arXiv subjects

Thierry Foglizzo

Publications and source records attributed to Thierry Foglizzo.

At least 19 recordsLinked to original sources

arXiv2024

Convective vortices in collapsing stars

Recent studies show that non-radial structures arising from massive star shell convection play an important role in shaping core-collapse supernova explosions. During the collapse phase, convective vortices generate acoustic waves that interact with the supernova shock. This amplifies turbulence in the post-shock region, contributing to explosion. We study how various physical parameters influence the evolution of these convective vortices during stellar collapse using simplified simulations. We model the collapsing star with a transonic Bondi flow and represent convection as solenoidal velocity perturbations. Our results are consistent with previous studies, demonstrating that the peak perturbation amplitude scales linearly with the pre-collapse convective Mach number and inversely with the angular wavenumber of convection. While the shell radius and width primarily determine the timescale of accretion, they have little impact on the peak perturbation amplitudes. Finally, we show that when the convective Mach number is below approximately 0.2, the dynamics remain within the linear regime.

astro-ph.SR↗

arXiv2024

Detectability of Late-time Supernova Neutrinos with Fallback Accretion onto Protoneutron star

We investigate the late-time neutrino emission powered by fallback mass accretion onto proto-neutron star (PNS), using neutrino radiation-hydrodynamic simulations with full Boltzmann neutrino transport. We follow the time evolution of accretion flow onto PNS until the system reaches a steady state. A standing shock wave is commonly formed in the accretion flow, whereas the shock radius varies depending on mass accretion rate and PNS mass. A sharp increase in temperature emerges in the vicinity of PNS ($\sim 10$ km), which characterizes neutrino emission. Both neutrino luminosity and average energy become higher with increasing mass accretion rate and PNS mass. The mean energy of emitted neutrinos is in the range of $10\lesssimε\lesssim20\,\mathrm{MeV}$, which is higher than that estimated from PNS cooling models ($\lesssim10\,\mathrm{MeV}$). Assuming a distance to core-collapse supernova of $10\,\mathrm{kpc}$, we quantify neutrino event rates for Super-Kamiokande (Super-K) and DUNE. The estimated detection rates are well above the background, and their energy-dependent features are qualitatively different from those expected from PNS cooling models. Another notable feature is that the neutrino emission is strongly flavor dependent, exhibiting that the neutrino event rate hinges on the neutrino oscillation model. We estimate them in the case with adiabatic Mikheev-Smirnov-Wolfenstein model, and show that the normal- and inverted mass hierarchy offer the large number of neutrino detection in Super-K and DUNE, respectively. Hence the simultaneous observation with Super-K and DUNE of the fallback neutrinos will provide a strong constraint on neutrino mass hierarchy.

astro-ph.HE↗

arXiv2023

Prospects for realtime characterization of core-collapse supernova and neutrino properties

Core-collapse supernovae (CCSNe) offer extremely valuable insights into the dynamics of galaxies. Neutrino time profiles from CCSNe, in particular, could reveal unique details about collapsing stars and particle behavior in dense environments. However, CCSNe in our galaxy and the Large Magellanic Cloud are rare and only one supernova neutrino observation has been made so far. To maximize the information obtained from the next Galactic CCSN, it is essential to combine analyses from multiple neutrino experiments in real time and transmit any relevant information to electromagnetic facilities within minutes. Locating the CCSN, in particular, is challenging, requiring disentangling CCSN localization information from observational features associated with the properties of the supernova progenitor and the physics of the neutrinos. Yet, being able to estimate the progenitor distance from the neutrino signal would be of great help for the optimisation of the electromagnetic follow-up campaign that will start soon after the propagation of the neutrino alert. Existing CCSN distance measurement algorithms based on neutrino observations hence rely on the assumption that neutrino properties can be described by the Standard Model. This paper presents a swift and robust approach to extract CCSN and neutrino physics information, leveraging diverse next-generation neutrino detectors to counteract potential measurement biases from Beyond the Standard Model effects.

astro-ph.HE↗

arXiv2023

Astrophysics with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultracompact stellar-mass binaries, massive black hole binaries, and extreme or intermediate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

gr-qc↗

arXiv2023

Standing Accretion Shock Instability in the Collapse of a Rotating Stellar Core

Hydrodynamical instabilities, such as the standing accretion-shock instability (SASI), play an essential role in the dynamics of core-collapse supernovae, with observable imprints in the neutrino and gravitational wave signals. Yet, the impact of stellar rotation on the development of SASI is poorly explored. We investigate the conditions favoring the growth of SASI in the presence of rotation through a perturbative analysis. The properties of SASI are compared in two stationary configurations, cylindrical and spherical equatorial, which mainly differ by their advection timescales from the shock to the proto-neutron star surface. Without rotation, the mode $m=1$, corresponding to a one-armed spiral SASI deformation, can be significantly more unstable in the spherical equatorial configuration. In fact, the shorter advection time in the spherical equatorial geometry allows for a larger contribution of the entropic-acoustic coupling from the region of adiabatic compression near the surface of the proto-neutron star. The angular momentum of the collapsing core favors the growth of prograde spiral modes $m=1$ and $m = 2$ in both geometries. Although the growth rate of the spiral instability is systematically faster in spherical geometry, its oscillation frequency is remarkably insensitive to the geometry. Such a contrast with non-rotating flows calls for a deeper understanding of the role of advection in the mechanism of spiral SASI. Our findings suggest that the resonant coupling of acoustic waves with their corotation radius may play a major role in the instability mechanism of collapsing cores with rotation. Elucidating this physical mechanism is essential to interpret the signal from future multi-messenger supernova observations.

astro-ph.HE↗

arXiv2023

Three-dimensional core-collapse supernovae with complex magnetic structures: II. Rotational instabilities and multi-messenger signatures

The gravitational collapse of rapidly rotating massive stars can lead to the onset of the low $T/\|W\|$ instability within the central proto-neutron star (PNS), which leaves strong signatures in both the gravitational wave (GW) and neutrino emission. Strong large-scale magnetic fields are usually invoked to explain outstanding stellar explosions of rapidly rotating progenitors, but their impact on the growth of such instability has not yet been cleared. We analyze a series of three-dimensional magnetohydrodynamic models to characterize the effects of different magnetic configurations on the development of the low $T/\|W\|$ and the related multi-messenger features. In the absence of magnetic fields, we observe the growth on dynamical time scales of the low $T/\|W\|$, associated with a strong burst of GW and a correlated modulation of the neutrino emission. However, models with a strong magnetic field show a quenching of the low $T/\|W\|$, due to a flattening of the rotation profile in the first $\sim100$ ms after shock formation caused by the magnetic transport of angular momentum. The associated GW emission is weakened by an order of magnitude, exhibits a broader spectral shape, and has no dominant feature associated with the PNS large-scale oscillation modes. Neutrino luminosities are damped along the equatorial plane due to a more oblate PNS, and the only clear modulation in the signal is due to SASI activity. Finally, magnetized models produce lower luminosities for $ν_e$ than for $\barν_e$, which is connected to a higher concentration of neutron-rich material in the PNS surroundings.

astro-ph.HE↗

arXiv2021

Insights into non-axisymmetric instabilities in three-dimensional rotating supernova models with neutrino and gravitational-wave signatures

We present a detailed analysis to clarify what determines the growth of the low-$T/|W|$ instability in the context of rapidly rotating core-collapse of massive stars. To this end, we perform three-dimensional core-collapse supernova (CCSN) simulations of a $27 M_{\odot}$ star including several updates in the general relativistic correction to gravity, the multi-energy treatment of heavy-lepton neutrinos, and the nuclear equation of state. Non-axisymmetric deformations are analyzed from the point of view of the time evolution of the pattern frequency and the corotation radius. The corotation radius is found to coincide with the convective layer in the proto neutron star (PNS). We propose a new mechanism to account for the growth of the low-$T/|W|$ instability in the CCSN environment. Near the convective boundary where a small Brunt-Väisälä frequency is expected, Rossby waves propagating in the azimuthal direction at mid latitude induce non-axisymmetric unstable modes, in both hemispheres. They merge with each other and finally become the spiral arm in the equatorial plane. We also investigate how the growth of the low-$T/|W|$ instability impacts the neutrino and gravitational-wave signatures.

astro-ph.HE↗

arXiv2020

Acoustic wave generation in collapsing massive stars with convective shells

The convection that takes place in the innermost shells of massive stars plays an important role in the formation of core-collapse supernova explosions. Upon encountering the supernova shock, additional turbulence is generated, amplifying the explosion. In this work, we study how the convective perturbations evolve during the stellar collapse. Our main aim is to establish their physical properties right before they reach the supernova shock. To this end, we solve the linearized hydrodynamics equations perturbed on a stationary background flow. The latter is approximated by the spherical transonic Bondi accretion, while the convective perturbations are modeled as a combination of entropy and vorticity waves. We follow their evolution from large radii, where convective shells are initially located, down to small radii, where they are expected to encounter the accretion shock above the proto-neutron star. Considering typical vorticity perturbations with a Mach number $\sim 0.1$ and entropy perturbations with magnitude $\sim 0.05 k_\mathrm{b}/\mathrm{baryon}$, we find that the advection of these perturbations down to the shock generates acoustic waves with a relative amplitude $δp/γp \lesssim 10\%$, in agreement with published numerical simulations. The velocity perturbations consist of contributions from acoustic and vorticity waves with values reaching $\sim 10\%$ of the sound speed ahead of the shock. The perturbation amplitudes decrease with increasing $\ell$ and initial radii of the convective shells.

astro-ph.SR↗

arXiv2018

The non-linear onset of neutrino-driven convection in two and three-dimensional core-collapse supernovae

A toy model of the post-shock region of core-collapse supernovae is used to study the non-linear development of turbulent motions driven by convection in the presence of advection. Our numerical simulations indicate that buoyant perturbations of density are able to trigger self-sustained convection only when the instability is not linearly stabilized by advection. Large amplitude perturbations produced by strong shock oscillations or combustion inhomogeneities before the collapse of the progenitor are efficiently shredded through phase mixing and generate a turbulent cascade. Our model enables us to investigate several physical arguments that had been proposed to explain the impact of the dimensionality on the onset of explosions in global simulations of core-collapse supernovae. Three-dimensional (3D) simulations are found to lead to higher entropy values than two-dimensional (2D) ones. We attribute this to greater turbulent mixing and dissipation of the kinetic energy into heat in 3D. Our results show that the increase of entropy is enhanced with finer numerical resolution and larger perturbation amplitude.

astro-ph.SR↗

arXiv2017

Are pulsars spun up or down by SASI spiral modes?

Pulsars may either be spun up or down by hydrodynamic instabilities during the supernova explosion of massive stars. Besides rapidly-rotating cases related to bipolar explosions, stellar rotation may affect the explosion of massive stars in the more common situations where the centrifugal force is minor. Using 2D simulations of a simplified setup in cylindrical geometry, we examine the impact of rotation on the Standing Accretion Shock Instability (SASI) and the corotation instability, also known as low-T/|W|. The influence of rotation on the saturation amplitude of these instabilities depends on the specific angular momentum in the accretion flow and the ratio of the shock to the neutron star radii. The spiral mode of SASI becomes more vigorous with faster rotation only if this ratio is large enough. A corotation instability develops at large rotation rates and impacts the dynamics more dramatically, leading to a strong one-armed spiral wave. Non-axisymmetric instabilities are able to redistribute angular momentum radially and affect the pulsar spin at birth. A systematic study of the relationship between the core rotation period of the progenitor and the initial pulsar spin is performed. Stellar rotation rates for which pulsars are spun up or down by SASI are estimated. Rapidly spinning progenitors are modestly spun down by spiral modes, less than $\sim 30\%$, when a corotation instability develops. Given the observational constraints on pulsar spin periods at birth, this suggests that rapid rotation might not play a significant hydrodynamic role in most core-collapse supernovae.

astro-ph.HE↗

arXiv2015

New insights on the spin-up of a neutron star during core-collapse

The spin of a neutron star at birth may be impacted by the asymmetric character of the explosion of its massive progenitor. During the first second after bounce, the spiral mode of the Standing Accretion Shock Instability (SASI) is able to redistribute angular momentum and spin-up a neutron star born from a non-rotating progenitor. Our aim is to assess the robustness of this process. We perform 2D numerical simulations of a simplified setup in cylindrical geometry to investigate the timescale over which the dynamics is dominated by a spiral or a sloshing mode. We observe that the spiral mode prevails only if the ratio of the initial shock radius to the neutron star radius exceeds a critical value. In that regime, both the degree of asymmetry and the average expansion of the shock induced by the spiral mode increase monotonously with this ratio, exceeding the values obtained when a sloshing mode is artificially imposed. With a timescale of 2-3 SASI oscillations, the dynamics of SASI takes place fast enough to affect the spin of the neutron star before the explosion. The spin periods deduced from the simulations are compared favorably to analytical estimates evaluated from the measured saturation amplitude of the SASI wave. Despite the simplicity of our setup, numerical simulations revealed unexpected stochastic variations, including a reversal of the direction of rotation of the shock. Our results show that the spin up of neutron stars by SASI spiral modes is a viable mechanism even though it is not systematic.

astro-ph.HE↗

arXiv2015

The explosion mechanism of core-collapse supernovae: progress in supernova theory and experiments

The explosion of core-collapse supernova depends on a sequence of events taking place in less than a second in a region of a few hundred kilometers at the center of a supergiant star, after the stellar core approaches the Chandrasekhar mass and collapses into a proto-neutron star, and before a shock wave is launched across the stellar envelope. Theoretical efforts to understand stellar death focus on the mechanism which transforms the collapse into an explosion. Progress in understanding this mechanism is reviewed with particular attention to its asymmetric character. We highlight a series of successful studies connecting observations of supernova remnants and pulsars properties to the theory of core-collapse using numerical simulations. The encouraging results from first principles models in axisymmetric simulations is tempered by new puzzles in 3D. The diversity of explosion paths and the dependence on the pre-collapse stellar structure is stressed, as well as the need to gain a better understanding of hydrodynamical and MHD instabilities such as SASI and neutrino-driven convection. The shallow water analogy of shock dynamics is presented as a comparative system where buoyancy effects are absent. This dynamical system can be studied numerically and also experimentally with a water fountain. The potential of this complementary research tool for supernova theory is analyzed. We also review its potential for public outreach in science museums.

astro-ph.HE↗

arXiv2014

Characterizing SASI- and Convection-Dominated Core-Collapse Supernova Explosions in Two Dimensions

The success of the neutrino mechanism of core-collapse supernovae relies on the supporting action of two hydrodynamic instabilities: neutrino-driven convection and the Standing Accretion Shock Instability (SASI). Depending on the structure of the stellar progenitor, each of these instabilities can dominate the evolution of the gain region prior to the onset of explosion, with implications for the ensuing asymmetries. Here we examine the flow dynamics in the neighborhood of explosion by means of parametric two-dimensional, time-dependent hydrodynamic simulations for which the linear stability properties are well understood. We find that systems for which the convection parameter is sub-critical (SASI-dominated) develop explosions once large-scale, high-entropy bubbles are able to survive for several SASI oscillation cycles. These long-lived structures are seeded by the SASI during shock expansions. Finite-amplitude initial perturbations do not alter this outcome qualitatively, though they can lead to significant differences in explosion times. Supercritical systems (convection-dominated) also explode by developing large-scale bubbles, though the formation of these structures is due to buoyant activity. Non-exploding systems achieve a quasi-steady state in which the time-averaged flow adjusts itself to be convectively sub-critical. We characterize the turbulent flow using a spherical Fourier-Bessel decomposition, identifying the relevant scalings and connecting temporal and spatial components. Finally, we verify the applicability of these principles on the general relativistic, radiation-hydrodynamic simulations of Mueller, Janka, & Heger (2012), and discuss implications for the three-dimensional case.

astro-ph.SR↗

arXiv2011

A Shallow Water Analogue of the Standing Accretion Shock Instability: Experimental Demonstration and Two-Dimensional Model

Despite the sphericity of the collapsing stellar core, the birth conditions of neutron stars can be highly non spherical due to a hydrodynamical instability of the shocked accretion flow. Here we report the first laboratory experiment of a shallow water analogue, based on the physics of hydraulic jumps. Both the experiment and its shallow water modeling demonstrate a robust linear instability and nonlinear properties of symmetry breaking, in a system which is one million times smaller and about hundred times slower than its astrophysical analogue.

astro-ph.HE↗

arXiv2011

On the linear growth mechanism driving the stationary accretion shock instability

During stellar core collapse, which eventually leads to a supernovae explosion, the stalled shock is unstable due to the standing accretion shock instability (SASI). This instability induces large-scale non spherical oscillations of the shock, which have crucial consequences on the dynamics and the geometry of the explosion. While the existence of this instability has been firmly established, its physical origin remains somewhat uncertain. Two mechanisms have indeed been proposed to explain its linear growth. The first is an advective-acoustic cycle, where the instability results from the interplay between advected perturbations (entropy and vorticity) and an acoustic wave. The second mechanism is purely acoustic and assumes that the shock is able to amplify trapped acoustic waves. Several arguments favouring the advective-acoustic cycle have already been proposed, however none was entirely conclusive for realistic flow parameters. In this article we give two new arguments which unambiguously show that the instability is not purely acoustic, and should be attributed to the advective-acoustic cycle. First, we extract a radial propagation timescale by comparing the frequencies of several unstable harmonics that differ only by their radial structure. The extracted time matches the advective-acoustic time but strongly disagrees with a purely acoustic interpretation. Second, we present a method to compute purely acoustic modes, by artificially removing advected perturbations below the shock. All these purely acoustic modes are found to be stable, showing that the advected wave is essential to the instability mechanism.

astro-ph.HE↗

arXiv2011

Dynamics of an Alfven surface in core collapse supernovae

We investigate the dynamics of an Alfven surface (where the Alfven speed equals the advection velocity) in the context of core collapse supernovae during the phase of accretion on the proto-neutron star. Such a surface should exist even for weak magnetic fields because the advection velocity decreases to zero at the center of the collapsing core. In this decelerated flow, Alfven waves created by the standing accretion shock instability (SASI) or convection accumulate and amplify while approaching the Alfven surface. We study this amplification using one dimensional MHD simulations with explicit physical dissipation. In the linear regime, the amplification continues until the Alfven wavelength becomes as small as the dissipative scale. A pressure feedback that increases the pressure in the upstream flow is created via a non linear coupling. We derive analytic formulae for the maximum amplification and the non linear coupling and check them with numerical simulations to a very good accuracy. We also characterize the non linear saturation of this amplification when compression effects become important, leading to either a change of the velocity gradient, or a steepening of the Alfven wave. Applying these results to core collapse supernovae shows that the amplification can be fast enough to affect the dynamics, if the magnetic field is strong enough for the Alfven surface to lie in the region of strong velocity gradient just above the neutrinosphere. This requires the presence of a strong magnetic field in the progenitor star, which would correspond to the formation of a magnetar under the assumption of magnetic flux conservation. An extrapolation of our analytic formula (taking into account the nonlinear saturation) suggests that the Alfven wave could reach an amplitude of B ~ 10^15 G, and that the pressure feedback could significantly contribute to the pressure below the shock.

astro-ph.SR↗

arXiv2010

Toward a magnetohydrodynamic theory of the stationary accretion shock: toy model of the advective-acoustic cycle in a magnetized flow

The effect of a magnetic field on the linear phase of the advective-acoustic instability is investigated, as a first step toward a magnetohydrodynamic (MHD) theory of the stationary accretion shock instability taking place during stellar core collapse. We study a toy model where the flow behind a planar stationary accretion shock is adiabatically decelerated by an external potential. Two magnetic field geometries are considered: parallel or perpendicular to the shock. The entropy-vorticity wave, which is simply advected in the unmagnetized limit, separates into five different waves: the entropy perturbations are advected, while the vorticity can propagate along the field lines through two Alfven waves and two slow magnetosonic waves. The two cycles existing in the unmagnetized limit, advective-acoustic and purely acoustic, are replaced by up to six distinct MHD cycles. The phase differences among the cycles play an important role in determining the total cycle efficiency and hence the growth rate. Oscillations in the growth rate as a function of the magnetic field strength are due to this varying phase shift. A vertical magnetic field hardly affects the cycle efficiency in the regime of super-Alfvenic accretion that is considered. In contrast, we find that a horizontal magnetic field strongly increases the efficiencies of the vorticity cycles that bend the field lines, resulting in a significant increase of the growth rate if the different cycles are in phase. These magnetic effects are significant for large-scale modes if the Alfven velocity is a sizable fraction of the flow velocity.

astro-ph.SR↗

arXiv2010

The saturation of SASI by parasitic instabilities

The Standing Accretion Shock Instability (SASI) is commonly believed to be responsible for large amplitude dipolar oscillations of the stalled shock during core collapse, potentially leading to an asymmetric supernovae explosion. The degree of asymmetry depends on the amplitude of SASI, which nonlinear saturation mechanism has never been elucidated. We investigate the role of parasitic instabilities as a possible cause of nonlinear SASI saturation. As the shock oscillations create both vorticity and entropy gradients, we show that both Kelvin-Helmholtz and Rayleigh-Taylor types of instabilities are able to grow on a SASI mode if its amplitude is large enough. We obtain simple estimates of their growth rates, taking into account the effects of advection and entropy stratification. In the context of the advective-acoustic cycle, we use numerical simulations to demonstrate how the acoustic feedback can be decreased if a parasitic instability distorts the advected structure. The amplitude of the shock deformation is estimated analytically in this scenario. When applied to the set up of Fernandez & Thompson (2009a), this saturation mechanism is able to explain the dramatic decrease of the SASI power when both the nuclear dissociation energy and the cooling rate are varied. Our results open new perspectives for anticipating the effect, on the SASI amplitude, of the physical ingredients involved in the modeling of the collapsing star.

astro-ph.SR↗