Angular Momentum Distribution Research Articles

Perfect fluid tori orbiting Kehagias–Sfetsos naked singularities

We construct perfect fluid tori in the field of the Kehagias–Sfetsos (K–S) naked singularities. These are spherically symmetric vacuum solutions of the modified Hořava quantum gravity, characterized by a dimensionless parameter $$\omega M^2$$ , combining the gravitational mass parameter M of the spacetime with the Hořava parameter $$\omega $$ , reflecting the role of the quantum corrections. In dependence on the value of $$\omega M^2$$ , the K–S naked singularities demonstrate a variety of qualitatively different behavior of their circular geodesics that is fully reflected in the properties of the toroidal structures, demonstrating clear distinction to the properties of the torii in the Schwarzschild spacetimes. In all of the K–S naked singularity spacetimes the tori are located above an “antigravity” sphere where matter can stay in a stable equilibrium position, which is relevant for the stability of the orbiting fluid toroidal accretion structures. The signature of the K–S naked singularity is given by the properties of marginally stable tori orbiting with the uniform distribution of the specific angular momentum of the fluid, $$l=$$ const. In the K–S naked singularity spacetimes with $$\omega M^2 > 0.2811$$ , doubled tori with the same $$l=$$ const can exist; mass transfer between the outer torus and the inner one is possible under appropriate conditions, while only outflow to the outer space is allowed in complementary conditions. In the K–S spacetimes with $$\omega M^2 < 0.2811$$ , accretion from cusped perfect fluid tori is not possible due to the non-existence of unstable circular geodesics.

THE INTERNAL ROTATION PROFILE OF THE B-TYPE STAR KIC 10526294 FROM FREQUENCY INVERSION OF ITS DIPOLE GRAVITY MODES

The internal angular momentum distribution of a star is key to determine its evolution. Fortunately, the stellar internal rotation can be probed through studies of rotationally-split non-radial oscillation modes. In particular, detection of non-radial gravity modes (g modes) in massive young stars has become feasible recently thanks to the Kepler space mission. Our aim is to derive the internal rotation profile of the Kepler B8V star KIC 10526294 through asteroseismology. We interpret the observed rotational splittings of its dipole g modes using four different approaches based on the best seismic models of the star and their rotational kernels. We show that these kernels can resolve differential rotation the radiative envelope if a smooth rotational profile is assumed and the observational errors are small. Based on Kepler data, we find that the rotation rate near the core-envelope boundary is well constrained to $163\pm89$ nHz. The seismic data are consistent with rigid rotation but a profile with counter-rotation within the envelope has a statistical advantage over constant rotation. Our study should be repeated for other massive stars with a variety of stellar parameters in order to deduce the physical conditions that determine the internal rotation profile of young massive stars, with the aim to improve the input physics of their models.

Radiochemical studies on nuclear fission at Trombay

Since the discovery of nuclear fission in the year 1939, both physical and radiochemical techniques have been adopted for the study of various aspects of the phenomenon. Due to the ability to separate individual elements from a complex reaction mixture with a high degree of sensitivity and selectivity, a chemist plays a significant role in the measurements of mass, charge, kinetic energy, angular momentum and angular distribution of fission products in various fissioning systems. At Trombay, a small group of radiochemists initiated the work on radiochemical studies of mass distribution in the early sixties. Since then, radiochemical investigations on various fission observables have been carried out at Trombay in n, p, α and heavy-ion-induced fissions. An attempt has been made to highlight the important findings of such studies in this paper, with an emphasis on medium energy and heavy-ion-induced fission.

Chaotic cold accretion on to black holes in rotating atmospheres

Chaotic cold accretion (CCA) profoundly differs from classic black hole accretion models. Using 3D high-resolution simulations, we probe the impact of rotation on the hot and cold accretion flow in a typical massive galaxy. In the hot mode, with or without turbulence, the pressure-dominated flow forms a geometrically thick rotational barrier, suppressing the accretion rate to ~1/3 of the Bondi rate. When radiative cooling is dominant, the gas loses pressure support and quickly circularizes in a cold thin disk. In the more common state of a turbulent and heated atmosphere, CCA drives the dynamics if the gas velocity dispersion exceeds the rotational velocity, i.e., turbulent Taylor number < 1. Extended multiphase filaments condense out of the hot phase via thermal instability and rain toward the black hole, boosting the accretion rate up to 100 times the Bondi rate. Initially, turbulence broadens the angular momentum distribution of the hot gas, allowing the cold phase to condense with prograde or retrograde motion. Subsequent chaotic collisions between the cold filaments, clouds, and a clumpy variable torus promote the cancellation of angular momentum, leading to high accretion rates. The simulated sub-Eddington accretion rates cover the range inferred from AGN cavity observations. CCA predicts inner flat X-ray temperature and $r^{-1}$ density profiles, as recently discovered in M 87 and NGC 3115. The synthetic H{\alpha} images reproduce the main features of cold gas observations in massive ellipticals, as the line fluxes and the filaments versus disk morphology. Such dichotomy is key for the long-term AGN feedback cycle. As gas cools, filamentary CCA develops and boosts AGN heating; the cold mode is thus reduced and the rotating disk remains the sole cold structure. Its consumption leaves the atmosphere in hot mode with suppressed accretion and feedback, reloading the cycle.

Defining the Observables for Quarks and Gluons Orbital Angular Momentum Distributions

We present a critical discussion of the observables that have been recently put forth to describe quarks and gluons orbital angular momentum distributions. Starting from a standard parameterization of the energy momentum tensor in QCD one can single out two forms of angular momentum, a so-called kinetic term, generally associated with the Ji decomposition, and a canonical term from the Jaffe Manohar decomposition. Orbital angular momentum has been connected to a Generalized Transverse Momentum Distribution (GTMD), for the canonical term, and to a twist three Generalized Parton Distribution for the kinetic term. We argue that while the latter appears as an azymuthal angular modulation in the longitudinal target spin asymmetry in deeply virtual Compton scattering, due to parity constraints, the GTMD associated with canonical angular momentum cannot be measured in a similar set of experiments.

COEVOLUTION BETWEEN SUPERMASSIVE BLACK HOLES AND BULGES IS NOT VIA INTERNAL FEEDBACK REGULATION BUT BY RATIONED GAS SUPPLY DUE TO ANGULAR MOMENTUM DISTRIBUTION

We reason that, without physical fine-tuning, neither the supermassive black holes (SMBHs) nor the stellar bulges can self-regulate or inter-regulate by driving away already fallen cold gas to produce the observed correlation between them. We suggest an alternative scenario where the observed mass ratios of the SMBHs to bulges reflect the angular momentum distribution of infallen gas such that the mass reaching the stable accretion disc is a small fraction of that reaching the bulge region, averaged over the cosmological time scales. We test this scenario using high resolution, large-scale cosmological hydrodynamic simulations (without AGN feedback), assuming the angular momentum distribution of gas landing in the bulge region to yield a Mestel disc that is supported by independent simulations resolving the Bondi radii of SMBHs. A mass ratio of $0.1-0.3\%$ between the very low angular momentum gas that free-falls to the sub-parsec region to accrete to the SMBH and the overall star formation rate is found. This ratio is found to increase with increasing redshift to within a factor of $\sim 2$, suggesting that the SMBH to bulge ratio is nearly redshift independent, with a modest increase with redshift, a testable prediction. Furthermore, the duty cycle of active galactic nuclei (AGN) with high Eddington ratios is expected to increase significantly with redshift. Finally, while SMBHs and bulges are found to coevolve on $\sim 30-150$Myr time scales or longer, there is indication that, on shorer time scales, the SMBH accretion rate and star formation may be less correlated.

Orbital tomography: Molecular band maps, momentum maps and the imaging of real space orbitals of adsorbed molecules

The frontier orbitals of molecules are the prime determinants of their chemical, optical and electronic properties. Arguably, the most direct method of addressing the (filled) frontier orbitals is ultra-violet photoemission spectroscopy (UPS). Although UPS is a mature technique from the early 1970s on, the angular distribution of the photoemitted electrons was thought to be too complex to be analysed quantitatively. Recently angle resolved UPS (ARUPS) work on conjugated molecules both, in ordered thick films and chemisorbed monolayers, has shown that the angular (momentum) distribution of the photocurrent from orbital emissions can be simply understood. The approach, based on the assumption of a plane wave final state is becoming known as orbital tomography. Here we will demonstrate, with selected examples of pentacene (5A) and sexiphenyl (6P), the potential of orbital tomography. First it will be shown how the full angular distribution of the photocurrent (momentum map) from a specific orbital is related to the real space orbital by a Fourier transform. Examples of the reconstruction of 5A orbitals will be given and the procedure for recovering the lost phase information will be outlined. We then move to examples of sexiphenyl where we interrogate the original band maps of thick sexiphenyl in the light of our understanding of orbital tomography that has developed since then. With comparison to theoretical simulations of the molecular band maps, the molecular conformation and orientation will be concluded. New results for the sexiphenyl monolayer on Al(110) will then be presented. From the band maps it will be concluded that the molecule is planarised and adopts a tilted geometry. Finally the momentum maps down to HOMO-11 will be analysed and real space orbitals reconstructed.

Pulse-shape effects in ionization of atomic hydrogen by short-pulse XUV intense laser radiation: A sensitivity study

The displacement effect studied in a recent paper [Ivanov et al., Phys. Rev. A 90, 043401 (2014)] in atomic ionization by a short XUV pulse is investigated in more detail. It is shown that achieving a significant displacement critically depends on the assumption of a plateau in the envelope function of the electric field, and that the ramp-on is fine-tuned in such a way that a drift velocity generated during the ramp-on phase can increase this displacement further. Seemingly minor variations in the electric fields defined in slightly different ways cause significant changes in the final results, in particular regarding the angular-momentum distribution of the ejected electron. In light of such a strong sensitivity seen in the predictions made with idealized pulse shapes and the likely difficulties of preparing such pulses experimentally, an experimental realization of the displacement effect will likely be a major challenge.

The parent populations of six groups identified from chemical tagging in the solar neighbourhood

We estimate the size and distribution of the parent populations for the 6 largest (at least 20 stars in the Solar neighborhood) chemical groups identified in the Chemical Tagging experiment by Mitschang et al.~2014. Stars in the abundance groups tend to lie near a boundary in angular momentum versus eccentricity space where the probability is highest for a star to be found in the Solar neighborhood and where orbits have apocenter approximately equal to the Sun's galactocentric radius. Assuming that the parent populations are uniformly distributed at all azimuthal angles in the Galaxy, we estimate that the parent populations of these abundance groups contain at least 200,000 members. The spread in angular momentum of the groups implies that the assumption of a uniform azimuthal distribution only fails for the two youngest groups and only for the highest angular momentum stars in them. The parent populations of three thin disk groups have narrow angular momentum distributions, but tails in the eccentricity and angular momentum distributions suggest that only a small fraction of stars have migrated and increased in eccentricity. In contrast, the parent populations of the thick disk groups exhibit both wide angular momentum and eccentricity distributions implying that both heating and radial migration has taken place.

Placement optimization of actuators and sensors for gyroelastic body

Gyroelastic body refers to a flexible structure with a distribution of stored angular momentum provided by fly wheels or control moment gyroscopes. The angular momentum devices can exert active torques to the structure for vibration suppression or shape control. This article mainly focuses on the placement optimization issue of the actuators and sensors on the gyroelastic body. The control moment gyroscopes and angular rate sensors are adopted as actuators and sensors, respectively. The equations of motion of the gyroelastic body incorporating the detailed actuator dynamics are linearized to a loosely coupled state-space model. Two optimization approaches are developed for both constrained and unconstrained gyroelastic bodies. The first is based on the controllability and observability matrices of the system. It is only applicable to the collocated actuator and sensor pairs. The second criterion is formulated from the concept of controllable and observable subspaces. It is capable of handling the cases of both collocated and noncollocated actuator and sensor pairs. The illustrative examples of a cantilevered beam and an unconstrained plate demonstrate the clear physical meaning and rationality of the two proposed methods.

DYNAMICAL PROPERTIES OF COLLISIONLESS STAR STREAMS

A sufficiently extended satellite in the tidal field of a host galaxy loses mass to create nearly symmetric leading and trailing tidal streams. We study the case in which tidal heating drives mass loss from a low mass satellite. The stream effectively has two dynamical components, a common angular momentum core superposed with episodic pulses with a broader angular momentum distribution. The pulses appear as spurs on the stream, oscillating above and below the stream centerline, stretching and blurring in configuration space as they move away from the cluster. Low orbital eccentricity streams are smoother and have less differential motion than high eccentricity streams. The tail of a high eccentricity stream can develop a fan of particles which wraps around at apocenter in a shell feature. We show that scaling the essentially stationary action-angle variables with the cube root of the satellite mass allows a low mass satellite stream to accurately predict the features in the stream from a satellite a thousand times more massive. As a practical astrophysical application, we demonstrate that narrow gaps in a moderate eccentricity stream, such as GD-1, blur out to 50\% contrast over approximately 6 radial periods. A high eccentricity stream, such as Pal~5, will blur small gaps in only radial 2 orbits as can be understood from the much larger dispersion of angular momentum in the stream.

Optical phased array radiating optical vortex with manipulated topological charges.

Optical antennas are key elements in quantum optics emitting and sensing, and behave wide range applications in optical domain. However, integration of optical antenna radiating orbital angular momentum is still a challenge in nano-scale. We theoretically demonstrate a sub-wavelength phased optical antenna array, which manipulates the distribution of the orbital angular momentum in the near field. Orbital angular momentum with topological charge of 4 can be obtained by controlling the phase distribution of the fundamental mode orbital angular momentum in each antenna element. Our results indicate this phased array may be utilized in high integrated optical communication systems.

Local stability of strongly magnetized black hole tori

We investigate the local stability of strongly magnetized relativistic tori orbiting Kerr black holes, for the case of a purely toroidal magnetic field topology. Our approach encompasses both solving the full dispersion relation numerically and solving a simplified analytic treatment. Both approaches indicate that such tori are subject to an unstable non-axisymmetric magnetorotational mode, regardless of such background properties as the gas-to-magnetic pressure ratio and specific angular momentum distribution. We demonstrate that our modal analysis matches well with what is seen in global, three-dimensional, general relativistic magnetohydrodynamic simulations.

Generation of no-diffraction hollow vertex beams with adjustable angular momentum by wave plate phase plates

In this article, a new scheme is proposed to generate approximately no-diffraction hollow vertex beams by wave plates. By selecting the appropriate thickness values of wave plates based on the properties of the double refraction, four-step-phase plates for o-light or e-light are formed. With linearly polarized light irradiated at the phase plate, the diffractions of o-light and e-light would overlap according to their intensities. By focusing effect of quasi-Galileo telescope system, a no-diffraction hollow vertex beam can be generated. In this scheme, the optical path is simple and convenient to adjust. Under the adaxial condition, the distributions of diffraction intensity and angular momentum of two wave plates at the numbers of cycles, s=1 and s=4, are numerically simulated according to Fresnel diffraction theory and classical electromagnetic field angular momentum theory. Simulation results indicate that the approximately no-diffraction hollow vertex beams can be generated by each of two phase plates within a long distance. The distributions of intensity and the angular momentum are essentially the same as those generated by spiral phase plates at the same number of cycles. The distributions of intensity and the angular momentum are different at different numbers of cycles s. If s increases, the diffraction bright ring radius increases, the intensity decreases and the average orbital angular momentum increases. At s=4, the length of no-diffraction region is significantly greater than at s=1 and the average orbital angular momentum is four times that at s=1. Within the no-diffraction region, the distribution of orbital angular momentum intensity varies with distance but the total angular momentum is constant. A phase compensator is inserted in the diffraction path to adjust the phase difference between o-light and e-light. Whereas the spin angular momentum of the diffraction light can be adjusted by them, and thus the total angular momentum intensity and average photon angular momentum can be adjusted. This scheme can be utilized to guide the cold atoms or molecules to obtain the adjustable torque throughout the interacting process of atoms and photons.

Study on the properties of vector beams generated by a curved wave plate in the strong-focusing regime

We propose a new scheme to generate the axisymmetric vector beam. A curved wave plate, designed by using the birefringence properties of a crystal, can generate two different phase distributions with respect to the o-light and e-light, and it then can transform the linearly polarized light into the radial or azimuthal vector beam with the property of rotational symmetry. The above scheme has the advantage of having a simple light path and can be adjusted conveniently. According to Richards-Wolf's model of the classical vector diffraction, we calculate the distributions of the diffracted electromagnetic field that is illuminated by a hollow Gaussian beam and focused by a lens with high numerical aperture. Results show that the hollow vector beam has a very high intensity and intensity gradient and longitudinal distribution of the electric or magnetic field even if illuminated by the laser with a power of 0.5 W. In addition, real-time adjusted distributions of the photon angular momentum can be generated. This scheme has good application prospects in the manipulations of the microscopic particles.

Observables for Quarks and Gluons Orbital Angular Momentum Distributions

We discuss the observables that have been recently put forth to describe quarks and gluons orbital angular momentum distributions. Starting from a standard parameterization of the energy momentum tensor in QCD one can single out two forms of angular momentum, a so-called kinetic term – Ji decomposition – or a canonical term – Jaffe-Manohar decomposition. Orbital angular momentum has been connected in each decomposition to a different observable, a Generalized Transverse Momentum Distribution (GTMD), for the canonical term, and a twist three Generalized Parton Distribution (GPD) for the kinetic term. While the latter appears as an azimuthal angular modulation in the longitudinal target spin asymmetry in deeply virtual Compton scattering, due to parity constraints, the GTMD associated with canonical angular momentum cannot be measured in a similar set of experiments.

Generation of vortex beams by the four-step phase plates

The generation and application of the vortex beams are part of the hot topics in the optical field. In this paper, the phase structure of the four-step phase plates, analyzed by Fourier series expansion method, is composed of a series of spiral phase plates. When the phase plate is directly irradiated by linearly polarized light, multi-order diffraction waves with different topological charge numbers are generated. Unlike vortex waves, the intensity distribution of the multi-order diffraction has a deviation from the axial symmetry due to the interference with each other. On this basis, a new scheme is proposed to generate vortex beams by the four-step phase plates. With the help of Mach-Zehnder interferometer, the diffraction waves generated by two pieces of the four-step phase plates overlap each other. By adjusting the phase difference of the Mach-Zehnder interferometer, some orders of diffraction waves generate destructive interference while the others generate constructive interference. Thus the linear polarized light can be converted into vortex beams. The diffraction intensity and angular momentum distributions of the four-step phase plates with different cycle numbers are numerically simulated and compared with the spiral phase plates, we can provethat the vortex beams can be obtained by simple four-step phase plates which are the same as those obtained by spiral phase plates. In addition, the four-step phase plates with a small cycle number can generate vortex beams with a large topological charge number and the fabrication difficulty of the phase plates is reduced.

Varying focal fields with asymmetric-sector-shaped vector beams

We theoretically and experimentally investigate the focusing properties of asymmetric-sector-shaped vector beams with localized linear polarization. Simulation results show that the shape-only modulation of the vector beam allows one to simultaneously change the intensity, phase, polarization, as well as spin angular momentum distributions of the focused field. Experimentally, we generate asymmetric-sector-shaped vector beams and study its intensity distributions and the polarization characteristics at the focal plane, which are in good agreements with the numerical simulations. The presented approach enriches the methods for focus engineering and inspires the manipulation of the angular momentum of light, which would be useful for optical manipulation.

Development of multiple tidal tails around globular clusters and dwarf satellite galaxies

The formation and evolution of tidal tails like those observed around some globular clusters and dwarf satellite galaxies is examined with an N-body simulation. In particular, we analyse in detail the evolving tidal features of a one-component satellite that is moving on a highly eccentric orbit in the external field of a host galaxy potential like our own. The results show that every time the satellite approaches apogalacticon, a fresh pair of tidal tails becomes notably prominent, and that eventually, the satellite possesses multiple tidal tails via repeating apocentre passages. Accordingly, the number of observed tidal arms can be used as a tracer of the number of orbital periods that such a system has completed around the centre of its host galaxy. By identifying the arm particles included in each of the first three consecutively formed pairs of tidal tails, we find that each pair of tidal tails is practically identical to one another regarding the energy and angular-momentum distributions. In addition, we demonstrate that the density profiles of these three pairs of tidal tails at their first apogalacticons after formation agree well with one another. It therefore follows that the multiplicity in tidal features originates from the repeated episode of tidal-arm formation in the course of the precessing motion of a satellite.

Optimal Gyricity Distribution for Space Structure Vibration Control

Many space vehicles have been launched with large flexible components such as booms and solar arrays. These large space structures may require active shape control given the possibility of lightly damped vibrations. Such vibrations can be controlled using a collection of control moment gyros, which consist of a spinning wheel in gimbals and produce a torque when the orientation of the wheel is changed. This study investigates the optimal distribution of these control moment gyros on large space structures for vibration suppression. Initially, a continuum model is adopted that represents the distribution of control moment gyros by a continuous distribution of stored angular momentum (gyricity). The optimal control solution for the gimbal motions (for a given gyricity distribution) is then optimized with respect to the gyricity distribution. Further investigation considers discrete parameter models of beam and a plate structures with evenly placed discrete pointwise control moment gyros. Numerical optimization of a suitable cost function allocates the amount of stored angular momentum possessed by these control moment gyros.