Momentum Dissipation Research Articles

Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of θ1 Orionis C

Abstract From observations made with the ESPaDOnS spectropolarimeter, recently installed on the 3.6-m Canada-France-Hawaii Telescope, we report the discovery of a strong magnetic field in the Of?p spectrum variable HD 191612 - only the second known magnetic O star (following θ1 Ori C). The stability of the observed Zeeman signature over four nights of observation, together with the non-rotational shape of line profiles, argues that the rotation period of HD 191612 is significantly longer than the 9-d value previously proposed. We suggest that the recently identified 538-d spectral variability period is the rotation period, in which case the observed line-of-sight magnetic field of −220 ± 38 G implies a large-scale field (assumed dipolar) with a polar strength of about −1.5 kG. If confirmed, this scenario suggests that HD 191612 is, essentially, an evolved version of the near-zero-age main-sequence magnetic O star θ1 Ori C, but with an even stronger field (about 15 kG at an age similar to that of θ1 Ori C). We suggest that the rotation rate of HD 191612, which is exceptionally slow by accepted O-star standards, could be due to angular momentum dissipation through a magnetically confined wind.

Energy and momentum dissipation through wave breaking

Wave breaking plays an important role in air‐sea interaction. Laboratory and field measurements suggest that the wave field dissipation can be significantly enhanced by wave breaking and acts as a source of energy for generating current and entraining air against the effect of buoyancy forces. In the present study, a breaking wave model is formulated by taking the acceleration threshold value of −g/2 as breaking criterion, and the statistics of breaking waves, such as breaking wave coverage per unit time, and the volume of breaking water per unit area of wave surface per unit time are estimated. Statistical models are also developed to assess energy and momentum dissipation through wave breaking. It is found that the energy and momentum dissipation rates decay as k−3 and k−2.5, respectively. The energy loss is mainly due to wave components at frequencies higher than the spectral peak frequency. Though energy loss due to wave components at frequencies lower than the spectral peak frequency is found, these wave components do not significantly lose energy after the breaking. The energy loss, when expressed as phase speed cb of breaking waves, is mainly in the range 0.20 cp < cb < 0.90 cp, where cp is the dominant wave phase speed, and the energy dissipation rate falls off rapidly toward shorter scales. This offers no support for the hypothesis of a “Kolmogorov cascade” in wind‐generated waves analogous to that in turbulence, with energy input from the wind at large scales and dissipation from the waves at the smallest scales. It is also demonstrated that, compared with empirical formula results, our model of energy (or momentum) dissipation rate lies between those models that estimate by empirical formula the rate of energy (or momentum) loss from a breaker with proportional coefficients 0.0085 and 0.0007. In addition, the peak frequency of our model energy (or momentum) dissipation rate downshifts to the lower‐frequency band as wind speed increases, whereas the peak of the empirical formula remains at the same frequency.

Special roles of loose neutron-halo nucleus structure on the fragmentation and momentum dissipation in heavy ion collisions

We studied the special roles of loose neutron-halo structure of 19B on the fragmentation and momentum dissipation by using isospin dependent quantum molecular dynamics. In order to protrude the special roles of neutron-halo structure on the reaction dynamics we compared the fragmentation multiplicity and nuclear stopping (momentum dissipations) induced by neutron-halo nucleus 19B with those induced by a same mass stable nucleus 19F under the same incident channel condition. We found that the loose neutron-halo structure of 19B increases the fragmentation multiplicity in lower beam energy region. On the contrary, the special halo structure of 19B weakens the nuclear stopping, compared to those induced by stable nucleus 19F. However both fragmentation multiplicity-difference and nuclear stopping-difference between the reactions induced by 19B and 19F gradually decrease and finally disappear as the increase of beam energy. These results suggest a good project for the experimental measurements of the fragmentation multiplicity and nuclear stopping to study the reaction dynamics induced by neutron-halo nuclei.

The isospin effects on the momentum dissipation induced by the Coulomb interacti on in the process of heavy-ion collissions

The isospin effects of Coulomb interaction on the momentum dissipation in the heavy ion collissions at an intermediate energy has been studied carefully within the isospin dependent quantum molecular dynamics model. The results show that Co ulomb interaction induces the reduction of momentum dissipation because it leads the shrinking of the mechanical and chemical instability domains, and the degre e of reduction depends sensitively on the form and strength of symmetry potentia l. However, the property that the momentum dissipation depends sensitively on th e isospin dependences of in-medium nucleon_nucleon cross section and insensitiv ely on the symmetry potential, is true for the cases with and without the Coulom b interaction.

On the spin-up by a rotating disk in a rotating stratified fluid

We investigate the response of a rotating stratified fluid to the local spin-up by a small rotating disk of radius $R$, with Rossby number $\hbox{\it Ro}\,{=}\,\omega_d/2\Omega$ around unity where $\omega_d$ is the rotating-disk vorticity and $\Omega$ the background rotation frequency. During an initial stage $\tau_{su}\,{=}\,O({E_k}^{-1/2} N^{-1})$ with Ekman number, $E_k\,{=}\,\nu/\Omega R^2$ ($\nu$ the kinematic viscosity and $N$ the buoyancy frequency), fluid ejected by the Ekman boundary layer mixes with ambient fluid, and forms an intermediate-density intrusion the radial spreading of which is arrested by background rotation. This flow resembles a concentric source–sink configuration with the sink represented by the Ekman layer above the disk and the source by the ejected fluid, which, by conservation of potential vorticity, leads to the formation of a cyclonic vortex embedded in an anti-cyclonic ring. In the next stage, the radial and axial diffusion of momentum dominate the flow evolution, and the flow is characterized by a balance between viscous dissipation of momentum and the amount of momentum applied by the rotating disk. Vorticity diffusion dominates the flow and smooths out the flow history when $E_k^{-1/2}(f/N)\,{<}\,3$, whereas the initial stage can be recognized as a separate flow stage when $E_k^{-1/2}(f/N)\,{>}\,3$. The stability of the density front is discussed.

Isospin Effect of Coulomb Interaction on Momentum Dissipationin Intermediate Energy Heavy Ion Collisions

We investigate the isospin effect of Coulomb interaction on the momentumdissipation or nuclear stopping in the intermediate energy heavy ioncollisions by using the isospin-dependent quantum molecular dynamics model.The calculated results show that the Coulomb interaction inducesobviously the reductions of the momentum dissipation. We also find thatthe variation amplitude of momentum dissipation induced by the Coulombinteraction depends sensitively on the form and strength of symmetrypotential. However, the isospin effect of Coulomb interaction on themomentum dissipation is less than that induced by the in-medium nucleon–nucleon cross section. In this case, Coulomb interaction does not changeobviously the isospin effect of momentum dissipation induced by thein-medium two-body collision. In particular, the Coulomb interaction ispreferable for standing up the isospin effect of in-medium nucleon–nucleon cross section on the momentum dissipation and reducing theisospin effect of symmetry potential on it, which is important forobtaining the feature about the sensitive dependence of momentumdissipation on the in-medium nucleon–nucleon cross section and weakly onthe symmetry potential.

Isospin effect of Coulomb interaction on the dissipation and fragmentation in intermediate energy heavy ion collisions

We investigate separately the isospin effects of Coulomb interaction and symmetry potential on the dissipation and fragmentation in the intermediate energy heavy ion collisions by using isospin-dependent quantum molecular dynamics model. The calculated results show that the Coulomb interaction induces the reductions of both isospin fractionation ratio and nuclear stopping (momentum dissipation). However, the Coulomb interaction not only does not change obviously the strong isospin effect of the symmetry potential on the isospin fractionation ratio but also does not change obviously that of in-medium two-body collision on the nuclear stopping. On the contrary, the symmetry potential induces the enhancement of the isospin fractionation ratio but it is insensitive to the nuclear stopping. Finally, the competition between the Coulomb interaction and symmetry potential induces the reductions of both isospin fractionation ratio and nuclear stopping for two forms of symmetry potentials in this paper.

The Evolution of Protoplanetary Disk Edges

We investigate gap formation in gaseous protostellar disks by a planet in a circular orbit in the limit of low disk viscosity. This regime may be appropriate to an aging disk after the epoch of planet formation. We find that the distance of the planet to the gap outer boundary can be between the location of the m = 2 and 1 outer Lindblad resonances. This distance is weakly dependent on both the planet's mass and disk viscosity. We find that the evolution of the disk edge takes place on two timescales. The first timescale is set by the spiral density waves driven by the planet. The second timescale depends on the viscosity of the disk. The disk approaches a state in which the outward angular momentum flux caused by the disk viscosity is balanced by the dissipation of spiral density waves that are driven at the Lindblad resonances. This occurs inefficiently, however, because of the extremely low gas density near the planet. We find that the distance between the planet and the peak density at the disk outer edge is only weakly dependent on the viscosity and planet mass; however, the ratio of the gas density near the planet to that in the disk (or the slope of density along the disk edge) is strongly dependent on both quantities. We find that the disk density profile along the edge scales approximately with the disk viscosity divided by the square of the planet mass. We account for this behavior with a simple scenario in which the dissipation of angular momentum from the spiral density waves is balanced against the diffusion in the steep edge of the disk.

Dissipation of angular momentum in light heavy-ion collision

The inclusive energy distributions of fragments (4$\leq$Z$\leq$7) emitted in the reactions $^{16}$O (116 MeV) + $^{27}$Al, $^{28}$Si, $^{20}$Ne (145 MeV) + $^{27}$Al, $^{59}$Co have been measured in the angular range $\theta_{lab} $= 10$^\circ$ - 65$^\circ$. Fusion-fission and deep inelastic components of the fragment emission have been extracted from the experimental data. The angular mometum dissipations in fully damped deep inelastic collisions have been estimated assming exit channel configuration similar to those for fusion-fission process. It has been found that, the angular momentum dissipations are more than those predicted by the empirical sticking limit in all cases. The deviation is found to increase with increasing charge transfer (lighter fragments). Qualitatively, this may be due to stronger friction in the exit channel. Moreover, for the heavier system $^{20}$Ne + $^{59}$Co, the overall magnitude of deviation is less as compared to those for the lighter systems, {\it i.e.}, $^{16}$O + $^{27}$Al, $^{28}$Si, $^{20}$Ne + $^{27}$Al. This may be due to lesser overlap in time scales of fusion and deep inelastic time scales for heavier systems.

Radiatively Inefficient Accretion Flows

Black hole accretion flows could be radiatively inefficient when a mass supply rate is either much less or much larger than the Eddington rate. Because of lack of radiative cooling, such flows are hot and geometrically thick. This article will review different models and different approaches to study radiatively inefficient accretion flows. Initially, the flows were studied as purely hydrodynamic viscous flows. It was shown analytically and numerically that the hydrodynamic flows are convectively unstable. Convection significantly changes the flow structure, flattening the radial density profile and decreasing the accretion rate. More recently, numerical MHD models of the flows have been developed by several groups. In these models the problems of angular momentum transport and dissipation of binding energy can be solved self-consistently. The MHD models show the flow’s strong dependence on the topology of the magnetic field, and some models show efficient convection.

Dual-porosity and kinematic wave approaches to assess the degree of preferential flow in an unsaturated soil

The purpose of this study was to assess the degree of preferential flow in an unsaturated soil column using two different models: the dual-porosity model, MACRO, and the kinematic wave approach (KWA) based on boundary-layer flow theory. The soil column experiments consisted of six infiltrations with intensities varying from 15 to 101 mm h−1. Bromide solution was also infiltrated at an intensity of 79 mm h−1 and a concentration of 80 mg l−1. Both MACRO and the KWA indicated the absence of pure preferential flow. The KWA indicated intermediate flow with dispersion of the wetting front with depth, whereas MACRO indicated flow dominated by the diffusion of capillary potential. These results shed light on the transition between flows dominated by momentum dissipation and by diffusion of capillary potential. The absence of pure macropore flow in the structured sandy soil is mainly due to efficient lateral mass exchange in this material.

Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei

(Abridged) We present a detailed study of the relation between circumnuclear dust morphology, host galaxy properties, and nuclear activity in nearby galaxies. We use our sample of 123 nearby galaxies with visible--near-infrared colormaps from the Hubble Space Telescope to create well-matched, paired samples of 28 active and 28 inactive galaxies, as well as 19 barred and 19 unbarred galaxies, that have the same host galaxy properties. Comparison of the barred and unbarred galaxies shows that grand design nuclear dust spirals are only found in galaxies with a large-scale bar. Tightly wound nuclear dust spirals, in contrast, show a strong tendency to avoid galaxies with large-scale bars. Comparison of the AGN and inactive samples shows that nuclear dust spirals, which may trace shocks and angular momentum dissipation in the ISM, occur with comparable frequency in both active and inactive galaxies. The only difference between the active and inactive galaxies is that several inactive galaxies appear to completely lack dust structure in their circumnuclear region, while none of the AGN lack this structure. The comparable frequency of nuclear spirals in active and inactive galaxies, combined with previous work that finds no significant differences in the frequency of bars or interactions between well-matched active and inactive galaxies, suggests that no universal fueling mechanism for low-luminosity AGN operates at spatial scales greater than ~100 pc radius from the galactic nuclei. The similarities instead suggest that the lifetime of nuclear activity is less than the characteristic inflow time from these spatial scales. An order-of-magnitude estimate of this inflow time is the dynamical timescale. This sets an upper limit of several million years to the lifetime of an individual episode of nuclear activity.

Magnetohydrodynamic-activity-induced toroidal momentum dissipation in collisionless regimes in tokamaks

It is shown that in the presence of magnetohydrodynamic (MHD) activities, such as resistive wall mode, the toroidal symmetry in tokamaks is broken. This results from the modification on |B|, and the variation of |B| on the distortion of the magnetic surface due to the MHD activities. Here, B is the magnetic field. The toroidal plasma viscosity in the collisionless regime for the nonresonant MHD modes is calculated. It scales like (δB/|B|)2 with δB, the typical magnitude of the perturbed magnetic field strength. The theory is applied to determine the toroidal flow speed in the presence of these modes.

Plasma and momentum transport processes in the vicinity of a magnetic island in a tokamak

In the vicinity of a magnetic island in tokamaks, the toroidal symmetry in the magnitude of the magnetic field |B| is broken. This leads to enhanced radial transport fluxes and momentum dissipation. The radial electric field can be determined from the quasineutrality condition, or equivalently the momentum equation, on the island magnetic surface. The equation that governs the radial electric field is nonlinear and can have bifurcated solutions. This may suppress turbulence fluctuations and improve plasma confinement. The theory remains valid for a rotating island with an appropriate re-interpretation of the radial electric field.

Drag force approach to the transport of colloids in unsaturated soils

A drag force approach to the transport of colloids in unsaturated soils is introduced. It is based on film flow which is due to momentum dissipation during porous media flow. Supporting results are from three types of colloid transport experiments: (1) sprinkling of a latex bead and methylene blue suspension in situ on the surface of a forest soil with subsequent bead collection underneath, (2) application of bead suspension to a column of undisturbed soil and purging of retained beads, and (3) application of a suspension with 5 bacteriophages to a different soil column. Relative colloid breakthrough concentrations in relation to the drag forces, P/P0, versus Fd(t)/A, were well pronounced during bead breakthrough, and bead diameters had no significant impact. In contrast, phage breakthrough showed a wide range of behavior. Previous bacteria breakthrough experiments support the approach; however, site‐specific soil properties should also be considered.

Anisotropic horizontal viscosity for ocean models

A consistent mathematical formulation of a horizontally anisotropic friction operator is developed for use as a subgrid-scale turbulence parameterization in ocean general circulation models. It is derived as the divergence of a deviatoric stress tensor that represents the traceless part of the Reynolds stress tensor. The stress tensor is assumed to depend linearly on the velocity gradients, has the property that no stresses are generated by uniform rotation, and the viscous coefficients can be chosen to ensure the friction terms are purely dissipative of kinetic energy. The anisotropy is determined by an arbitrary field of horizontal unit vectors n ̂ that specify a preferred direction at each point in space which breaks the transverse isotropy. This direction may be aligned with east–west (e.g., to describe anisotropy associated with the beta effect), with gradients of topography, with the mean flow, or with any other preferred direction which is expected to break the horizontal isotropy of turbulent momentum dissipation. The general anisotropic formulation involves four viscous coefficients, compared to only two in the isotropic case. A reduced two-coefficient anisotropic form is derived by neglecting terms in the energy dissipation rate proportional to the horizontal velocity divergence, which in geophysical flows is small compared to typical horizontal velocity gradients. This form involves different viscosities in the directions parallel and perpendicular to n ̂ . Experimental measurements of Reynolds stresses in self-preserving shear flows are shown to exhibit this type of anisotropy when n ̂ is aligned with the mean flow. The two-coefficient form is evaluated in numerical simulations in a periodic zonal channel. A functional discretization of the operator which ensures positive-definite dissipation of kinetic energy is also presented in an appendix.

Temporal and spatial relaxation of electrons in low temperature plasmas

A brief introduction into the large importance of relaxation processes in the electron gas for the understanding of various transient processes in low temperature plasmas is given. Based on comprehensive, strictly non-hydrodynamic studies on the electron relaxation by using kinetic approaches the large variety of the temporal and spatial electron relaxation in atomic and molecular plasmas is elaborated and demonstrated under different plasma conditions. The analysis has been performed particularly with respect to the relaxation behavior of the electron energy distribution function and the associated energy and momentum dissipation in electron collisions as well as the resultant characteristic overall relaxation times and relaxation lengths. Especially the temporal and spatial relaxation, respectively, is considered under field-free conditions in decaying plasmas, under the action of constant electric fields leading to the formation of steady states and under the action of temporally or spatially periodic fields. Moreover, the complex spatiotemporal transient behavior of the electron gas into a spatially structured, time-independent state is considered. To simplify the representation, all results on the temporal and spatial relaxation have been deduced using corresponding two-term approximations, but could be performed as well with available multi-term approaches without effecting significant changes in the deduced basic relaxation characteristics.

Steady Baroclinic Flow through Ridges with Narrow Gaps*

Abstract The steady baroclinic flow in a basin containing a meridional barrier representing a midocean ridge is studied in the linear, quasigeostrophic limit of a two-layer model. Thermal damping and a simple friction provide dissipation of thickness (heat) and momentum. The ridge is pierced by two gaps in the upper layer but only a single gap in the lower layer. The flow in the model is forced by specified upwelling at the upper surface and by a specified cross-isopycnal velocity at the interface in addition to the autogenerated cross-isopycnal velocity associated with the thermal damping. The forcing may be either broad in longitude or narrowly confined. The nature of the geometry of the model ridge mixes the baroclinic and barotropic response to the forcing, and this has profound consequences for the resulting circulation. In particular, when the baroclinic interaction of the two layers is strong, the recirculation region to the east of the ridge, previously discovered in earlier barotropic models of t...

A conceptual model of preferential flow systems in forested hillslopes: evidence of self‐organization

AbstractPreferential flow paths are known to be important conduits of subsurface stormflow in forest hillslopes. Earlier research on preferential flow paths focused on vertical transport; however, lateral transport is also evident in steep forested slopes underlain by bedrock or till. Macropores consisting of decayed and live roots, subsurface erosion, surface bedrock fractures, and animal burrows form the basis of a ‘backbone’ for lateral preferential flow in such sites. Evidence from field studies in Japan indicates that although individual macropore segments are generally &lt;0·5 m in length, they have a tendency to self‐organize into larger preferential flow systems as sites become wetter. Staining tests show clear evidence of interconnected macropore flow segments, including: flow within decayed root channels and subsurface erosion cavities; flow in small depressions of the bedrock substrate; fracture flow in weathered bedrock; exchange between macropores and mesopores; and flow at the organic horizon–mineral soil interface and in buried pockets of organic material and loose soil. Here we develop a three‐dimensional model for preferential flow systems based on distributed attributes of macropores and potential connecting nodes (e.g. zones of loose soil and buried organic matter). We postulate that the spatially variable and non‐linear preferential flow response observed at our Japan field site, as well as at other sites, is attributed to discrete segments of macropores connecting at various nodes within the regolith. Each node is activated by local soil water conditions and is influenced strongly by soil depth, permeability, pore size, organic matter distribution, surface and substrate topography, and possibly momentum dissipation. This study represents the first attempt to characterize the spatially distributed nature of preferential flow paths at the hillslope scale and presents strong evidence that these networks exhibit complex system behaviour. Copyright © 2001 John Wiley &amp; Sons, Ltd.

From Infall to Rotation around Young Stellar Objects: A Transitional Phase with a 2000 AU Radius Contracting Disk?

Evidence for a transitional stage in the formation of a low-mass star is reported, intermediate between the fully embedded and the T Tauri phases. Millimeter aperture synthesis observations in the HCO+ J = 1-0 and 3-2, HCN 1-0, 13CO 1-0, and C18O 1-0 transitions reveal distinctly different velocity fields around two embedded, low-mass young stellar objects. The 0.6 M☉ of material around TMC 1 (IRAS 04381+2517) closely follows inside-out collapse in the presence of a small amount of rotation (~3 km s-1 pc-1), while L1489 IRS (IRAS 04016+2610) is surrounded by a 2000 AU radius, flared disk containing 0.02 M☉. This disk shows Keplerian rotation around a ~0.65 M☉ star and infall at 1.3(r/100 AU)-0.5 km s-1, or, equivalently, sub-Keplerian motions around a central object between 0.65 and 1.4 M☉. Its density is characterized by a radial power law and an exponential vertical scale height. The different relative importance of infall and rotation around these two objects suggests that rotationally supported structures grow from collapsing envelopes over a few times 105 yr to sizes of a few thousand AU, and then decrease over a few times 104 yr to several hundred AU typical for T Tauri disks. In this scenario, L1489 IRS represents a transitional phase between embedded young stellar objects and T Tauri stars with disks. The expected duration of this phase of ~5% of the embedded stage is consistent with the current lack of other known objects like L1489 IRS. Alternative explanations cannot explain L1489 IRS's large disk, such as formation from a cloud core with an unusually large velocity gradient or a binary companion that prevents mass accretion onto small scales. It follows that the transfer and dissipation of angular momentum is key to understanding the formation of disks from infalling envelopes.