Sufficient Angular Momentum Research Articles

Implications of post-kick jets in core-collapse supernovae

ABSTRACTI examine the assumption that the jets that shape the axisymmetrical morphological features of core-collapse supernova (CCSN) remnants are post-kick jets, i.e. the neutron star (NS) launches these jets after the explosion and after it acquired its natal kick velocity. I find that this assumption implies that the pre-collapse cores of CCSN progenitors have sufficient angular momentum fluctuations to support jittering jets that explode the star. From the finding that the shaping-jets neither tend to be aligned with the kick velocity nor to be perpendicular to it I argue that the assumption that the shaping-jets are post-kick jets has the following implications. (1) The NS accretes mass at a radius of $r_{\rm acc} \approx 5000 {~\rm km}$ from the centre of the explosion at $\approx 10 {~\rm s}$ after explosion. (2) The required angular momentum fluctuations of the accreted gas to explain the medium values of jets-kick angles are also sufficient to support an intermittent pre-kick accretion disc, just before and during the explosion. Such an intermittent accretion disc is likely to launch jets that explode the star in the frame of the jittering jets explosion mechanism. This suggests that most likely the shaping-jets are the last jets in the jittering jets explosion mechanism rather than post-kick jets. (3) The jittering jets explosion mechanism expects that black holes have small natal kick velocities.

Planetary bioengineering on Earth to return and maintain the atmospheric carbon dioxide to pre-industrial levels: Assessing potential mechanisms

We are all familiar with the episodes in the deep time history of Earth that enabled life to emerge in such abundance. Episodes like the formation of a Moon large enough and near enough to cause tides in the Earth’s waters and rocks, a core of sufficient iron with sufficient angular momentum to generate a protective magnetosphere around Earth, and assumption of a planetary axis angle that generates the ecological variation of our seasonal cycles. The living things that did arise on this planet have been modifying their habitats on Earth since they first appeared. Modifications that include the greening of Earth by photosynthetic organisms, which turned a predominantly reducing atmosphere into an oxidising one, the consequent precipitation of iron oxides into iron ore strata, and the formation of huge deposits of limestone by calcifying organisms. The episodes on which we wish to concentrate are 1) the frequent involvement of marine calcifiers (coccolithophores, foraminifera, molluscs, crustacea, corals, echinoderms), that have been described as ecosystem engineers modifying habitats in a generally positive way for other organisms, and 2) the frequent involvement of humans in changing the Earth’s biosphere in a generally negative way for other organisms. The fossil record shows that ancestral marine calcifiers had the physiology to cope with both acidified oceans and great excesses of atmospheric CO2 periodically throughout the past 500 million years, creating vast remains of shells as limestone strata in the process. So, our core belief is that humankind must look to the oceans for a solution to present-day climate change. The marine calcifiers of this planet have a track record of decisively modifying both oceans and atmospheres but take millions of years to do it. On the other hand, humanity works fast; in just a few thousand years we have driven scores of animals and plants to extinction, and in just a few hundred years we have so drastically modified our atmosphere that, arguably, we stand on the verge of extinction ourselves. Of all Earth’s ecosystems, those built around biological calcifiers, which all convert organic carbon into inorganic limestone, are the only ones that offer the prospect of permanent net removal of CO2 from our atmosphere. These are the carbon-removal biotechnologies we should be seeking to exploit.

The impact of stellar evolution on rotating star clusters: the gravothermal-gravogyro catastrophe and the formation of a bar of black holes

ABSTRACT We present results from a suite of eight direct N-body simulations, performed with nbody6++ GPU, representing realistic models of rotating star clusters with up to 1.1 × 105 stars. Our models feature primordial (hard) binaries, a continuous mass spectrum, differential rotation, and tidal mass-loss induced by the overall gravitational field of the host galaxy. We explore the impact of rotation and stellar evolution on the star cluster dynamics. In all runs for rotating star clusters, we detect a previously predicted mechanism: an initial phase of violent relaxation followed by the so-called gravogyro catastrophe. We find that the gravogyro catastrophe reaches a finite amplitude, which depends in strength on the level of the bulk rotation, and then levels off. After this phase, the angular momentum is transferred from high-mass to low-mass particles in the cluster (both stars and compact objects). Simultaneously, the system becomes gravothermally unstable and collapses, thus undergoing the so-called gravothermal-gravogyro catastrophe. Comparing models with and without stellar evolution, we find an interesting difference. When stellar evolution is not considered, the whole process proceeds at a faster pace. The population of heavy objects tends to form a triaxial structure that rotates in the cluster centre. When stellar evolution is considered, we find that such a rotating bar is populated by stellar black holes and their progenitors. The triaxial structure becomes axisymmetric over time, but we also find that the models without stellar evolution suffer repeated gravogyro catastrophes as sufficient angular momentum and mass are removed by the tidal field.

Ultra-delayed Neutrino-driven Explosion of Rotating Massive-star Collapse

Abstract Long-term neutrino-radiation hydrodynamics simulations in full general relativity are performed for the collapse of rotating massive stars that are evolved from He-stars with initial masses of 20 and 32 M ⊙. It is shown that if the collapsing stellar core has sufficient angular momentum, the rotationally supported proto-neutron star (PNS) survives for seconds accompanying the formation of a massive torus of mass larger than 1 M ⊙. Subsequent mass accretion onto the central region produces a massive and compact central object, and eventually enhances the neutrino luminosity beyond 1053 erg s−1, resulting in a very delayed neutrino-driven explosion, in particular toward the polar direction. The kinetic energy of the explosion can be appreciably higher than 1052 erg for a massive progenitor star and compatible with that of energetic supernovae like broad-line type-Ic supernovae. By the subsequent accretion, the massive PNS collapses eventually into a rapidly spinning black hole, which could be a central engine for gamma-ray bursts if a massive torus surrounds it.

Conditions for accretion disc formation and observability of wind-accreting X-ray binaries

Abstract We explore the effect of anisotropic wind driving on the properties of accretion onto black holes (BHs) in close binaries. We specifically focus on line-driven winds, which are common in high-mass X-ray binaries (HMXBs). In close binary systems, the tidal force from the companion star can modify the wind structure in two different ways. One is the reduction of wind terminal velocity due to the weaker effective surface gravity. The other is the reduction in mass flux due to gravity darkening (GD). We incorporate these effects into the so-called CAK theory in a simple way and investigate the wind flow around the accretor on the orbital scale. We find that a focused accretion stream is naturally formed when the Roche lobe filling factor is ${\gtrsim}0.8$ –0.9, analogous to that of wind Roche lobe overflow, but only when the velocity reduction is taken into account. The formation of a stream is necessary to bring in sufficient angular momentum to form an accretion disc around the BH. GD effects reduce the amount of accreted angular momentum, but not enough to prevent the formation of a disc. Based on these results, we expect there to be a discrete step in the observability of HMXBs depending on whether the donor Roche lobe filling factor is below or above ${\sim}$ 0.8–0.9.

The Betelgeuse Project. III. Merger Characteristics

Abstract We previously proposed that Betelgeuse might have been spun up by accreting a companion of about 1 M ⊙. Here we explore in more detail the possible systematics of such a merger and a larger range of accreted masses. We use the stellar evolutionary code mesa to add angular momentum to a primary star in core helium burning, core carbon burning, or shell carbon burning. Our models provide a reasonable “natural” explanation for why Betelgeuse has a large, but sub-Keplerian equatorial velocity. They eject sufficient mass and angular momentum in rotationally induced mass loss to reproduce the observed ratio of the equatorial velocity to escape velocity of Betelgeuse, ≈0.23, within a factor of 3 nearly independent of the primary mass, the secondary mass, and the epoch at which merger occurs. Our models suggest that merger of a primary of somewhat less than 15 M ⊙ with secondaries from 1 to 10 M ⊙ during core helium burning or core carbon burning could yield the equatorial rotational velocity of ∼15 km s−1 attributed to Betelgeuse. For accreting models, a wave of angular momentum is halted at the composition boundary at the edge of the helium core. The inner core is thus not affected by the accretion of the companion in these simulations. Accretion has relatively little effect on the production of magnetic fields in the inner core. Our results do not prove, but do not negate, that Betelgeuse might have ingested a companion of several M ⊙.

Be Star Disks: Powered by a Nonzero Central Torque

Abstract Be stars are rapidly rotating B stars with Balmer emission lines that indicate the presence of a Keplerian, rotationally supported, circumstellar gas disk. Current disk models, referred to as “decretion disks,” make use of the zero-torque inner boundary condition typically applied to accretion disks, with the “decretion” modeled by adding mass to the disk at a radius of about 2% larger than the inner disk boundary. We point out that, in this model, the rates at which mass and energy need to be added to the disk are implausibly large. What is required is that the disk has not only a source of mass but also a continuing source of angular momentum. We argue that the disk evolution may be more physically modeled by application of the nonzero torque inner boundary condition of Nixon & Pringle, which determines the torque applied at the boundary as a fraction of the advected angular momentum flux there and approaches the accretion and decretion disk cases in the appropriate limits. We provide supporting arguments for the suggestion that the origin of the disk material is small-scale magnetic flaring events on the stellar surface, which, when combined with rapid rotation, can provide sufficient mass to form, and sufficient angular momentum to maintain, a Keplerian Be star disk. We discuss the origin of such small-scale magnetic fields in radiative stars with differential rotation. We conclude that small-scale magnetic fields on the stellar surface, may be able to provide the necessary mass flux and the necessary time-dependent torque on the disk inner regions to drive the observed disk evolution.

Minutes-delayed Jets from a Neutron Star Companion in Core-collapse Supernovae

Abstract This paper presents cases in which a neutron star (NS; or a black hole) companion to a type-Ib or type-Ic (stripped-envelope) core-collapse supernova (CCSN) accretes mass from the explosion ejecta and launches jets minutes to hours after explosion. The NS orbits at a pre-explosion radius of a ≃ 1−5 R ⊙. When the ejecta velocity drops to , the ejecta gas that the NS accretes possesses sufficient specific angular momentum to form an accretion disk around the NS. The NS accretes a fraction of of the ejecta mass through an accretion disk over a time period of 10 minutes–few hr. If the jets carry about 10% of the accretion energy, then their total energy is a fraction of about 0.003−0.03 of the kinetic energy of the ejecta. The implications of these jets from an NS (or a black hole) companion to a CCSN are the shaping of the inner ejecta to have a bipolar morphology, energizing the light curve of the CCSN, and in some cases the possible enrichment of the inner ejecta with r-process elements.

Design and experimental research on ultrasonic levitated spherical rotor gyroscope

This study proposes a spherical rotor gyroscope based on the near-field acoustic levitation (NFAL) principle, which utilizes high-frequency vibration to levitate objects at a short distance near the driving surface. The ultrasonic suspended gyroscope consists of a piezoelectrically excited stator and a spherical rotor. The stator was excited to generate ultrasonic vibration under the action of the inverse piezoelectric effect, provide non-contact support for the spherical rotor, and drive it to rotate with the traveling wave vibration. When the spherical rotor obtained sufficient angular momentum, the driving voltages were changed to induce the standing wave vibration and thus provide a stable levitation force field for the rotor. At this stage, the rotor was only subjected to the effects of gravity and levitation force and the spherical rotor rotated at a high speed to obtain gyroscopic inertia. Herein, the finite element model of the stator was established for dynamic analysis, and an acoustic–structure coupling model was established to analyze the non-contact supporting force of the gyroscope. The prototype was manufactured, and the stator was tested for vibration, and test platforms for levitation height, rotation speed, and gyroscopic inertia of the non-contact ultrasonic levitated spherical rotor gyroscope were built. The stator vibration performance was consistent with the simulation analysis result. The levitation height could be as high as tens of microns at the operating frequency. The rotation speed could reach 5600 r/min, and the gyroscopic inertia was verified. Therefore, the feasibility of NFAL for a levitated gyroscope was verified.

Post-AGB Discs from Common-Envelope Evolution

Post-asymptotic giant branch (post-AGB) stars with discs are all binaries. Many of these binaries have orbital periods between 100 and 1000 days so cannot have avoided mass transfer between the AGB star and its companion, likely through a common-envelope type interaction. We report on preliminary results of our project to model circumbinary discs around post-AGB stars using our binary population synthesis code binary_c. We combine a simple analytic thin-disc model with binary stellar evolution to estimate the impact of the disc on the binary, and vice versa, fast enough that we can model stellar population and hence explore the rather uncertain parameter space involved with disc formation. We find that, provided the discs form with sufficient mass and angular momentum, and have an inner edge that is relatively close to the binary, they can both prolong the life of their parent post-AGB star and pump the eccentricity of orbits of their inner binaries.

Gedanken experiments to destroy a black hole. II. Kerr-Newman black holes cannot be overcharged or overspun

We consider gedanken experiments to destroy an extremal or nearly extremal Kerr-Newman black hole by causing it to absorb matter with sufficient charge and/or angular momentum as compared with energy that it cannot remain a black hole. It was previously shown by one of us that such gedanken experiments cannot succeed for test particle matter entering an extremal Kerr-Newman black hole. We generalize this result here to arbitrary matter entering an extremal Kerr-Newman black hole, provided only that the non-electromagnetic contribution to the stress-energy tensor of the matter satisfies the null energy condition. We then analyze the gedanken experiments proposed by Hubeny and others to over-charge and/or over-spin an initially slightly non-extremal Kerr-Newman black hole. Analysis of such gedanken experiments requires that we calculate all effects on the final mass of the black hole that are second-order in the charge and angular momentum carried into the black hole, including all self-force effects. We obtain a general formula for the full second order correction to mass, $\delta^2 M$, which allows us to prove that no gedanken experiments of the generalized Hubeny type can ever succeed in over-charging and/or over-spinning a Kerr-Newman black hole, provided only that the non-electromagnetic stress-energy tensor satisfies the null energy condition. Our analysis is based upon Lagrangian methods, and our formula for the second-order correction to mass is obtained by generalizing the canonical energy analysis of Hollands and Wald to the Einstein-Maxwell case. Remarkably, we obtain our formula for $\delta^2 M$ without having to explicitly compute self-force or finite size effects. Indeed, in an appendix, we show explicitly that our formula incorporates both the self-force and finite size effects for the special case of a charged body slowly lowered into an uncharged black hole.

Neutron transfer in the C13+Au197 reaction from gold isotope residuals

Residual gold nuclei were produced via neutron transfer at multiple energies using a 130-MeV $^{13}\mathrm{C}$ beam incident on a stacked-foil target consisting of alternating layers of $^{197}\mathrm{Au}$ and $^{27}\mathrm{Al}$. Production cross sections, over an energy range of 56 to 129 MeV, for seven gold isotopes and two gold isomers were determined through activation analysis. By using the Wilczy\ifmmode \acute{n}\else \'{n}\fi{}ski binary transfer model with a modified version of the recoil formula and a standard evaporation model, we were able to reproduce the isotopic production cross sections at high beam energy, with some disagreement at lower beam energies. This limiting angular momentum model does not predict the transfer of sufficient angular momentum to reproduce the observed isomeric populations.

WASP-92b, WASP-93b and WASP-118b: three new transiting close-in giant planets

We present the discovery of three new transiting giant planets, first detected with the WASP telescopes, and establish their planetary nature with follow up spectroscopy and ground-based photometric light curves. WASP-92 is an F7 star, with a moderately inflated planet orbiting with a period of 2.17 d, which has Rp = 1.461 ± 0.077RJ and Mp = 0.805 ± 0.068MJ. WASP-93b orbits its F4 host star every 2.73 d and has Rp = 1.597 ± 0.077RJ and Mp = 1.47 ± 0.029MJ. WASP-118b also has a hot host star (F6) and is moderately inflated, where Rp = 1.440 ± 0.036RJ and Mp = 0.514 ± 0.020MJ and the planet has an orbital period of 4.05 d. They are bright targets (V = 13.18, 10.97 and 11.07, respectively) ideal for further characterization work, particularly WASP-118b, which is being observed by K2 as part of campaign 8. The WASP-93 system has sufficient angular momentum to be tidally migrating outwards if the system is near spin–orbit alignment, which is divergent from the tidal behaviour of the majority of hot Jupiters discovered.

Optimal technique for maximal forward rotating vaults in men’s gymnastics

In vaulting a gymnast must generate sufficient linear and angular momentum during the approach and table contact to complete the rotational requirements in the post-flight phase. This study investigated the optimization of table touchdown conditions and table contact technique for the maximization of rotation potential for forwards rotating vaults. A planar seven-segment torque-driven computer simulation model of the contact phase in vaulting was evaluated by varying joint torque activation time histories to match three performances of a handspring double somersault vault by an elite gymnast. The closest matching simulation was used as a starting point to maximize post-flight rotation potential (the product of angular momentum and flight time) for a forwards rotating vault. It was found that the maximized rotation potential was sufficient to produce a handspring double piked somersault vault. The corresponding optimal touchdown configuration exhibited hip flexion in contrast to the hyperextended configuration required for maximal height. Increasing touchdown velocity and angular momentum lead to additional post-flight rotation potential. By increasing the horizontal velocity at table touchdown, within limits obtained from recorded performances, the handspring double somersault tucked with one and a half twists, and the handspring triple somersault tucked became theoretically possible.

FOLLOWING BLACK HOLE SCALING RELATIONS THROUGH GAS-RICH MERGERS

We present black hole mass measurements from kinematic modeling of high-spatial resolution integral field spectroscopy of the inner regions of nine nearby (ultra-)luminous infrared galaxies in a variety of merger stages. These observations were taken with OSIRIS and laser guide star adaptive optics on the Keck I and Keck II telescopes, and reveal gas and stellar kinematics inside the spheres of influence of these supermassive black holes. We find that this sample of black holes are overmassive (∼107–9 ) compared to the expected values based on black hole scaling relations, and suggest that the major epoch of black hole growth occurs in early stages of a merger, as opposed to during a final episode of quasar-mode feedback. The black hole masses presented are the dynamical masses enclosed in ∼25 pc, and could include gas which is gravitationally bound to the black hole but has not yet lost sufficient angular momentum to be accreted. If present, this gas could in principle eventually fuel active galactic nucleus feedback or be itself blown out from the system.

The influence of touchdown conditions and contact phase technique on post-flight height in the straight handspring somersault vault

In vaulting the gymnast must generate sufficient linear and angular momentum during the approach and table contact in order to complete the rotational requirements in the post-flight phase. This study investigated the effects of touchdown conditions and contact technique on peak post-flight height of a straight handspring somersault vault. A planar seven-segment torque-driven computer simulation model of the contact phase in vaulting was evaluated by varying joint torque activation time histories to match three performances of a straight handspring somersault vault by an elite gymnast. The closest matching simulation was used as a starting point to optimise peak post-flight height of the mass centre for a straight handspring somersault. It was found that optimising either the touchdown conditions or the contact technique increased post-flight height by 0.1m whereas optimising both together increased post-flight height by 0.4m above that of a simulation matching the recorded performance. Thus touchdown technique and contact technique make similar contributions to post-flight height in the straight handspring somersault vault. Increasing touchdown velocity and angular momentum lead to additional post-flight height although there was a critical value of vertical touchdown velocity beyond which post-flight height decreased.

Triggering jet-driven explosions of core-collapse supernovae by accretion from convective regions

We find that convective regions of collapsing massive stellar cores possess sufficient stochastic angular momentum to form intermittent accretion disks around the newly born neutron star (NS) or black hole (BH), as required by the jittering-jets model for core-collapse supernova (CCSN) explosions. To reach this conclusion we derive an approximate expression for stochastic specific angular momentum in convection layers of stars, and using the mixing-length theory apply it to four stellar models at core-collapse epoch. In all models, evolved using the stellar evolution code MESA, the convective helium layer has sufficient angular momentum to form an accretion disk. The mass available for disk formation around the NS or BH is 0.1-1.2Mo; stochastic accretion of this mass can form intermittent accretion disks that launch jets powerful enough to explode the star according to the jittering-jets model. Our results imply that even if no explosion occurs after accretion of the inner ~2-5Mo of the core onto the NS or BH (the mass depends on the stellar model), accretion of outer layers of the core will eventually lead to an energetic supernova explosion.

Gravitational wave signatures from low-mode spiral instabilities in rapidly rotating supernova cores

We study properties of gravitational waves (GWs) from the rotating core collapse of a $15{M}_{\ensuremath{\bigodot}}$ star by performing three-dimensional general-relativistic hydrodynamic simulations with an approximate neutrino transport. By parametrically changing the precollapse angular momentum, we focus on the effects of rotation on the GW signatures in the early postbounce evolution. Regarding three-flavor neutrino transport, we solve the energy-averaged set of radiation energy and momentum based on the Thorne's momentum formalism. In addition to the gravitational quadrupole radiation from matter motions, we take into account GWs from anisotropic neutrino emission. With these computations, our results present supporting evidence for the previous anticipation that nonaxisymmetric instabilities play an essential role in determining the postbounce GW signatures. During prompt convection, we find that the waveforms show narrow-band and highly quasiperiodic signals which persist until the end of simulations. We point out that such features reflect the growth of the one-armed spiral modes. The typical frequency of the quasiperiodic waveforms can be well explained by the propagating acoustic waves between the stalled shock and the rotating proto-neutron star surface, which suggests the appearance of the standing-accretion-shock instability. Although the GW signals exhibit strong variability between the two polarizations and different observer directions, they are within the detection limits of next-generation detectors such as KAGRA and Advanced LIGO, if the source with sufficient angular momentum is located in our Galaxy.

LONG HINDLIMBS CONTRIBUTE TO AIR-RIGHTING PERFORMANCE IN FALLING TREE FROGS

Tree frogs are able to climb or even jump on leaves and branches using their toe pads adhering to and detaching from surfaces, but have to be in the face of the risk of falling down due to fatigue or a slip. While falling down from a great height, air-righting response was observed, which prevents tree frogs from injury caused by back hitting the ground, however the mechanism underlying is unknown. Using a high-speed camera with a plane mirror, we captured the body and limb kinematics of falling tree frogs as they performed rapid air-righting response in three-dimension. The kinematics were then characterized into three stages and substituted into a mathematical multi-body model to simulate the air-righting dynamics caused by the kinematics. The results show that a head-down tree frog performs rapid air-righting with specific series of actions in hindlimb motion, which generates sufficient local angular momentum for the frog to rotate its body upright and keep the total moment of momentum conserved. The applications of these results to small landing robots and multi-segment spacecraft are anticipated.

THE LARGE-SCALE MAGNETIC FIELDS OF THIN ACCRETION DISKS

Large scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared to the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number Pm is around unity. In this work, we revisit this problem considering the angular momentum of the disk is removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, beta~1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full 3D numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong MRI turbulence surrounding them.