ABSTRACT
We investigate the explosion of stars with zero-age main-sequence masses between 20 and 35 M⊙ and varying degrees of rotation and magnetic fields including ones commonly considered ...progenitors of gamma-ray bursts (GRBs). The simulations, combining special relativistic magnetohydrodynamics, a general relativistic approximate gravitational potential, and two-moment neutrino transport, demonstrate the viability of different scenarios for the post-bounce evolution. Having formed a highly massive proto-neutron star (PNS), several models launch successful explosions, either by the standard supernova mechanism based on neutrino heating and hydrodynamic instabilities or by magnetorotational processes. It is, however, quite common for the PNS to collapse to a black hole (BH) within a few seconds. Others might produce proto-magnetar-driven explosions. We explore several ways to describe the different explosion mechanisms. The competition between the time-scales for advection of gas through the gain layer and heating by neutrinos provides an approximate explanation for models with insignificant magnetic fields. The fidelity of this explosion criterion in the case of rapid rotation can be improved by accounting for the strong deviations from spherical symmetry and mixing between pole and equator. We furthermore study an alternative description including the ram pressure of the gas falling through the shock. Magnetically driven explosions tend to arise from a strongly magnetized region around the polar axis. In these cases, the onset of the explosion corresponds to the equality between the advection time-scale and the time-scale for the propagation of Alfvén waves through the gain layer.
ABSTRACT
We explore the influence of non-axisymmetric modes on the dynamics of the collapsed core of rotating, magnetized high-mass stars in three-dimensional simulations of a rapidly rotating star ...with an initial mass of $M_{\rm {\small ZAMS}} = 35 \, \mathrm{M}_{\odot }$ endowed with four different pre-collapse configurations of the magnetic field, ranging from moderate to very strong field strength and including the field predicted by the stellar evolution model. The model with the weakest magnetic field achieves shock revival due to neutrino heating in a gain layer characterized by a large-scale, hydrodynamic m = 1 spiral mode. Later on, the growing magnetic field of the proto neutron star launches weak outflows into the early ejecta. Their orientation follows the evolution of the rotational axis of the proto neutron star, which starts to tilt from the original orientation due to the asymmetric accretion flows impinging on its surface. The models with stronger magnetization generate mildly relativistic, magnetically driven polar outflows propagating over a distance of 104 km within a few $100 \, \textrm {ms}$. These jets are stabilized against disruptive non-axisymmetric instabilities by their fast propagation and by the shear of their toroidal magnetic field. Within the simulation times of around $1 \, \textrm {s}$, the explosions reach moderate energies and the growth of the proto neutron star masses ceases at values substantially below the threshold for black hole formation, which, in combination with the high rotational energies, might suggest a possible later proto-magnetar activity.
ABSTRACT
We assess the variance of the post-collapse evolution remnants of compact, massive, low-metallicity stars, under small changes in the degrees of rotation and magnetic field of selected ...pre-supernova cores. These stellar models are commonly considered progenitors of long gamma-ray bursts. The fate of the protoneutron star (PNS) formed after the collapse, whose mass may continuously grow due to accretion, critically depends on the poloidal magnetic field strength at bounce. Should the poloidal magnetic field be sufficiently weak, the PNS collapses to a black hole (BH) within a few seconds. Models on this evolutionary track contain promising collapsar engines. Poloidal magnetic fields smooth over large radial scales (e.g. dipolar fields) or slightly augmented with respect to the original pre-supernova core yield long-lasting PNSs. In these models, BH formation is avoided or staved off for a long time, hence, they may produce protomagnetars (PMs). Some of our PM candidates have been run for $\lesssim 10\,$ s after core bounce, but they have not entered the Kelvin–Helmholtz phase yet. Among these models, some display episodic events of spin-down during which we find properties broadly compatible with the theoretical expectations for PMs ($M_\rm {\small PNS}\approx 1.85{-}2.5\, \mathrm{M}_{\odot }$, $\bar{P}_\rm {\small PNS}\approx 1.5 {-} 4\,$ ms, and $b^{\rm surf}_\rm {\small PNS}\lesssim 10^{15}\,$ G) and their very collimated supernova ejecta have nearly reached the stellar surface with (still growing) explosion energies $\gtrsim {2} \times 10^{51}\, \textrm {erg}$.
Abstract
Using axisymmetric simulations coupling special relativistic magnetohydrodynamics (MHD), an approximate post-Newtonian gravitational potential and two-moment neutrino transport, we show ...different paths for the formation of either protomagnetars or stellar mass black holes. The fraction of prototypical stellar cores which should result in collapsars depends on a combination of several factors, among which the structure of the progenitor star and the profile of specific angular momentum are probably the foremost. Along with the implosion of the stellar core, we also obtain supernova-like explosions driven by neutrino heating and hydrodynamic instabilities or by magneto-rotational effects in cores of high-mass stars. In the latter case, highly collimated, mildly relativistic outflows are generated. We find that after a rather long post-collapse phase (lasting ≳1 s) black holes may form in cases both of successful and failed supernova-like explosions. A basic trend is that cores with a specific angular momentum smaller than that obtained by standard, one-dimensional stellar evolution calculations form black holes (and eventually collapsars). Complementary, protomagnetars result from stellar cores with the standard distribution of specific angular momentum obtained from prototypical stellar evolution calculations including magnetic torques and moderate to large mass-loss rates.
ABSTRACT
The final collapse of the cores of massive stars can lead to a wide variety of outcomes in terms of electromagnetic and kinetic energies, nucleosynthesis, and remnants. The association of ...this wide spectrum of explosion and remnant types with the properties of the progenitors remains an open issue. The rotation and magnetic fields in Wolf–Rayet stars of subsolar metallicity may explain extreme events such as superluminous supernovae and gamma-ray bursts powered by proto-magnetars or collapsars. Continuing with numerical studies of magnetorotational core collapse, including detailed neutrino physics, we focus on progenitors with zero-age main-sequence masses in the range between 5 and 39 ${\rm M}_{\odot }$. The pre-collapse stars are 1D models employing prescriptions for the effects of rotation and magnetic fields. Eight of the 10 stars we consider are the results of chemically homogeneous evolution owing to enhanced rotational mixing . All but one of them produce explosions driven by neutrino heating (more likely for low-mass progenitors up to 8 ${\rm M}_{\odot }$) and non-spherical flows or by magnetorotational stresses (more frequent above 26 ${\rm M}_{\odot }$). In most of them and for the one non-exploding model, ongoing accretion leads to black hole formation. Rapid rotation makes subsequent collapsar activity plausible. Models not forming black holes show proto-magnetar-driven jets. Conditions for the formation of nickel are more favourable in magnetorotationally driven models, although our rough estimates fall short of the requirements for extremely bright events if these are powered by radioactive decay. However, the approximate light curves of our models suggest that a proto-magnetar or black hole spin-down may fuel luminous transients (with peak luminosities $\sim 10^{43-44}\, \textrm {erg}$).
Nucleosynthesis in magneto-rotational supernovae Reichert, M; Obergaulinger, M; Eichler, M ...
Monthly notices of the Royal Astronomical Society,
03/2021, Letnik:
501, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We present the nucleosynthesis of magneto-rotational supernovae (MR-SNe) including neutrino-driven and magneto-rotational-driven ejecta based, for the first time, on 2D simulations with ...accurate neutrino transport. The models analysed here have different rotation and magnetic fields, allowing us to explore the impact of these two key ingredients. The accurate neutrino transport of the simulations is critical to analyse the slightly neutron-rich and proton-rich ejecta that are similar to the, also neutrino-driven, ejecta in standard supernovae. In the model with strong magnetic field, the r-process produces heavy elements up to the third r-process peak (A ∼ 195), in agreement with previous works. This model presents a jet-like explosion with proton-rich jets surrounded by neutron-rich material where the r-process occurs. We have estimated a lower limit for 56Ni of $2.5\times 10^{-2} \, \mathrm{M}_\odot$, which is still well below the expected hypernova value. Longer simulations including the accretion disc evolution are required to get a final prediction. In addition, we have found that the late evolution is critical in a model with weak magnetic field in which late-ejected neutron-rich matter produces elements up to the second r-process peak. Even if we cannot yet provide conclusions for hypernova nucleosynthesis, our results agree with observations of old stars and radioactive isotopes in supernova remnants. This makes MR-SNe a good additional scenario to neutron star mergers for the synthesis of heavy elements and brings us closer to understand their origin and the role of MR-SNe in the early Galaxy nucleosynthesis.
ABSTRACT
Magnetic fields can play a major role in the dynamics of outstanding explosions associated to violent events such as gamma-ray bursts and hypernovae, since they provide a natural mechanism ...to harness the rotational energy of the central proto-neutron star and power relativistic jets through the stellar progenitor. As the structure of such fields is quite uncertain, most numerical models of MHD-driven core-collapse supernovae consider an aligned dipole as initial magnetic field, while the field’s morphology can actually be much more complex. We present three-dimensional simulations of core-collapse supernovae with more realistic magnetic structures, such as quadrupolar fields and, for the first time, an equatorial dipolar field. Configurations other than an aligned dipole produce weaker explosions and less collimated outflows, but can at the same time be more efficient in extracting the rotational energy from the PNS. This energy is then stored in the surroundings of the PNS, rather than powering the polar jets. A significant axial dipolar component is also produced by models starting with a quadrupolar field, pointing to an effective dynamo mechanism operating in proximity of the PNS surface.
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.
ABSTRACT We present the first special relativistic, axisymmetric hydrodynamic simulations of black hole-torus systems (approximating general relativistic gravity) as remnants of binary-neutron star ...(NS-NS) and neutron star-black hole (NS-BH) mergers, in which the viscously driven evolution of the accretion torus is followed with self-consistent energy-dependent neutrino transport and the interaction with the cloud of dynamical ejecta expelled during the NS-NS merging is taken into account. The modeled torus masses, BH masses and spins, and the ejecta masses, velocities, and spatial distributions are adopted from relativistic merger simulations. We find that energy deposition by neutrino annihilation can accelerate outflows with initially high Lorentz factors along polar low-density funnels, but only in mergers with extremely low baryon pollution in the polar regions. NS-BH mergers, where polar mass ejection during the merging phase is absent, provide sufficiently baryon-poor environments to enable neutrino-powered, ultrarelativistic jets with terminal Lorentz factors above 100 and considerable dynamical collimation, favoring short gamma-ray bursts (sGRBs), although their typical energies and durations might be too small to explain the majority of events. In the case of NS-NS mergers, however, neutrino emission of the accreting and viscously spreading torus is too short and too weak to yield enough energy for the outflows to break out from the surrounding ejecta shell as highly relativistic jets. We conclude that neutrino annihilation alone cannot power sGRBs from NS-NS mergers.
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