Angular Mo Research Articles

Quasi-periodic oscillations from relativistic ray-traced hydrodynamical tori

We simulate an oscillating purely hydrodynamical torus with constant specific angular mo- mentum around a Schwarzschild black hole. The goal is to search for quasi-periodic oscil- lations (QPOs) in the light curve of the torus. The initial torus setup is subjected to radial, vertical and diagonal (combination of radial and vertical) velocity perturbations. The hydro- dynamical simulations are performed using the general relativistic magnetohydrodynamics code Cosmos++ and ray-traced using the GYOTO code. We found that a horizontal velocity perturbation triggers the radial and plus modes, while a vertical velocity perturbation trig- gers the vertical and X modes. The diagonal perturbation gives a combination of the modes triggered in the radial and vertical perturbations.

Dynamics of an electric dipole moment in a stochastic electric field

The mean-field dynamics of an electric dipole moment in a deterministic and a fluctuating electric field is solved to obtain the average over fluctuations of the dipole moment and the angular momentum as a function of time for a Gaussian white-noise stochastic electric field. The components of the average electric dipole moment and the average angular momentum along the deterministic electric-field direction do not decay to zero, despite fluctuations in all three components of the electric field. This is in contrast to the decay of the average over fluctuations of a magnetic moment in a stochastic magnetic field with Gaussian white noise in all three components. The components of the average electric dipole moment and the average angular momentum perpendicular to the deterministic electric-field direction oscillate with time but decay to zero, and their variance grows with time.

Energy currents for quasi-monochromatic fields

Relations for the components of Poynting vector are obtained for quasi- monochromatic waves. It is shown that the behaviour of the transverse Poynting vector component is similar to that for the coherent waves. The total angular mo- mentum of the quasi-monochromatic wave can be separated into orbital and spin parts. Using the example of a Gaussian beam, we show that the spin angular mo- mentum is associated with coherence characteristics of the optical beam.

Chaos in Compact Binaries with Frequency Map Analysis

The dynamics of compact binaries is very complicated because of spin-orbit cou- pling and spin-spin coupling. With Laskar's frequency map analysis (FMA) and frequency diffusion as an indicator, we found that misalignment of the spins and orbital angular mo- mentum has a great effect on the dynamics, and for systems with different mass ratios β ≡ m2/m1 chaos occurs at different spin-orbit configurations. For equal-mass binaries (β =1 ), chaos occurs when the spins nearly cancel each other out. For some other systems (for example β ∼ 1/2), the binaries are irregular,even chaotic, when the spins are perpendic- ular to the orbital angular momentum. For the case where gravitational radiation is taken into account, we give an analytic estimation for the frequency diffusion based on the decay of the orbit, which is roughly consistent with our simulations. This means the FMA is not suitable as a chaos indicator for weak chaotic cases with dissipative terms.

Can ion-neutral damping help to form spicules?

The possible mechanism of generation of spicules by Alfvenic disturbances is studied in dissipative MHD where dissipation is mainly caused by ion-neutral collision damping, as suggested by Haerendel (1992). Ion-neutral damping becomes non-negligible in the upper chromosphere at high cyclic frequencies of typically greater than 0.1 Hz, and the potential role played by this effect in both forming and supporting solar spicules is investigated. The propagation of randomly generated Alfvenic disturbances on vertically open solar magnetic flux tubes is considered. The flux tubes are taken to be axisymmetric and initially untwisted with the field strength declining from 1600 G in the photo- sphere to 20 G in the corona. Their propagation is investigated by numerically solving a set of fully nonlinear, dissipative 1.5D MHD equations with waves being generated by a continuous random driver introduced into the equation of angular mo- mentum in the low atmosphere of the Sun. This work is a continuation of James et al. (2003) which studied the results for a continuous, monochromatic sinusoidal driver. As with the previous study, spicule-like structures were formed. The formation was again found to be primarily caused by the impact of a series of slow shocks generated by the continuous interaction between the upward propagating driven disturbance and the downward propagating disturbances reflected by the transition region. The formation was aided by the increased ther- mal pressure gradient caused by Joule heating due to ion-neutral collisions. There is some indication that an analogue of the momentum transfer effect suggested by Haerendel (1992) for simple sinusoidal waves is at work, but this effect on it's own is at best only of a similar order as the reduction in height caused by including damping in the first place. However, the effect is highly sensitive to the level of ionisation and therefore to the energy balance. Including the effects of thermal conduction and radiation may well lead to different results and thus it would be premature to dismiss the mechanism completely at this point. Significant damping and heating was again observed, strengthening the previously made suggestions that ion-neutral damping may play a more important role in the dynamics of the upper chromosphere than normally assumed in numerical simulations (where it is often neglected completely), although a treatment of radiative losses must be included before this can be confirmed. The heating provided by ion-neutral damping may be an appropriate counter to the low temperatures suffered by other mechanisms better able to reproduce spicule dynamics.

On shear-induced turbulence in rotating stars

We review various prescriptions which have been proposed for the turbulent transport of matter and angular mo- mentum in differentially rotating stellar radiation zones. A new prescription is presented for the horizontal transport associated with the anisotropic shear turbulence which is produced by the differential rotation in latitude; this β-viscosity is drawn from torque measurements in the classical Couette-Taylor experiment (Richard & Zahn 1999, A&A, 347, 734). Its implementation in a stellar evolution code leads to enhanced mixing, as illustrated by models of a rotating main-sequence star of 1.5 solar mass.

Very extended shapes in the A ∼ 110 region

Very extended shapes in the A ∼ 110 region

Angular-momentum-insensitive quantum-defect theory for diatomic systems

With recent developments such as the analytic solutions of the Schrodinger equation for 1/r 6 and 1/r 3 potentials @1-3#, our understanding of two-atom systems is already conceptually comparable to the quantum-defect theory ~QDT! of atomic Rydberg spectra @4#. Namely, the slow atomic scattering and the rovibrational spectrum of a particu- lar angular momentum can be understood in terms of the long-range solutions and a parameter that is a slowly varying function of energy in the threshold region @5-7,1-3#. In this work, we show that a two-atom system possesses an even greater degree of systematics in the following sense. Unlike the QDT for atomic Rydberg spectra in which different angular-momentum states have different quantum defects with no general relationships among them, a single param- eter is sufficient to characterize slow atomic collisions for practically all angular momenta. The same parameter also characterizes the rovibrational spectra in the threshold re- gion, including states of different angular momenta. In other words, in addition to the relationship between the bound spectrum and scattering, as expected from traditional QDT formulations @4#, a two-atom system has also the unique property that scattering of different angular momenta are re- lated, and so are the bound spectra of different angular mo- menta. The origin of this relationship between different angular- momentum states is not difficult to understand and is due to a combination of the following three properties of a typical molecular system. ~i! Atoms are strongly repulsive at short distances. ~ii! Atoms are heavy compared to electrons. ~iii!

Carbon–carbon π antibonding effects on the thermochemistry of alkanes, elucidated by angular overlap and MO calculations

It is postulated that the enthalpies of formation of alkanes can be rationalised quantitatively in terms of an additive bond energy scheme with a constant C–H term (taking the methane value of 415.8 kJ mol−1) while the C–C bond energy is dependent upon the π-antibonding effects of the highest occupied molecular orbitals (HOMOs). This antibonding effect can be treated by angular overlap methods with two adjustable parameters, eπ′(HH) (which represents the magnitude of the effect where the 2pπ orbitals concerned are engaged in σ-overlap only with hydrogen 1s orbitals) and eπ′(HC) (where one 2pπ orbital is σ-bonded to hydrogen atoms and the other to carbon atoms); eπ′(CC) is deemed to be zero, so that the C–C bond energy in the absence of any π antibonding effect takes the diamond value (357.4 kJ mol−1). The values for eπ′(CH) and eπ′(HH) are obtained from the experimental enthalpies of formation of C2–C6 alkanes having no 1,4 steric interactions. It is then possible to calculate bond energy terms for each of the types of bonds CX –CY [X, Y = P(primary), S(secondary), T(tertiary), Q(quaternary)]. These yield calculated enthalpies of formation for alkanes CnH2n+2 (n = 2–8) in excellent agreement with experiment in the absence of significant steric interactions. Corrections for 1,4 steric repulsion are needed for CS–CQ , CT–CT and CT–CQ bonds but not for CS–CT . The HOMOs in CH4−nMen (n = 2, 3) obtained from semi-empirical MO calculations (PM3) have the same relative orbital energies (b2>a1>b1 for n = 2, a1>e for n = 3) as found in the angular overlap calculations, and there is a clear correlation between the antibonding effects as measured by MO eigenvalues and the empirical thermochemical parameters. There is also an unexpectedly good agreement between the atomic fractional charges in alkanes calculated by Mulliken population analysis of the PM3 MOs and those obtained by electronegativity equilibration.

Gravitational Collapse of Rotating Gaseous Ellipsoids

view Abstract Citations (56) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gravitational Collapse of Rotating Gaseous Ellipsoids Fujimoto, M. Abstract The work of Rossner on the finite-amplitude oscillation of a Maclaurin ellipsoid is extended to the adiabatic and non-adiabatic gravitational collapse of a rotating uniform gaseous ellipsoid. The problem is made tractable by introducing an appropriate temperature distribution in the initial ellipsoid and by assuming a special formula for the cooling rate of the gas. Solutions can be represented by trajectories in the (al,a2,a3)-space, where ai, a2, and a~ are the lengths of the three principal axes of the ellipsoid. The adiabatic collapse is characterized by the same trajectories that represent non-linear adiabatic pulsations. They never reach any plane a, = 0 (j = 1, 2, 3) but are confined within a closed zero-velocity surface. The trajectories in the (ai,a2,a3)-space which represent the non-adiabatic collapse make one or more loops, depending on the cooling rate of the gas and the total angular momentum. They finally reach the plane a3 = 0; thus the ellipsoid ultimately becomes a flat disk. A small, finite, non-axisymmetric perturbation applied to an axisymmetric ellipsoid grows in the course of the collapse. For large angular momentum the resultant figure is a very thin, needle-like ellipsoid. An ellipsoid which initially has three unequal principal axes displays a peculiar gravitational collapse under the constraint of the circulation integral. The configuration may be elongated even if the angular mo- mentum is very small Publication: The Astrophysical Journal Pub Date: May 1968 DOI: 10.1086/149569 Bibcode: 1968ApJ...152..523F full text sources ADS |