scholarly journals Magnetorotational core collapse of possible GRB progenitors – I. Explosion mechanisms

2020 ◽  
Vol 492 (4) ◽  
pp. 4613-4634 ◽  
Author(s):  
M Obergaulinger ◽  
M Á Aloy
Keyword(s):  
Magnetic Fields ◽  
Time Scale ◽  
Main Sequence ◽  
Gamma Ray ◽  
Polar Axis ◽  
Hydrodynamic Instabilities ◽  
Alternative Description ◽  
Ram Pressure ◽  
General Relativistic ◽  
Relativistic Magnetohydrodynamics

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.

ON MAGNETIC SELF-COLLIMATION OF RELATIVISTIC JETS

2010 ◽  
Vol 19 (06) ◽  
pp. 689-694
Author(s):  
N. GLOBUS ◽  
V. CAYATTE ◽  
C. SAUTY
Keyword(s):  
Black Hole ◽  
Total Energy ◽  
Ideal Fluid ◽  
Polar Axis ◽  
Kerr Metric ◽  
Relativistic Jets ◽  
Simple Criterion ◽  
General Relativistic ◽  
Time Splitting ◽  
Relativistic Magnetohydrodynamics

We present a semi-analytical model using the equations of general relativistic magnetohydrodynamics (GRMHD) for jets emitted by a rotating black hole. We assume steady axisymmetric outflows of a relativistic ideal fluid in Kerr metrics. We express the conservation equations in the frame of the FIDucial Observer (FIDO or ZAMO) using a 3+1 space–time splitting. Calculating the total energy variation between a non-polar field line and the polar axis, we extend to the Kerr metric the simple criterion for the magnetic collimation of jets obtained for a nonrotating black hole by Meliani et al.10 We show that the black role rotation induced a more efficient magnetic collimation of the jet.

scholarly journals On monolithic supermassive stars

2020 ◽  
Vol 494 (2) ◽  
pp. 2236-2243 ◽  
Author(s):  
Tyrone E Woods ◽  
Alexander Heger ◽  
Lionel Haemmerlé
Keyword(s):  
Early Universe ◽  
Stellar Evolution ◽  
Time Scale ◽  
Main Sequence ◽  
Massive Stars ◽  
Rotating Stars ◽  
One Dimensional ◽  
General Relativistic ◽  
The One ◽  
Limiting Case

ABSTRACT Supermassive stars have been proposed as the progenitors of the massive ($\sim \!10^{9}\, \mathrm{M}_{\odot }$) quasars observed at z ∼ 7. Prospects for directly detecting supermassive stars with next-generation facilities depend critically on their intrinsic lifetimes, as well as their formation rates. We use the one-dimensional stellar evolution code kepler to explore the theoretical limiting case of zero-metallicity non-rotating stars, formed monolithically with initial masses between $10$ and $190\, \mathrm{kM}_{\odot }$. We find that stars born with masses between $\sim\! 60$ and $\sim\! 150\, \mathrm{kM}_{\odot }$ collapse at the end of the main sequence, burning stably for $\sim\! 1.5\, \mathrm{Myr}$. More massive stars collapse directly through the general relativistic instability after only a thermal time-scale of $\sim\! 3$–$4\, \mathrm{kyr}$. The expected difficulty in producing such massive thermally relaxed objects, together with recent results for currently preferred rapidly accreting formation models, suggests that such ‘truly direct’ or ‘dark’ collapses may not be typical for supermassive objects in the early Universe. We close by discussing the evolution of supermassive stars in the broader context of massive primordial stellar evolution and the possibility of supermassive stellar explosions.

scholarly journals Presupernova Evolution of Rotating Massive Stars and the Rotation Rate of Pulsars

2004 ◽  
Vol 215 ◽  
pp. 591-600 ◽  
Author(s):  
A. Heger ◽  
S. E. Woosley ◽  
N. Langer ◽  
H. C. Spruit
Keyword(s):  
Angular Momentum ◽  
Neutron Stars ◽  
Rotation Rate ◽  
Main Sequence ◽  
Gamma Ray ◽  
Massive Stars ◽  
Hydrodynamic Instabilities ◽  
Order Of Magnitude ◽  
Advanced Stages ◽  
Explosion Mechanism

Rotation in massive stars has been studied on the main sequence and during helium burning for decades, but only recently have realistic numerical simulations followed the transport of angular momentum that occurs during more advanced stages of evolution. The results affect such interesting issues as whether rotation is important to the explosion mechanism, whether supernovae are strong sources of gravitational radiation, the star's nucleosynthesis, and the initial rotation rate of neutron stars and black holes. We find that when only hydrodynamic instabilities (shear, Eddington-Sweet, etc.) are included in the calculation, one obtains neutron stars spinning at close to critical rotation at their surface – or even formally in excess of critical. When recent estimates of magnetic torques (Spruit 2002) are added, however, the evolved cores spin about an order of magnitude slower. This is still more angular momentum than observed in young pulsars, but too slow for the collapsar model for gamma-ray bursts.

scholarly journals Flux expulsion with dynamics

2016 ◽  
Vol 791 ◽  
pp. 568-588 ◽  
Author(s):  
Andrew D. Gilbert ◽  
Joanne Mason ◽  
Steven M. Tobias
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Lorentz Force ◽  
Differential Rotation ◽  
Time Scale ◽  
Scaling Laws ◽  
Dynamical Regime ◽  
Kinematic Evolution ◽  
Flux Expulsion ◽  
The Magnetic Field

In the process of flux expulsion, a magnetic field is expelled from a region of closed streamlines on a $TR_{m}^{1/3}$ time scale, for magnetic Reynolds number $R_{m}\gg 1$ ($T$ being the turnover time of the flow). This classic result applies in the kinematic regime where the flow field is specified independently of the magnetic field. A weak magnetic ‘core’ is left at the centre of a closed region of streamlines, and this decays exponentially on the $TR_{m}^{1/2}$ time scale. The present paper extends these results to the dynamical regime, where there is competition between the process of flux expulsion and the Lorentz force, which suppresses the differential rotation. This competition is studied using a quasi-linear model in which the flow is constrained to be axisymmetric. The magnetic Prandtl number $R_{m}/R_{e}$ is taken to be small, with $R_{m}$ large, and a range of initial field strengths $b_{0}$ is considered. Two scaling laws are proposed and confirmed numerically. For initial magnetic fields below the threshold $b_{core}=O(UR_{m}^{-1/3})$, flux expulsion operates despite the Lorentz force, cutting through field lines to result in the formation of a central core of magnetic field. Here $U$ is a velocity scale of the flow and magnetic fields are measured in Alfvén units. For larger initial fields the Lorentz force is dominant and the flow creates Alfvén waves that propagate away. The second threshold is $b_{dynam}=O(UR_{m}^{-3/4})$, below which the field follows the kinematic evolution and decays rapidly. Between these two thresholds the magnetic field is strong enough to suppress differential rotation, leaving a magnetically controlled core spinning in solid body motion, which then decays slowly on a time scale of order $TR_{m}$.

scholarly journals iharm3D: Vectorized General Relativistic Magnetohydrodynamics

2021 ◽  
Vol 6 (66) ◽  
pp. 3336
Author(s):  
Ben Prather ◽  
George Wong ◽  
Vedant Dhruv ◽  
Benjamin Ryan ◽  
Joshua Dolence ◽  
...  
Keyword(s):  
General Relativistic ◽  
Relativistic Magnetohydrodynamics

scholarly journals Gamma-Ray Bursts as Sources of Strong Magnetic Fields

2016 ◽  
pp. 481-528
Author(s):  
Jonathan Granot ◽  
Tsvi Piran ◽  
Omer Bromberg ◽  
Judith L. Racusin ◽  
Frédéric Daigne
Keyword(s):  
Magnetic Fields ◽  
Gamma Ray ◽  
Gamma Ray Bursts ◽  
Strong Magnetic Fields

scholarly journals Short-Term Spectral Variability in the Herbig Ae Star Ab Aur: Preliminary Analysis of Chromospheric Lines

1993 ◽  
Vol 137 ◽  
pp. 665-668
Author(s):  
Torsten Böhm ◽  
Claude Catala
Keyword(s):  
Magnetic Fields ◽  
Acoustic Waves ◽  
Main Sequence ◽  
Main Question ◽  
Spectral Variability ◽  
Surface Magnetic Field ◽  
External Agent ◽  
Static Contraction ◽  
Magnetic Origin ◽  
Herbig Ae Stars

The Herbig Ae stars are PMS objects of intermediate mass. Their location in the H-R diagram indicates that they are in the radiative phase of their quasi-static contraction toward the main sequence, i.e. that they do not possess outer convective zones, according to the standard stellar evolution theory (Iben, 1965; Gilliland, 1986). In spite of the expected absence of subphotospheric convective envelopes, these stars show remarkable signs of activity: emission in the Mg II h and k lines, presence of the CIV resonance lines at 1550 A and He I 5875.7 A line, Ca II IR triplet in emission, etc... Considering that stellar activity, witnessed by the same type of indicators in other parts of the H-R diagram, is generally attributed to dynamo magnetic fields and/or acoustic waves generated in the convection zone, these active phenomena are quite paradoxical in the Herbig Ae stars.The main question concerns the origin of their activity: is this activity linked to phenomena occurring within the stars, like e.g. dynamo-generated magnetic fields, or to an external agent, like e.g. a boundary layer between an accretion disk and the stellar surface? We already have some indirect clues that the activity of the Herbig Ae stars might be of magnetic origin (Praderie et al., 1986; Catala et al. 1986) observed a rotational modulation of lines formed in the wind of AB Aur, prototype of the Herbig Ae stars. By analogy with the solar wind, they proposed that the modulation might be due to the corotation of azimuthal structures in the wind, controlled by a surface magnetic field.

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