ABSTRACT
The magnetic field is believed to play an important role in at least some core-collapse supernovae (CCSN) if its magnitude reaches $10^{15}\, \rm {G}$, which is a typical value for a ...magnetar. In the presence of fast rotation, such a strong magnetic field can drive powerful jet-like explosions if it has the large-scale coherence of a dipole. The topology of the magnetic field is, however, probably much more complex with strong multipolar and small-scale components and the consequences for the explosion are so far unclear. We investigate the effects of the magnetic field topology on the dynamics of CCSN and the properties of the forming proto-neutron star (PNS) by comparing pre-collapse fields of different multipolar orders and radial profiles. Using axisymmetric special relativistic MHD simulations and a two-moment neutrino transport, we find that higher multipolar magnetic configurations lead to generally less energetic explosions, slower expanding shocks, and less collimated outflows. Models with a low order multipolar configuration tend to produce more oblate PNS, surrounded in some cases by a rotationally supported toroidal structure of neutron-rich material. Moreover, magnetic fields which are distributed on smaller angular scales produce more massive and faster rotating central PNS, suggesting that higher order multipolar configurations tend to decrease the efficiency of the magnetorotational launching mechanism. Even if our dipolar models systematically display a far more efficient extraction of the rotational energy of the PNS, fields distributed on smaller angular scales are still capable of powering magnetorotational explosions and shape the evolution of the central compact object.
We present the new code alcar developed to model multidimensional, multienergy-group neutrino transport in the context of supernovae and neutron-star mergers. The algorithm solves the evolution ...equations of the zeroth- and first-order angular moments of the specific intensity, supplemented by an algebraic relation for the second-moment tensor to close the system. The scheme takes into account frame-dependent effects of the order
$\mathcal {O}(v/c)$
as well as the most important types of neutrino interactions. The transport scheme is significantly more efficient than a multidimensional solver of the Boltzmann equation, while it is more accurate and consistent than the flux-limited diffusion method. The finite-volume discretization of the essentially hyperbolic system of moment equations employs methods well-known from hydrodynamics. For the time integration of the potentially stiff moment equations we employ a scheme in which only the local source terms are treated implicitly, while the advection terms are kept explicit, thereby allowing for an efficient computational parallelization of the algorithm. We investigate various problem set-ups in one and two dimensions to verify the implementation and to test the quality of the algebraic closure scheme. In our most detailed test, we compare a fully dynamic, one-dimensional core-collapse simulation with two published calculations performed with well-known Boltzmann-type neutrino-hydrodynamics codes and we find very satisfactory agreement.
We study the amplification of magnetic fields in the collapse and the post-bounce evolution of the core of a non-rotating star of 15 M⊙ in axisymmetry. To this end, we solve the coupled equations of ...magnetohydrodynamics and neutrino transport in the two-moment approximation. The pre-collapse magnetic field is strongly amplified by compression in the infall. Initial fields of the order of 1010 G translate into protoneutron star fields similar to the ones observed in pulsars, while stronger initial fields yield magnetar-like final field strengths. After core bounce, the field is advected through the hydrodynamically unstable neutrino-heating layer, where non-radial flows due to convection and the standing accretion shock instability amplify the field further. Consequently, the resulting amplification factor of the order of 5 is the result of the number of small-eddy turnovers taking place within the time-scale of advection through the post-shock layer. Due to this limit, most of our models do not reach equipartition between kinetic and magnetic energy and, consequently, evolve similarly to the non-magnetic case, exploding after about 800 ms when a single or few high-entropy bubbles persist over several dynamical time-scales. In the model with the strongest initial field we studied, 1012 G, for which equipartition between flow and field is achieved, the magnetic tension favours a much earlier development of such long-lived high-entropy bubbles and enforces a fairly ordered large-scale flow pattern. Consequently, this model, after exhibiting very regular shock oscillations, explodes much earlier than non-magnetic ones.
ABSTRACT
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 analyse 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 multimessenger 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 ∼100 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 Standing Accretion Shock Instability activity. Finally, magnetized models produce lower luminosities for νe than for $\bar{\nu }_e$, which is connected to a higher concentration of neutron-rich material in the PNS surroundings.
ABSTRACT
Magnetorotational supernovae are a rare type of core-collapse supernovae where the magnetic field and rotation play a central role in the dynamics of the explosion. We present the ...post-processed nucleosynthesis of state-of-the-art neutrino-MHD supernova models that follow the post explosion evolution for few seconds. We find three different dynamical mechanisms to produce heavy r-process elements: (i) a prompt ejection of matter right after core bounce, (ii) neutron-rich matter that is ejected at late times due to a reconfiguration of the protoneutronstar shape, (iii) small amount of mass ejected with high entropies in the centre of the jet. We investigate total ejecta yields, including the ones of unstable nuclei such as 26Al, 44Ti, 56Ni, and 60Fe. The obtained 56Ni masses vary between $0.01\!-\!1\, \mathrm{M_\odot }$. The latter maximum is compatible with hypernova observations. Furthermore, all of our models synthesize Zn masses in agreement with observations of old metal-poor stars. We calculate simplified light curves to investigate whether our models can be candidates for superluminous supernovae. The peak luminosities obtained from taking into account only nuclear heating reach up to a few $\sim 10^{43} \, \mathrm{erg\, s^{-1}}$. Under certain conditions, we find a significant impact of the 66Ni decay chain that can raise the peak luminosity up to $\sim 38{{\ \rm per \, cent}}$ compared to models including only the 56Ni decay chain. This work reinforces the theoretical evidence on the critical role of magnetorotational supernovae to understand the occurrence of hypernovae, superluminous supernovae, and the synthesis of heavy elements.
Whether the magnetorotational instability (MRI) can amplify initially weak magnetic fields to dynamically relevant strengths in core-collapse supernovae is still a matter of active scientific debate. ...Recent numerical studies have shown that the first phase of MRI growth dominated by channel flows is terminated by parasitic instabilities of the Kelvin–Helmholtz type that disrupt MRI channel flows and quench further magnetic field growth. However, it remains to be properly assessed by what factor the initial magnetic field can be amplified and how it depends on the initial field strength and the amplitude of the perturbations. Different termination criteria leading to different estimates of the amplification factor were proposed within the parasitic model. To determine the amplification factor and test which criterion is a better predictor of the MRI termination, we perform three-dimensional shearing-disc and shearing-box simulations of a region close to the surface of a differentially rotating protoneutron star in non-ideal magnetohydrodynamics with two different numerical codes. We find that independently of the initial magnetic field strength, the MRI channel modes can amplify the magnetic field by, at most, a factor of 100. Under the conditions found in protoneutron stars, a more realistic value for the magnetic field amplification is of the order of 10. This severely limits the role of the MRI channel modes as an agent amplifying the magnetic field in protoneutron stars starting from small seed fields. A further amplification should therefore rely on other physical processes, such as for example an MRI-driven turbulent dynamo.
Abstract
We investigate the post-explosion phase in core-collapse supernovae with 2D hydrodynamical simulations and a simple neutrino treatment. The latter allows us to perform 46 simulations and ...follow the evolution of the 32 explosion models during several seconds. We present a broad study based on three progenitors (11.2, 15, and 27
M
⊙
), different neutrino heating efficiencies, and various rotation rates. We show that the first seconds after shock revival determine the final explosion energy, remnant mass, and properties of ejected matter. Our results suggest that a continued mass accretion increases the explosion energy even at late times. We link the late-time mass accretion to initial conditions such as rotation strength and shock deformation at explosion time. Only some of our simulations develop a neutrino-driven wind (NDW) that survives for several seconds. This indicates that NDWs are not a standard feature expected after every successful explosion. Even if our neutrino treatment is simple, we estimate the nucleosynthesis of the exploding models for the 15
M
⊙
progenitor after correcting the neutrino energies and luminosities to get a more realistic electron fraction.
ABSTRACT
Motivated by the many associations of gamma-ray bursts (GRBs) with energetic supernova (SN) explosions, we study the propagation of relativistic jets within the progenitor star in which an ...SN shock wave may be launched briefly before the jets start to propagate. Based on analytic considerations and verified with an extensive set of 2D axisymmetric relativistic hydrodynamic simulations, we have estimated a threshold intrinsic jet luminosity, $L_{\rm j}^{\rm thr}$, for successfully launching a jet. This threshold depends on the structure of the progenitor and, thus, it is sensitive to its mass and to its metallicity. For a prototype host of cosmological long GRBs, a low-metallicity star of 35 M⊙, it is $L_{\rm j}^{\rm thr}\simeq 1.35\times 10^{49}$ erg s−1. The observed equivalent isotropic gamma-ray luminosity, $L_{\rm \gamma ,iso,BO} \simeq 4 \epsilon _\gamma L_{\rm j} \theta _{\rm BO}^{-2}$, crucially depends on the jet opening angle after breakout, θBO, and on the efficiency for converting the intrinsic jet luminosity into γ-radiation, εγ. Highly energetic jets can produce low-luminosity events if either their opening angle after the breakout is large, which is found in our models, or if the conversion efficiency of kinetic and internal energy into radiation is low enough. Beyond this theoretical analysis, we show how the presence of an SN shock wave may reduce this luminosity threshold by means of numerical simulations. We foresee that the high-energy transients released by jets produced near the luminosity threshold will be more similar to llGRBs or X-ray flashes than to GRBs.
The magnetorotational instability (MRI) can be a powerful mechanism amplifying the magnetic field in core-collapse supernovae. Whether initially weak magnetic fields can be amplified by this ...instability to dynamically relevant strengths is still a matter of debate. One of the main uncertainties concerns the process that terminates the growth of the instability. Parasitic instabilities of both Kelvin–Helmholtz and tearing-mode type have been suggested to play a crucial role in this process, disrupting MRI channel flows and quenching magnetic field amplification. We perform two-dimensional and three-dimensional sheering-disc simulations of a differentially rotating protoneutron star layer in non-ideal magnetohydrodynamics with unprecedented high numerical accuracy, finding that Kelvin–Helmholtz parasitic modes dominate tearing modes in the regime of large hydrodynamic and magnetic Reynolds numbers, as encountered close to the surface of protoneutron stars. They also determine the maximum magnetic field stress achievable during the exponential growth of the MRI. Our results are consistent with the theory of parasitic instabilities based on a local stability analysis. To simulate the Kelvin–Helmholtz instabilities properly, a very high numerical resolution is necessary. Using ninth-order spatial reconstruction schemes, we find that at least eight grid zones per MRI channel are necessary to simulate the growth phase of the MRI and reach an accuracy of ∼10 per cent in the growth rate, while more than ∼60 zones per channel are required to achieve convergent results for the value of the magnetic stress at MRI termination.
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