Angular Momentum
Distribution by Scientific Domains 
  




Distribution within Physics and Astronomy 
  



Kinds of Angular Momentum
Terms modified by Angular Momentum
Selected Abstracts
Comparative study between N body and Fokker,Planck simulations for rotating star clusters , I. Equalmass system
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Eunhyeuk Kim
ABSTRACT
We have carried out N body simulations for rotating star clusters with equal mass and compared the results with Fokker,Planck models.
These two different approaches are found to produce fairly similar results, although there are some differences with regard to the detailed aspects.
We confirmed the acceleration of the core collapse of a cluster due to an initial nonzero angular momentum and found a similar evolutionary trend in the central density and velocity dispersion in both simulations.
The degree of acceleration depends on the initial angular momentum.
Angular momentum is being lost from the cluster due to the evaporation of stars with a large angular momentum on a relaxation timescale.
[source]
An objective finite element approximation of the kinematics of geometrically exact rods and its use in the formulation of an energy,momentum conserving scheme in dynamics
INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 12 2002
I. Romero
Abstract
We present in this paper a new finite element formulation of geometrically exact rod models in the threedimensional dynamic elastic range.
The proposed formulation leads to an objective (or frameindifferent under superposed rigid body motions) approximation of the strain measures of the rod involving finite rotations of the director frame, in contrast with some existing formulations.
This goal is accomplished through a direct finite element interpolation of the director fields defining the motion of the rod's crosssection.
Furthermore, the proposed framework allows the development of timestepping algorithms that preserve the conservation laws of the underlying continuum Hamiltonian system.
The conservation laws of linear and angular momenta are inherited by construction, leading to an improved approximation of the rod's dynamics.
Several numerical simulations are presented illustrating these properties.
Copyright © 2002 John Wiley & Sons, Ltd.
[source]
Numerical instabilities in the computation of pseudopotential matrix elements
JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2006
Christoph van Wüllen
Abstract
Steep high angular momentum Gaussian basis functions in the vicinity of a nucleus whose inner electrons are replaced by an effective core potential may lead to numerical instabilities when calculating matrix elements of the core potential.
Numerical roundoff errors may be amplified to an extent that spoils any result obtained in such a calculation.
Effective core potential matrix elements for a model problem are computed with high numerical accuracy using the standard algorithm used in quantum chemical codes and compared to results of the MOLPRO program.
Thus, it is demonstrated how the relative and absolute errors depend an basis function angular momenta, basis function exponents and the distance between the offcenter basis function and the center carrying the effective core potential.
Then, the problem is analyzed and closed expressions are derived for the expected numerical error in the limit of large basis function exponents.
It is briefly discussed how other algorithms would behave in the critical case, and they are found to have problems as well.
The numerical stability could be increased a little bit if the type 1 matrix elements were computed without making use of a partial wave expansion. © 2005 Wiley Periodicals, Inc., J Comput Chem 27: 135,141 2006
[source]
The SAURON project , VII.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
Integralfield absorption, emissionline kinematics of 24 spiral galaxy bulges
ABSTRACT
We present observations of the stellar and gas kinematics for a representative sample of 24 Sa galaxies obtained with our custombuilt integralfield spectrograph SAURON operating on the William Herschel Telescope.
The data have been homogeneously reduced and analysed by means of a dedicated pipeline.
All resulting data cubes were spatially binned to a minimum mean signaltonoise ratio of 60 per spatial and spectral resolution element.
Our maps typically cover the bulgedominated region.
We find a significant fraction of kinematically decoupled components (12/24), many of them displaying central velocity dispersion minima.
They are mostly aligned and corotating with the main body of the galaxies, and are usually associated with dust discs and rings detected in unsharpmasked images.
Almost all the galaxies in the sample (22/24) contain significant amounts of ionized gas which, in general, is accompanied by the presence of dust.
The kinematics of the ionized gas are consistent with circular rotation in a disc corotating with respect to the stars.
The distribution of mean misalignments between the stellar and gaseous angular momenta in the sample suggests that the gas has an internal origin.
The [O iii]/H, ratio is usually very low, indicative of current star formation, and shows various morphologies (ringlike structures, alignments with dust lanes or amorphous shapes).
The star formation rates (SFRs) in the sample are comparable with that of normal disc galaxies.
Low gas velocity dispersion values appear to be linked to regions of intense star formation activity.
We interpret this result as stars being formed from dynamically cold gas in those regions.
In the case of NGC 5953, the data suggest that we are witnessing the formation of a kinematically decoupled component from cold gas being acquired during the ongoing interaction with NGC 5954.
[source]
Symmetry breaking and Wigner molecules in fewelectron quantum dots
PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2006
Constantine Yannouleas
Abstract
We discuss symmetry breaking in twodimensional quantum dots resulting from strong interelectron repulsion relative to the zeropoint kinetic energy associated with the confining potential.
Such symmetry breaking leads to the emergence of crystalline arrangements of electrons in the dot.
The socalled Wigner molecules form already at fieldfree conditions.
The appearance of rotating Wigner molecules in circular dots under high magnetic field, and their relation to magic angular momenta and quantumHalleffect fractional fillings is also discussed.
Recent calculations for two electrons in an elliptic quantum dot, using exact diagonalization and an approximate generalizedHeitler,London treatment, show that the electrons can localize and form a molecular dimer for screened interelectron repulsion.
The calculated singlettriplet splitting (J ) as a function of the magnetic field (B ) agrees with cotunneling measurements; its behavior reflects the effective dissociation of the dimer for large B . Knowledge of the dot shape and of J (B ) allows determination of two measures of entanglement (concurrence and von Neumann entropy for indistinguishable fermions), whose behavior correlates also with the dissociation of the dimer.
The theoretical value for the concurrence at B = 0 agrees with the experimental estimates. (© 2006 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)
[source]
Dependence of s waves on continuous dimension: The quantum oscillator and free systems
FORTSCHRITTE DER PHYSIK/PROGRESS OF PHYSICS, Issue 12 2006
K.B. Wolf
Abstract
Wavefunctions with rotational symmetry (i.e., zero angular momentum) in D dimensions, are called s waves.
In quantum quadratic systems (free particle, harmonic and repulsive oscillators), their radial parts obey Schrödinger equations with a fictitious centrifugal (for integer D , 4) or centripetal (for D = 2) potential.
These Hamiltonians close into the threedimensional Lorentz algebra so(2,1), whose exceptional interval corresponds to the critical range of continuous dimensions 0 < D < 4, where they exhibit a oneparameter family of selfadjoint extensions in ,2(,+).
We study the characterization of these extensions in the harmonic oscillator through their spectra which , except for the Friedrichs extension , are not equally spaced, and we build their time evolution Green function.
The oscillator is then contracted to the free particle in continuous D dimensions, where the extension structure is mantained in the limit of continuous spectra.
Finally, we compute the free time evolution of the expectation values of the Hamiltonian, dilatation generator, and square radius between three distinct sets of ,heat'diffused localized eigenstates.
This provides a simple grouptheoretic description of the purported contraction/expansion of Gaussianring s waves in D > 0 dimensions.
[source]
Time asymmetric quantum theory , III.
FORTSCHRITTE DER PHYSIK/PROGRESS OF PHYSICS, Issue 6 2003
Decaying states, the causal Poincaré semigroup
Abstract
These Gamow kets span an irreducible representation space for Poincaré transformations which, similar to the Wigner representations for stable particles, are characterized by spin (angular momentum of the partial wave amplitude) and complex mass (position of the resonance pole).
The Poincaré transformations of the Gamow kets, as well as of the LippmannSchwinger plane wave scattering states, form only a semigroup of Poincaré transformations into the forward light cone.
Their transformation properties are derived.
From these one obtains an unambiguous definition of resonance mass and width for relativistic resonances.
The physical interpretation of these transformations for the Born probabilities and the problem of causality in relativistic quantum physics is discussed.
[source]
Variations in the Earth's gravity field caused by torsional oscillations in the core
GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2004
Mathieu Dumberry
SUMMARY
We investigate whether a component of the flow in the Earth's fluid core, namely torsional oscillations, could be detected in gravity field data at the surface and whether it could explain some of the observed time variations in the elliptical part of the gravity field (J2).
Torsional oscillations are azimuthal oscillations of rigid coaxial cylindrical surfaces and have typical periods of decades.
This type of fluid motion supports geostrophic pressure gradients, which produce deformations of the core,mantle boundary.
Because of the density discontinuity between the core and the mantle, such deformations produce changes in the gravity field that, because of the flow geometry, are both axisymmetric and symmetric about the equator.
Torsional oscillations are thus expected to produce time variations in the zonal harmonics of even degree in the gravity field.
Similarly, the changes in the rotation rates of the mantle and inner core that occur to balance the change in angular momentum carried by the torsional oscillations also produce zonal variations in gravity.
We have built a model to calculate the changes in the gravity field and in the rotation rates of the mantle and inner core produced by torsional oscillations.
We show that the changes in the rotation rate of the inner core produce changes in J2 that are a few orders of magnitude too small to be observed.
The amplitudes of the changes in J2 from torsional oscillations are 10 times smaller than the temporal changes that are observed to occur about a linear secular trend.
However, provided the mechanism responsible for these changes in J2 is identified and that this contribution is removed from the data, it may be possible in the future to detect the lowest harmonic degrees of the torsional oscillations in the gravity field data.
We also show that torsional oscillations have contributed to the linear secular change in J2 by about ,0.75 × 10,12 per year in the last 20 years.
Finally, the associated change in the vertical ground motion at the surface of the Earth that is predicted by our mechanism is of the order of 0.2 mm, which is too small to be detected with the current precision in measurements.
[source]
Optimal transportation meshfree approximation schemes for fluid and plastic flows
INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 12 2010
B. Li
Abstract
We develop an optimal transportation meshfree (OTM) method for simulating general solid and fluid flows, including fluid,structure interaction.
The method combines concepts from optimal transportation theory with materialpoint sampling and maxent meshfree interpolation.
The proposed OTM method generalizes the Benamou,Brenier differential formulation of optimal mass transportation problems to problems including arbitrary geometries and constitutive behavior.
The OTM method enforces mass transport and essential boundary conditions exactly and is free from tension instabilities.
The OTM method exactly conserves linear and angular momentum and its convergence characteristics are verified in standard benchmark problems.
We illustrate the range and scope of the method by means of two examples of application: the bouncing of a gasfilled balloon off a rigid wall; and the classical Tayloranvil benchmark test extended to the hypervelocity range.
Copyright © 2010 John Wiley & Sons, Ltd.
[source]
A variationally consistent mesh adaptation method for triangular elements in explicit Lagrangian dynamics
INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 9 2010
Sudeep K. Lahiri
Abstract
In this paper a variational formulation for mesh adaptation procedures, involving local mesh changes for triangular meshes, is presented.
Such local adaptive changes are very well suited for explicit methods as they do not involve significant computational expense.
They also greatly simplify the projection of field variables from the old to the new meshes.
Crucially, the variational nature of the formulation used to derive the equilibrium equations at steps where adaptation takes place ensures that conservation of linear and angular momentum is obtained (Int. J. Numer.
Meth.
Engng 2000; 49:1295,1325).
Several examples in 2D showing the application of the proposed adaptive algorithms are used to demonstrate the validity of the methodology proposed.
Copyright © 2009 John Wiley & Sons, Ltd.
[source]
Correlation studies in weakly confining quantum dot potentials
INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2008
Peter Kimani
Abstract
We investigate the electron correlation in fewelectron closedshell atomic systems and similarly in fewelectron quantum dots under weak confinement.
As usual we start with restricted Hartree,Fock (HF) calculations and add electron correlation in steps in a series of approximations based on the single particle Green's function approach: (i) secondorder Green function (GF); (ii) 2ph TammDancoff approximation (TDA); and (iii) an extended version thereof which introduces groundstate correlation into the TDA.
Our studies exhibit similarities and differences between weakly confined quantum dots and standard atomic systems.
The calculations support the application of HF, GF, and TDA techniques in the modeling of threedimensional quantum dot systems.
The observed differences emphasize the significance of confinement and electronic features unique to quantum dots, such as the increased binding of electrons with higher angular momentum and thus,compared to atomic systems,modified shellfilling sequences. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
[source]
Quantum wave packet calculation of reaction probabilities, cross sections, and rate constants for the C(1D) + HD reaction
INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005
Fahrettin Gogtas
Abstract
The timedependent real wave packet method has been used to study the C(1D) + HD reaction.
The statetostate and statetoall reactive scattering probabilities for a broad range of energies are calculated at zero total angular momentum.
The probabilities for J > 0 are estimated from accurately computed J = 0 probabilities by using the J shifting approximation.
The integral cross sections for a large energy range, and thermal rate constants are calculated. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005
[source]
Cover Picture: Laser & Photon.
LASER & PHOTONICS REVIEWS, Issue 4 2008
Rev.: 2(4)/200
Light can carry both spin and orbital angular momentum.
The orbital angular momentum arises from "twisted" phasefronts which can be made visible in interferograms.
Still a young research area, the investigation of orbital angular momentum of light now encompasses areas as diverse as testing of quantum information protocols, manipulation of ultracold atoms and applications in micromachining. (Cover picture: S. FrankeArnold, L. Allen, and M. Padgett, pp.
299,313, in this issue)
[source]
Magnetoelectric effect on frequency mixing in atoms
LASER PHYSICS LETTERS, Issue 7 2004
V. D. Ovsiannikov
Abstract
Steady electric and magnetic fields can stimulate frequency mixing of two laser waves in ensemble of free atoms.
In addition to coherence conditions, the steady fields may induce additional resonance singularities essentially enhancing the cross section for scattering the sumfrequency wave.
Interference between different components of the electric and magneticfield induced frequency summation amplitudes may cause significant effects on the efficiency of conversion.
The dependence on the incident wave polarization and the atomic resonance structure is calculated analytically for the frequency mixing in atoms with a singlet structure of the ground and resonance states.
Numerical estimates for the quantitative characteristics of the effect are presented for helium, alkaliearth and mercury atoms in their ground n1S0 state in the case of the twophoton resonance on excited singlet states with angular momentum 0, 1, and 2. (© 2004 by ASTRO, Ltd. Published exclusively by WILEYVCH Verlag GmbH & Co. KGaA)
[source]
On the polarization properties of the micromaser pumped by the atoms with degenerate levels
LASER PHYSICS LETTERS, Issue 3 2004
V. A. Reshetov
Abstract
The polarization properties of the micromaser field pumped by the atoms with the resonant levels, which are degenerate in the projections of the total angular momentum on the quantization axis, are studied numerically.
The standard micromaser operation, when the atoms enter the cavity excited to the upper resonant level, and the micromaser operation under the coherent pump, when the atoms enter the cavity in a superposition of resonant atomic levels, are considered.
The treatment is based on the master equation for the density matrix of the micromaser field, which takes into account the degeneracy of atomic levels. (© 2004 by HMS Consultants.
Inc. Published exclusively by WILEYVCH Verlag GmbH & Co.KGaA)
[source]
Blowup analysis, existence and qualitative properties of solutions for the twodimensional Emden,Fowler equation with singular potential
MATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 18 2007
Daniele Bartolucci
Abstract
Motivated by the study of a twodimensional point vortex model, we analyse the following Emden,Fowler type problem with singular potential:
where V(x) = K(x)/x2, with ,,(0, 1), 0angular momentum in the system and obtain the existence of axially symmetric one peak nonradial blowup solutions.
Copyright © 2007 John Wiley & Sons, Ltd.
[source]
The determination of planetary structure in tidally relaxed inclined systems
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010
Rosemary A. Mardling
ABSTRACT
The recent discovery of a transiting shortperiod planet on a slightly noncircular orbit with a massive highly eccentric companion orbiting the star HATP13 offers the possibility of probing the structure of the shortperiod planet.
The ability to do this relies on the system being in a quasiequilibrium state in the sense that the eccentricities are constant on the usual secular timescale (typically, a few thousand years), and decay on a timescale which is much longer than the age of the system.
Since the equilibrium eccentricity is effectively a function only of observable system parameters and the unknown Love number of the shortperiod planet, the latter can be determined with accurate measurements of the planet's eccentricity and radius.
However, this analysis relies on the assumption that the system is coplanar, a situation which seems unlikely given the high eccentricity of the outer planet.
Here we generalize our recent analysis of this fixedpoint phenomenon to mutually inclined systems in which the outer body dominates the total angular momentum, and show that (1) the fixed point of coplanar systems is replaced by a limit cycle in eb,, space, where eb is the eccentricity of the inner planet and , is the angle between the periapse lines, with the average value of eb, e(av)b, decreasing and its amplitude of variation increasing with increasing mutual inclination.
This behaviour significantly reduces the ability to unambiguously determine the Love number of the shortperiod planet if the mutual inclination is higher than around 10°. (2) We show that for Q values less than 106, the HATP13 system cannot have a mutual inclination between 54° and 126° because Kozai oscillations coupled with tidal dissipation would act to quickly move the inclination outside this range, and (3) that the behaviour of retrograde systems is the mirror image of that for prograde systems in the sense that (almost) identical limit cycles exist for a given mutual inclination and , minus this value. (4) We derive a relationship between e(av)b, the equilibrium radius of the shortperiod planet, its Q value and its core mass, and show that given current estimates of eb and the planet radius, as well as the lower bound placed on the Q value by the decay rate of e(av)b, the HATP13 system is likely to be close to prograde coplanar, or have a mutual inclination between 130°
and 135°.
Lower rather than higher core masses are favoured. (5) An expression for the timescale for decay of the mutual inclination is derived, revealing that it evolves towards a nonzero value as long as eb > 0 on a timescale which is much longer than the age of the system. (6) We conclude with a scattering scenario for the origin of the HATP13 system and show that almost identical initial conditions can result in significantly different outer planet eccentricities, stellar obliquities and planet radii.
The implications for systems with high stellar obliquities such as HATP7 and WASP17 are briefly discussed.
[source]
Gas dynamics of the central few parsec region of NGC 1068 fuelled by the evolving nuclear star cluster
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
M. Schartmann
ABSTRACT
Recently, highresolution observations with the help of the nearinfrared adaptive optics integral field spectrograph Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the Very Large Telescope proved the existence of massive and young nuclear star clusters in the centres of a sample of Seyfert galaxies.
With the help of threedimensional highresolution hydrodynamical simulations with the Pluto code, we follow the evolution of such clusters, especially focusing on stellar mass loss feeding gas into the ambient interstellar medium and driving turbulence.
This leads to a vertically wide distributed clumpy or filamentary inflow of gas on large scales (tens of parsec), whereas a turbulent and very dense disc builds up on the parsec scale.
In order to capture the relevant physics in the inner region, we treat this disc separately by viscously evolving the radial surface density distribution.
This enables us to link the tens of parsecscale region (accessible via SINFONI observations) to the (sub)parsecscale region (observable with the midinfrared interferometer instrument and via water maser emission).
Thereby, this procedure provides us with an ideal testbed for data comparison.
In this work, we concentrate on the effects of a parametrized turbulent viscosity to generate angular momentum and mass transfer in the disc and additionally take star formation into account.
Most of the input parameters are constrained by available observations of the nearby Seyfert 2 galaxy NGC 1068, and we discuss parameter studies for the free parameters.
At the current age of its nuclear starburst of 250 Myr, our simulations yield disc sizes of the order of 0.8,0.9 pc, gas masses of 106 M, and mass transfer rates of 0.025 M, yr,1 through the inner rim of the disc.
This shows that our largescale torus model is able to approximately account for the disc size as inferred from interferometric observations in the midinfrared and compares well to the extent and mass of a rotating disc structure as inferred from water maser observations.
Several other observational constraints are discussed as well.
[source]
Azimuthally symmetric theory of gravitation , I. On the perihelion precession of planetary orbits
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2010
G. G. Nyambuya
ABSTRACT
From a purely nongeneral relativistic standpoint, we solve the empty space Poisson equation (,2,= 0) for an azimuthally symmetric setting (i.e. for a spinning gravitational system like the Sun).
We seek the general solution of the form ,=,(r, ,).
This general solution is constrained such that in the zerothorder approximation it reduces to Newton's wellknown inverse square law of gravitation.
For this general solution, it is seen that it has implications on the orbits of test bodies in the gravitational field of this spinning body.
We show that to secondorder approximation, this azimuthally symmetric gravitational field is capable of explaining at least two things: (i) the observed perihelion shift of solar planets; (ii) the fact that the mean Earth,Sun distance must be increasing (this resonates with the observations of two independent groups of astronomers, who have measured that the mean Earth,Sun distance must be increasing at a rate between about 7.0 ± 0.2 m century,1 and 15.0 ± 0.3 m cy,1).
In principle, we are able to explain this result as a consequence of the loss of orbital angular momentum; this loss of orbital angular momentum is a direct prediction of the theory.
Further, we show that the theory is able to explain at a satisfactory level the observed secular increase in the Earth year (1.70 ± 0.05 ms yr,1).
Furthermore, we show that the theory makes a significant and testable prediction to the effect that the period of the solar spin must be decreasing at a rate of at least 8.00 ± 2.00 s cy,1.
[source]
Quantitative analysis of clumps in the tidal tails of star clusters
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
A. Just
ABSTRACT
Tidal tails of star clusters are not homogeneous but show welldefined clumps in observations as well as in numerical simulations.
Recently, an epicyclic theory for the formation of these clumps was presented.
A quantitative analysis was still missing.
We present a quantitative derivation of the angular momentum and energy distribution of escaping stars from a star cluster in the tidal field of the Milky Way and derive the connection to the position and width of the clumps.
For the numerical realization we use starbystar N body simulations.
We find a very good agreement of theory and models.
We show that the radial offset of the tidal arms scales with the tidal radius, which is a function of cluster mass and the rotation curve at the cluster orbit.
The mean radial offset is 2.77 times the tidal radius in the outer disc.
Near the Galactic Centre the circumstances are more complicated, but to lowest order the theory still applies.
We have also measured the Jacobi energy distribution of bound stars and showed that there is a large fraction of stars (about 35 per cent) above the critical Jacobi energy at all times, which can potentially leave the cluster.
This is a hint that the mass loss is dominated by a selfregulating process of increasing Jacobi energy due to the weakening of the potential well of the star cluster, which is induced by the mass loss itself.
[source]
Thermal effects of circumplanetary disc formation around protogas giant planets
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
M. N. Machida
ABSTRACT
The formation of a circumplanetary disc and accretion of angular momentum on to a protoplanetary system are investigated using threedimensional hydrodynamical simulations.
The local region around a protoplanet in a protoplanetary disc is considered with sufficient spatial resolution: the region from outside the Hill sphere to the Jovian radius is covered by the nestedgrid method.
To investigate the thermal effects of the circumplanetary disc, various equations of state are adopted.
Large thermal energy around the protoplanet slightly changes the structure of the circumplanetary disc.
Compared with a model adopting an isothermal equation of state, in a model with an adiabatic equation of state, the protoplanet's gas envelope extends farther, and a slightly thick disc appears near the protoplanet.
However, different equations of state do not affect the acquisition process of angular momentum for the protoplanetary system.
Thus, the specific angular momentum acquired by the system is fitted as a function only of the protoplanet's mass.
A large fraction of the total angular momentum contributes to the formation of the circumplanetary disc.
The disc forms only in a compact region in very close proximity to the protoplanet.
Adapting the results to the Solar system, the protoJupiter and Saturn have compact discs in the region of r < 21rJup(r < 0.028 rH,Jup) and r < 66rSat(r < 0.061rH,Sat), respectively, where rJup(rH,Jup) and rSat(rH,Sat) are the Jovian and Saturnian (Hill) radius, respectively.
The surface density has a peak in these regions due to the balance between centrifugal force and gravity of the protoplanet.
The size of these discs corresponds well to the outermost orbit of regular satellites around Jupiter and Saturn.
Regular satellites may form in such compact discs around protogas giant planets.
[source]
Dynamical response to supernovainduced gas removal in spiral galaxies with dark matter halo
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Hiroko Koyama
ABSTRACT
We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies.
A thin baryonic disc having the Kuzmin density profile embedded in a spherical dark matter halo having a density profile proposed by Navarro, Frenk & White is considered.
For the purpose of comparison, we also consider the homogeneous and r,1 profiles for dark matter in a truncated spherical halo.
Assuming for simplicity that the dark matter distribution is not affected by massloss from discs and the change of baryonic disc matter distribution is homologous, we evaluate the effects of dynamical response in the resulting discs.
We found that the dynamical response only for an adiabatic approximation of massloss can simultaneously account for the rotation velocity and disc size as observed particularly in dwarf spiral galaxies, thus reproducing the Tully,Fisher relation and the size versus magnitude relation over the full range of magnitude.
Furthermore, we found that the mean specific angular momentum in discs after the massloss becomes larger than that before the massloss, suggesting that the massloss would most likely occur from the central disc region where the specific angular momentum is low.
[source]
Bulges versus discs: the evolution of angular momentum in cosmological simulations of galaxy formation
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Jesus Zavala
ABSTRACT
We investigate the evolution of angular momentum in simulations of galaxy formation in a cold dark matter universe.
We analyse two model galaxies generated in the N body/hydrodynamic simulations of Okamoto et al.
Starting from identical initial conditions, but using different assumptions for the baryonic physics, one of the simulations produced a bulgedominated galaxy and the other one a discdominated galaxy.
The main difference is the treatment of star formation and feedback, both of which were designed to be more efficient in the discdominated object.
We find that the specific angular momentum of the discdominated galaxy tracks the evolution of the angular momentum of the dark matter halo very closely: the angular momentum grows as predicted by linear theory until the epoch of maximum expansion and remains constant thereafter.
By contrast, the evolution of the angular momentum of the bulgedominated galaxy resembles that of the central, most bound halo material: it also grows at first according to linear theory, but 90 per cent of it is rapidly lost as pregalactic fragments, into which gas had cooled efficiently, merge, transferring their orbital angular momentum to the outer halo by tidal effects.
The discdominated galaxy avoids this fate because the strong feedback reheats the gas, which accumulates in an extended hot reservoir and only begins to cool once the merging activity has subsided.
Our analysis lends strong support to the classical theory of disc formation whereby tidally torqued gas is accreted into the centre of the halo conserving its angular momentum.
[source]
Distant future of the Sun and Earth revisited
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
K.P. Schröder
ABSTRACT
We revisit the distant future of the Sun and the Solar system, based on stellar models computed with a thoroughly tested evolution code.
For the solar giant stages, mass loss by the cool (but not dustdriven) wind is considered in detail.
Using the new and wellcalibrated massloss formula of Schröder & Cuntz, we find that the mass lost by the Sun as a red giant branch (RGB) giant (0.332 M,, 7.59 Gyr from now) potentially gives planet Earth a significant orbital expansion, inversely proportional to the remaining solar mass.
According to these solar evolution models, the closest encounter of planet Earth with the solar cool giant photosphere will occur during the tipRGB phase.
During this critical episode, for each timestep of the evolution model, we consider the loss of orbital angular momentum suffered by planet Earth from tidal interaction with the giant Sun, as well as dynamical drag in the lower chromosphere.
As a result of this, we find that planet Earth will not be able to escape engulfment, despite the positive effect of solar mass loss.
In order to survive the solar tipRGB phase, any hypothetical planet would require a presentday minimum orbital radius of about 1.15 au.
The latter result may help to estimate the chances of finding planets around white dwarfs.
Furthermore, our solar evolution models with detailed massloss description predict that the resulting tipAGB (asymptotic giant branch) giant will not reach its tipRGB size.
Compared to other solar evolution models, the main reason is the more significant amount of mass lost already in the RGB phase of the Sun.
Hence, the tipAGB luminosity will come short of driving a final, dustdriven superwind, and there will be no regular solar planetary nebula (PN).
The tipAGB is marked by a last thermal pulse, and the final mass loss of the giant may produce a circumstellar (CS) shell similar to, but rather smaller than, that of the peculiar PN IC 2149 with an estimated total CS shell mass of just a few hundredths of a solar mass.
[source]
Comparative study between N body and Fokker,Planck simulations for rotating star clusters , I. Equalmass system
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Eunhyeuk Kim
ABSTRACT
We have carried out N body simulations for rotating star clusters with equal mass and compared the results with Fokker,Planck models.
These two different approaches are found to produce fairly similar results, although there are some differences with regard to the detailed aspects.
We confirmed the acceleration of the core collapse of a cluster due to an initial nonzero angular momentum and found a similar evolutionary trend in the central density and velocity dispersion in both simulations.
The degree of acceleration depends on the initial angular momentum.
Angular momentum is being lost from the cluster due to the evaporation of stars with a large angular momentum on a relaxation timescale.
[source]
The baryonic and dark matter properties of highredshift gravitationally lensed disc galaxies
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
P. Salucci
ABSTRACT
We present a detailed study of the structural properties of four gravitationally lensed disc galaxies at z= 1.
Modelling the rotation curves on subkpc scales, we derive the values for the disc mass, the reference dark matter density and core radius, and the angular momentum per unit mass.
The derived models suggest that the rotation curve profile and amplitude are best fitted with a dark matter component similar to those of local spiral galaxies.
The stellar component also has a similar lengthscale, but with substantially smaller masses than similarly luminous disc galaxies in the local Universe.
Comparing the average dark matter density inside the optical radius, we find that the disc galaxies at z= 1 have larger densities (by up to a factor of ,7) than similar disc galaxies in the local Universe.
Furthermore, the angular momentum per unit mass versus reference velocity is well matched to the local relation, suggesting that the angular momentum of the disc remains constant between high redshifts and the present day.
Though statistically limited, these observations point towards a spirals' formation scenario in which stellar discs are slowly grown by the accretion of angular momentum conserving material.
[source]
Kinematics of hypervelocity stars in the triaxial halo of the Milky Way
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Qingjuan Yu
ABSTRACT
Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic Centre have unique kinematic properties compared to other halo stars.
Their trajectories will deviate from being exactly radial because of the asymmetry of the Milky Way potential produced by the flattened disc and the triaxial dark matter halo, causing a change of angular momentum that can be much larger than the initial small value at injection.
We study the kinematics of HVSs and propose an estimator of dark halo triaxiality that is determined only by instantaneous position and velocity vectors of HVSs at large Galactocentric distances (r, 50 kpc).
We show that, in the case of a substantially triaxial halo, the distribution of deflection angles (the angle between the stellar position and velocity vector) for HVSs on bound orbits is spread uniformly over the range 10°,180°.
Future astrometric and deep widefield surveys should measure the positions and velocities of a significant number of HVSs, and provide useful constraints on the shape of the Galactic dark matter halo.
[source]
Radiative torques: analytical model and basic properties
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007
A. Lazarian
ABSTRACT
We attempt to get a physical insight into grain alignment processes by studying basic properties of radiative torques (RATs).
For this purpose we consider a simple toy model of a helical grain that reproduces well the basic features of RATs.
The model grain consists of a spheroidal body with a mirror attached at an angle to it.
Being very simple, the model allows analytical description of RATs that act upon it.
We show a good correspondence of RATs obtained for this model and those of irregular grains calculated by ddscat.
Our analysis of the role of different torque components for grain alignment reveals that one of the three RAT components does not affect the alignment, but induces only for grain precession.
The other two components provide a generic alignment with grain long axes perpendicular to the radiation direction, if the radiation dominates the grain precession, and perpendicular to magnetic field, otherwise.
The latter coincides with the famous predictions of the Davis,Greenstein process, but our model does not invoke paramagnetic relaxation.
In fact, we identify a narrow range of angles between the radiation beam and the magnetic field, for which the alignment is opposite to the Davis,Greenstein predictions.
This range is likely to vanish, however, in the presence of thermal wobbling of grains.
In addition, we find that a substantial part of grains subjected to RATs gets aligned with low angular momentum, which testifies that most of the grains in diffuse interstellar medium do not rotate fast, that is, rotate with thermal or even subthermal velocities.
This tendency of RATs to decrease grain angular velocity as a result of the RAT alignment decreases the degree of polarization, by decreasing the degree of internal alignment, that is, the alignment of angular momentum with the grain axes.
For the radiationdominated environments, we find that the alignment can take place on the timescale much shorter than the time of gaseous damping of grain rotation.
This effect makes grains a more reliable tracer of magnetic fields.
In addition, we study a selfsimilar scaling of RATs as a function of ,/aeff.
We show that the selfsimilarity is useful for studying grain alignment by a broad spectrum of radiation, that is, interstellar radiation field.
[source]
The minimum mass ratio of W UMatype binary systems
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
B. Arbutina
ABSTRACT
When the total angular momentum of a binary system Jtot=Jorb+Jspin is at a certain critical (minimum) value, a tidal instability occurs which eventually forces the stars to merge into a single, rapidly rotating object.
The instability occurs when Jorb= 3Jspin, which in the case of contact binaries corresponds to a minimum mass ratio qmin, 0.071,0.078.
The minimum mass ratio is obtained under the assumption that stellar radii are fixed and independent.
This is not the case with contact binaries where, according to the Roche model, we have R2=R2(R1, a, q).
By finding a new criterion for contact binaries, which arises from dJtot= 0, and assuming k21,k22 for the component's dimensionless gyration radii, a theoretical lower limit qmin= 0.094,0.109 for overcontact degree f= 0,1 is obtained.
[source]
Tidal mass loss from collisionless systems
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007
Marios Kampakoglou
ABSTRACT
We examine the problem tidally induced mass loss from collisionless systems, such as dark matter haloes.
We develop a model for tidal mass loss, based upon the phasespace distribution of particles, which accounts for how both tidal and Coriolis torques perturb the angular momentum of each particle in the system.
This allows us to study how both the density profile and velocity anisotropy affect the degree of mass loss , we present basic results from such a study.
Our model predicts that mass loss is a continuous process even in a static tidal field, a consequence of the fact that mass loss weakens the potential of the system making it easier for further mass loss to occur.
We compare the predictions of our model with N body simulations of idealized systems in order to check its validity.
We find reasonable agreement with the N body simulations except for in the case of very strong tidal fields, where our results suggest that a higher order perturbation analysis may be required.
The continuous tidally induced mass loss predicted by our model can lead to substantial reduction in satellite mass in cases where the traditional treatment predicts no mass loss.
As such, our results may have important consequences for the orbits and survival of lowmass satellites in dark matter haloes.
[source]

