up to $2.7\times10^{51}\,\mathrm{erg}$ in the SFHx model. Due to the high binding energy of the metal core, BH collapse by fallback is eventually unavoidable, but partial mass ejection may be possible. The $100\,\mathrm{M}_\odot$ models have not achieved shock revival or undergone BH collapse by the end of the simulation. All models exhibit relatively strong gravitational-wave emission both in the high-frequency g-mode emission band and at low frequencies. The SFHx and SFHo models show clear emission from the standing accretion shock instability. For our models, we estimate maximum detection distances of up to $\mathord{\sim}46\,\mathrm{kpc}$ with LIGO and $\mathord{\sim} 850\,\mathrm{kpc}$ with Cosmic Explorer.

astro-ph.HE↗

arXiv2020

The chemical signature of jet-driven hypernovae

Hypernovae powered by magnetic jets launched from the surface of rapidly rotating millisecond magnetars are one of the leading models to explain broad-lined Type Ic supernovae (SNe Ic-BL), and have been implicated as an important source of metal enrichment in the early Universe. We investigate the nucleosynthesis in such jet-driven hypernovae using a parameterised, but physically motivated, approach that analytically relates an artificially injected jet energy flux to the power available from the energy in differential rotation in the proto-neutron star. We find ejected $^{56}\mathrm{Ni}$ masses of $0.05\,\mathrm{M}_\odot - 0.45\,\mathrm{M}_\odot$ in our most energetic models with explosion energy $>10^{52}\,\mathrm{erg}$. This is in good agreement with the range of observationally inferred values for SNe Ic-BL. The $^{56}\mathrm{Ni}$ is mostly synthesised in the shocked stellar envelope, and is therefore only moderately sensitive to the jet composition. Jets with a high electron fraction $Y_\mathrm{e}=0.5$ eject more $^{56}\mathrm{Ni}$ by a factor of 2 than neutron-rich jets. We can obtain chemical abundance profiles in good agreement with the average chemical signature observed in extremely metal-poor (EMP) stars presumably polluted by hypernova ejecta. Notably, $\mathrm{[Zn/Fe]} \gtrsim 0.5$ is consistently produced in our models. For neutron-rich jets, there is a significant r-process component, and agreement with EMP star abundances in fact requires either a limited contribution from neutron-rich jets or a stronger dilution of r-process material in the interstellar medium than for the slow SN ejecta outside the jet. The high $\mathrm{[C/Fe]}\gtrsim 0.7$ observed in many EMP stars cannot be consistently achieved due to the large mass of iron in the ejecta, however, and remains a challenge for jet-driven hypernovae based on the magneto-rotational mechanism.

astro-ph.HE↗