Disk Asymmetries Research Articles

Structure of the Earth’s circumsolar dust ring

Interplanetary dust particles from comets and asteroids pervade the Solar System and become temporarily trapped into orbital resonances with Earth, leading to a circumsolar dust ring. Using the unique vantage point of the Spitzer Space Telescope from its Earth-trailing solar orbit, we have measured for the first time the azimuthal structure of the Earth’s resonant dust ring. There is a relative paucity of particles within 0.1 AU of the Earth, followed by an enhancement in a cloud that is centered 0.2 AU behind Earth with a width of 0.08 AU along the Earth’s orbit. The North ecliptic pole is ∼3% brighter at 8 μm wavelength when viewed from inside the enhancement. The presence of azimuthal asymmetries in debris disks around other stars is considered strong evidence for planets. By measuring the properties of the Earth’s resonant ring, we can provide “ground truth” to models for interactions of planets and debris disks, possibly leading to improved predictions for detectability of life-bearing planets. The low amplitude of the azimuthal asymmetry in the Earth’s circumsolar ring suggests significant contributions to the zodiacal light from particles that are large (>30 μm) or have large orbital eccentricity that makes capture into mean motion resonances inefficient.

Storm fronts over galaxy discs: models of how waves generate extraplanar gas and its anomalous kinematics

The existence of partially ionized, diffuse gas and dust clouds at kiloparsec scale distances above the central planes of edge-on, galaxy discs was an unexpected discovery about 20 yrs ago. Subsequent observations showed that this EDIG (extended or extraplanar diffuse interstellar gas) has rotation velocities approximately 10-20% lower than those in the central plane, and have been hard to account for. Here we present results of hydrodynamic models, with radiative cooling and heating from star formation. We find that in models with star formation generated stochastically across the disc an extraplanar gas layer is generated as long as the star formation is sufficiently strong. However, this gas rotates at nearly the same speed as the mid-plane gas. We then studied a range of models with imposed spiral or bar waves in the disc. EDIG layers were also generated in these models, but primarily over the wave regions, not over the entire disc. Because of this partial coverage, the EDIG clouds move radially, as well as vertically, with the result that observed kinematic anomalies are reproduced. The implication is that the kinematic anomalies are the result of three-dimensional motions when the cylindrical symmetry of the disc is broken. Thus, the kinematic anomalies are the result of bars or strong waves, and more face-on galaxies with such waves should have an asymmetric EDIG component. The models also indicate that the EDIG can contain a significant fraction of cool gas, and that some star formation can be triggered at considerable heights above the disc midplane. We expect all of these effects to be more prominent in young, forming discs, to play a role in rapidly smoothing disc asymmetries, and in working to self-regulate disc structure.

IMAGING THE DEBRIS DISK OF HD 32297 WITH A PHASE-MASK CORONAGRAPH AT HIGH STREHL RATIO

We present the first Ks-band image of the scattered light from HD 32297's debris disk. HD 32297 is an A5 star known to possess a nearly edge-on disk with a high fractional luminosity Ld/L⋆ ≈ 0.003 in the infrared. Our image was obtained using phase-mask coronagraphy on the 1.6 m well-corrected subaperture on the 5 m Palomar Hale telescope, and reaches an inner working angle of 400 mas. We confirm the previously reported disk asymmetry and central hole. The gray J − Ks disk color appears similar on the two sides of the disk, which would demand a minimum grain size of at least a few microns, larger than thermal arguments suggest. One possible explanation is a planetesimal clump in a resonant orbit with an outward migrating giant planet, located at larger radii than the warm dust emitting at longer wavelengths. Our observations also clearly demonstrate the benefits of operating in the extreme adaptive optics regime with a coronagraph able to reach a very small inner working angle.

Les facteurs de risque du glaucome primitif à angle ouvert

To review the literature for risk factors involved in primary open-angle glaucoma (POAG). Articles and reviews studying the epidemiology and assessment of risk factors in POAG were collected from PubMed. The literature findings were divided into three categories: (1) What are the risks of developing POAG in healthy subjects? Significant factors include advancing age, family history of glaucoma, African origin, and elevated intraocular pressure; (2) What are the risks of developing POAG in ocular hypertensive patients? Significant factors include age, family history of glaucoma, reduced central corneal thickness, pronounced elevated IOP, and IOP fluctuations; and (3) What are the risks for progression in known cases of POAG? Significant factors include advancing age, elevated IOP fluctuations, IOP, elevated cup-over-disk ratio, disc asymmetry, and disk hemorrhages. A working knowledge of the risk factors for developing POAG and disease progression is essential to good clinical practice. Therefore, one could ideally concentrate resources on populations at risk for developing the disease as well as more actively treat those patients at risk for progression.

Vortex motion in magnetic disks with different geometric asymmetry

An asymmetric disk achieves control over the clockwise and counterclockwise vortex states in a magnetic disk with an in-plane magnetic field. In this study, the influence of different geometric asymmetry on the vortex motion in 800nm disks has been studied. The excised angle, defined as half of the center angle corresponding to the excised arc, is flatted varying from 15° to 90°. For each asymmetric degree disk, the vortex motion is investigated through anisotropic magnetoresistance measurement and hysteresis loops recorded by focused magneto-optic Kerr effect magnetometry. The vortex nucleation and annihilation fields show strong dependence and different sensitivity on the asymmetry of disk. An interestingly evident switching mode change is also observed at particular excised angle. Numerical simulations, corresponding to realistically identical system, are calculated and agree well with the experimental results.

A survey of investigations used for the management of glaucoma in hospital service in the United Kingdom

This study surveys the current use of investigations for the management of glaucoma in hospital practice by UK ophthalmologists. A total of 1007 questionnaires were posted to all active NHS consultant ophthalmologists. They were asked to indicate the type of hospital (whether university (UTH) or general (DGH) hospital), glaucoma specialist status, and availability and use of automated perimetry, disc photography, HRT, GDx, OCT, and pachymetry. A total of 493 completed questionnaires were received and 469 were analysed: 284 (60.6%) DGH, 185 (39.4%) UTH, 144 (30.7%) glaucoma specialists. There was good availability of automated perimetry (467, 99.6%), disc photography (420, 89.6%), pachymetry (374, 79.7%), OCT (212, 45.2%), HRT (206, 43.9%), and GDx (59, 12.6%). A total of 308 (65.7%) consultants had at least one digital imaging instrument available. The majority of consultants used SAP (347, 74.0%) and SITA-fast (282, 60.1%) for glaucoma suspects, and for monitoring glaucoma (283, 60.3% and 197, 42.0%, respectively). Some used Esterman (155, 33.0%) and Goldmann fields (90, 19.2%) in addition to SAP and SITA-fast for glaucoma suspects. Few consultants used short-wavelength automated perimetry and frequency-doubled perimetry. Of the three imaging tests, HRT was the most commonly used investigation for disc asymmetry, early glaucoma, glaucomatous progression, ocular hypertension, normal tension glaucoma, and unreliable visual fields (P<0.0001). Where pachymeters were available, 333 (89.0%) consultants and 117 (98.3%) glaucoma specialists used pachymetry in glaucoma management. There was some variation in the use of investigations for the diagnosis and management of glaucoma, reflecting the range of techniques available. SAP, SITA-fast, and pachymetry were the most commonly utilised investigations followed by HRT. Glaucoma specialist status, type of hospital, and presence of research influenced the availability and use of all investigations, except visual fields.

Optic Disc Asymmetry: Normal Tension Glaucoma or a Space-occupying Lesion?

Optic disc asymmetry is a common finding. In the majority of cases automated visual fields are normal. However, in a minority, a visual field defect may lead to a diagnosis of normal tension glaucoma. Rarely, the optic discs and visual field deteriorate irrespective of the normal or reduced intraocular pressure.

Discovery of Extreme Asymmetry in the Debris Disk Surrounding HD 15115

We report the first scattered light detection of a dusty debris disk surrounding the F2 V star HD 15115 using the Hubble Space Telescope in the optical and Keck adaptive optics in the near-infrared. The most remarkable property of the HD 15115 disk relative to other debris disks is its extreme length asymmetry. The east side of the disk is detected to ~315 AU radius, whereas the west side of the disk has radius >550 AU. We find a blue optical to near-infrared scattered light color relative to the star that indicates grain scattering properties similar to the AU Mic debris disk. The existence of a large debris disk surrounding HD 15115 adds further evidence for membership in the β Pic moving group, which was previously argued based on kinematics alone. Here we hypothesize that the extreme disk asymmetry is due to dynamical perturbations from HIP 12545, an M star east of HD 15115 that shares a common proper motion vector, heliocentric distance, galactic space velocity, and age.

개폐균열이 존재하는 비대칭 회전부 및 비등방 고정부를 갖는 회전체의 모드해석

This paper describes the new modal analysis method to detect the presence of the breathing crack in a general rotor system with disk asymmetry and stator anisotropy. It is proposed that the modal analysis using directional frequency response functions (dFRFs), which, accounting for the directivity in modes, clears the heavily over-lapping of other harmonics occurring from non-isotropic properties in addition to those due to crack, can provide an effective method to detect the modes by a crack. The simulations from the simple general rotor model show that the r-dFRFs (reverse dFRFs) for asymmetry confirms a good indicator of the presence of the breathing crack and the instability is primarily influenced by the shaft asymmetry than the breathing crack.

A Dynamical Simulation of the Debris Disk around HD 141569A

We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. The disk has two conspicuous spiral rings and two large-scale spiral arms. We explore the roles of radiation pressure from the central star, gas drag from the gas disk, and the tidal forces from two nearby stars in creating and maintaining these structures. The disk's color, scattering function, and infrared emission suggest that submicron-sized grains dominate the dust population observed in scattered light. CO observations indicate the presence of up to 60 M? of gas. The dust grains are subject to the competing effects of expulsive radiation pressure (? > 1, where ? is the ratio of the radiation and gravitational forces) and retentive gas drag. We use a simple one-dimensional axisymmetric model to show that the presence of the gas helps confine the dust and that a broad ring of dust is produced if a central hole exists in the disk. This model also suggests that the disk is in a transient, excited dynamical state, as the observed dust creation rate applied over the age of the star is inconsistent with submillimeter mass measurements. We model in two dimensions the effects of a flyby encounter between the disk and a binary star in a prograde, parabolic, coplanar orbit. We track the spatial distribution of the disk's gas, planetesimals, and dust. We conclude that the surface density distribution reflects the planetesimal distribution for a wide range of parameters. Our most viable model features a disk with initial radius 400 AU, a gas mass of 50 M?, and ? = 4 and suggests that the system is being observed within 4000 yr of the flyby periastron. The model reproduces some features of HD 141569A's disk, such as a broad single ring and large spiral arms, but it does not reproduce the observed multiple spiral rings or disk asymmetries nor the observed clearing in the inner disk. For the latter, we consider the effect of a 5MJ planet in an eccentric orbit on the planetesimal distribution of HD 141569A.

The Basic Dynamical Mechanism in Spiral Galaxies

This paper explicates the most fundamental mechanism that rules spiral galaxies. Although spiral galaxies are complex systems for which we do not yet have a complete understanding, the dark matter being the most severe unknown, it is possible to pinpoint the few physical factors that determine their most important properties, such as bars and spiral arms. Dynamics linked to the dissipative nature of gas and its transformation into stars provides clues that spiral galaxies are driven by dissipation close to a state of marginal stability with respect to the dynamics in the galaxy plane. Here, we present numerical evidence suggesting that warps play a similar role but in the transverse direction. N-body simulations show that typical galactic disks are also marginally stable with respect to a bending instability, leading to typical observed warps. The frequent occurrence of warps and asymmetries in the outer galactic disks, like bars in the inner disks, give new constraints on the dark matter, but this time in the outer disks.

Numerical Modeling of Dusty Debris Disks

Infrared and submillimeter observations of nearby Vega-like stars have revealed a number of clumpy, asymmetric dust debris disks. Previous studies using semianalytical and numerical methods have suggested planetary companions of various mass as the likely cause of most examples of disk asymmetry. In this paper, we modify an N-body symplectic gravitational integrator to include radiation terms and conduct medium-resolution parameter searches to identify likely planetary candidates in observed Vega-like systems. We also present high-resolution models of Vega and Eridani, comparing our results to those of previous authors, and a new model for Fomalhaut.

Stellar Kinematics of Boxy Bulges: Large-Scale Bars and Inner Disks

Major-axis long-slit stellar kinematics was obtained for 30 edge-on spiral galaxies, 24 with a boxy/peanut-shaped (B/PS) bulge. B/PS bulges are present in >45% of highly inclined systems and much work suggests that they are the edge-on projection of thick bars. Profiles of the mean stellar velocity V, the velocity dispersion sigma, and the asymmetric (h3) and symmetric (h4) deviations from a Gaussian are presented. Comparing those with N-body bar diagnostics, we find bar signatures in 80% of our sample. B/PS bulge galaxies typically show a double-hump rotation curve with an intermediate dip or plateau. They often show a flat central velocity dispersion profile accompanied by a secondary peak or plateau and >=40% have a local central sigma minimum. The h3 profiles display up to 3 slope reversals and h3 is normally correlated with V over the presumed bar length, contrary to expectations from axisymmetric disks. Those characteristic bar signatures strengthen the case for a close link between B/PS bulges and bars. h3 is anti-correlated with V in the very center of most galaxies, indicating that they additionally harbor cold and dense decoupled nearly axisymmetric central stellar disks. These coincide with previously identified star-forming ionized-gas disks in the gas-rich systems, and we argue that they formed out of gas inflow driven by the bar. As suggested by N-body models, h3 appears to be a reliable tracer of asymmetries in disks, allowing to discriminate between axisymmetric and barred disks seen in projection. B/PS bulges (and thus a large fraction of all bulges) appear to be made-up mostly of disk material which has acquired a large vertical extent through bar-driven vertical instabilities. Their formation is thus probably dominated by secular evolution processes rather than merging. [Abridged]

A stellar fly-by simulation giving β Pic's disc asymmetries

We have numerically investigated the dynamics of how a close stellar fly-by encounter of a symmetrical circumstellar planetesimal disc can give rise to the many kinds of asymmetries and substructures attributed to the dusty disc of β Pic. We find three recognizable groupings of test particles that can be related to the morphology of the β Pic disc. These are: highly eccentric and inclined orbit particles that reach apocentre in the southwest, moderately eccentric and inclined orbit particles that reach apocentre in the northeast, and a relatively unperturbed region inside ∽200 AU radius.

Structure of the Mid‐Infrared–emitting Disk around WL 16

WL16 is a unique member of the embedded young stellar population in the nearby rho Ophiuchi cloud core: its extended (900 AU diameter), high surface brightness disk is visible only at mid-infrared wavelengths. We present diffraction-limited (~ 0.3 arcsec) images, from 7.9 to 24.5 microns, of WL16 acquired at the Keck II telescope. We find the central star to be a 250 Lsun, 4 Msun, Herbig Ae star, seen through foreground material of the rho Oph cloud core that provides an extinction of Av=31+/-1 magnitudes. WL16's disk is detected through all nine observed passbands. We confirm, therefore, that the emitting particles are composed of both polycyclic aromatic hydrocarbons (PAHs) and very small (5--100 Angstroms) graphitic grains (VSGs). We confirm a resolved (1.5 diameter) core component at 7.9 and 8.8 microns, due to emission from positively charged PAHs. An enhancement in the emission at 12.5 microns at the disk's edges is found for the first time, and signals the presence of larger (>= 50-80 carbon atoms) PAHs and/or more hydrogenated PAHs than those found in the bulk of the disk. We find a disk asymmetry, observed at all nine mid-infrared wavelengths, at projected radii 1-2.5 (corresponding to 125 AU <= r <= 300 AU) from the central source.

Dynamics and Growth of Dust Grains in the Protoplanetary Nebula

In the standard scenario of planet formation, the planetesimals grow from primordial dust grains of interstellar origin embedded in a gas disk, the so-called protoplanetary nebula. Large-scale gaseous structures, such as spiral waves and vortices, are likely to survive for a large number of rotation periods and can strongly influence the dynamics and growth of the dust grains. This has been stressed for the first time by P. Barge & J. Sommeria (1995, A&A, 295, L1) in the case of the vortices with a new scenario for the formation of the planetesimals and the rapid growth of the giant planet cores. This dissertation is concerned with the action of the mean flow of the gas on the dynamics of the solid dust particles. To reach this goal, we implemented a direct time-dependent integration code that models the particle dynamics under gravitational and viscous forces in a zero-thickness disk. A parallel version of this highorder accuracy particle-mesh code has been performed to carry out the potential role of large-scale vortices and spiral perturbations in the protoplanetary disks. The results show that the particles do not drift inward to the star as occurs in a standard symmetrical nebula. On the one hand, vortices tend to capture a large number of the particles. The effectiveness of this size-selective concentration mechanism depends on the value of the drag coefficient and on the distance from the Sun but also on the elongation of the vortex and its characteristic lifetime. We also found analytical expressions for the capture time as well as capture constraints as a function of the friction parameter, the elongation of the vortex, and the impact parameter. On the other hand, in the case of spiral perturbations, the velocity field can be described by a set of concentric streamlines as is usual in the kinematical description of a spiral wave. Moreover, since protoplanetary disks are nearly Keplerian, the spiral mode is likely the most probable and the most m p 1 robust one (E. Lee & J. Goodman 1999, MNRAS, 308, 684). In such a model, the streamlines are confocal ellipses and the particle trajectories become nearly coincident with them. With such a velocity field, we find that the particle trajectories end up in stable elliptic orbits and that the resulting dust surface density is significantly increased in the vicinity of the spiral pattern, incidentally facilitating the growth of the particles. We find also that a ringlike swarm of particles, with a given size distribution, tends to spread into a spiral-like pattern. This pattern results from the size segregation of the particles under the friction drag force. In summary, our results show that small disk asymmetries, due to giant vortices or spiral patterns, are able to confine the solid material within some small regions of the protoplanetary nebula. By increasing the lifetime and the surface density of the solid particles, these confining mechanisms offer new possibilities for the formation of the planetesimals and the giant planet cores and could help also in explaining a rapid formation of the extrasolar giant planets.

Properties of tidally-triggered vertical disk perturbations

We present a detailed analysis of the properties of warps and tidally-triggered perturbations perpen- dicular to the plane of 47 interacting/merging edge-on spiral galaxies. The derived parameters are compared with those obtained for a sample of 61 non-interacting edge-on spirals. The entire optical (R-band) sample used for this study was presented in two previous papers. We nd that the scale height of disks in the interacting/merging sample is characterized by perturbations on both large (' disk cut-o radius) and short ('z0) scales, with ampli- tudes of the order of 280 pc and 130 pc on average, respectively. The size of these large (short) -scale instabilities corresponds to 14% (6%) of the mean disk scale height. This is a factor of 2 (1.5) larger than the value found for non-interacting galaxies. A hallmark of nearly all tidally distorted disks is a scale height that increases system- atically with radial distance. The frequent occurrence and the signicantly larger size of these gradients indicate that disk asymmetries on large scales are a common and persistent phenomenon, while local disturbances and bending instabilities decline on shorter timescales. Nearly all (93%) of the interacting/merging and 45% of the non-interacting galaxies studied are noticeably warped. Warps of interacting/merging galaxies are 2.5 times larger on average than those observed in the non-interacting sample, with sizes of the order of 340 pc and 140 pc, respectively. This indicates that tidal distortions do considerably contribute to the formation and size of warps. However, they cannot entirely explain the frequent occurrence of warped disks.

Self‐consistent Response of a Galactic Disk to an Elliptical Perturbation Halo Potential

We calculate the self-consistent response of an axisymmetric galactic disk perturbed by an elliptical halo potential of harmonic number m = 2 and obtain the net disk ellipticity. Such a potential is commonly expected to arise due to galactic tidal encounter and also during the galaxy formation process. The self-gravitational potential corresponding to the self-consistent, nonaxisymmetric density response of the disk is obtained by inversion of the Poisson equation for a thin disk. This response potential is shown to oppose the perturbation potential because physically the disk self-gravity resists the imposed potential. This results in a reduction in the net ellipticity of the perturbation halo potential in the disk plane. The reduction factor denoting this decrease is independent of the strength of the perturbation potential and has a typical minimum value of ~0.75-0.9 for a wide range of galaxy parameters. The reduction is most important at 1.4 exponential disk scale lengths and is progressively less so at higher radii. For the solar neighborhood region of the Galaxy, the reduction factor is 0.8. Beyond twice the Holmberg radius, the reduction is negligible, and there the disk asymmetry in the atomic hydrogen gas traces the true ellipticity of the halo potential. The reduction is negligible at all radii for higher harmonics (m ≥ 3) of the halo potential. On correcting for the negative disk response, the true ellipticity of the halo potential for a typical spiral galaxy is shown to be higher by ~20% than the typical halo ellipticity of ≤0.1 deduced in the literature from observations of isophotal or kinematical asymmetry of disks.

Superhumps in low-mass X-ray binaries

We propose a mechanism for the superhump modulations observed in optical photometry of at least two black-hole X-ray transients (SXTs). As in extreme mass-ratio cataclysmic variables (CVs), superhumps are assumed to result from the presence of the 3:1 orbital resonance in the accretion disc. This causes the disc to become non-axisymmetric and precess. However, the mechanism for superhump luminosity variations in low-mass X-ray binaries (LMXBs) must differ from that in CVs, where it is attributed to a tidally-driven modulation of the disc's viscous dissipation, varying on the beat between the orbital and disc precession period. By contrast in LMXBs, tidal dissipation in the outer accretion disc is negligible: the optical emission is overwhelmingly dominated by reprocessing of intercepted central X-rays. Thus a different origin for the superhump modulation is required. Recent observations and numerical simulations indicate that in an extreme mass-ratio system the disc area changes on the superhump period. We deduce that the superhumps observed in SXTs arise from a modulation of the reprocessed flux by the changing area. Therefore, unlike the situation in CVs, where the superhump amplitude is inclination-independent, superhumps should be best seen in low-inclination LMXBs, whereas an orbital modulation from the heated face of the secondary star should be more prominent at high inclinations. Modulation at the disc precession period (10 s of days) may indicate disc asymmetries such as warping. We comment on the orbital period determinations of LMXBs, and the possibility and significance of possible permanent superhump LMXBs.

Rings in the Planetesimal Disk of beta Pictoris.

The nearby main-sequence star beta Pictoris is surrounded by an edge-on disk of dust produced by the collisional erosion of larger planetesimals. Here we report the discovery of substructure within the northeast extension of the disk midplane that may represent an asymmetric ring system around beta Pic. We present a dynamical model showing that a close stellar flyby with a quiescent disk of planetesimals can create such rings, along with previously unexplained disk asymmetries. Thus we infer that beta Pic's planetesimal disk was highly disrupted by a stellar encounter in the last hundred thousand years.