8,478 research outputs found
Visuo-vestibular interaction in the reconstruction of travelled trajectories
We recently published a study of the reconstruction of passively travelled trajectories from optic flow. Perception was prone to illusions in a number of conditions, and not always veridical in the others. Part of the illusionary reconstructed trajectories could be explained by assuming that subjects base their reconstruction on the ego-motion percept built during the stimulus' initial moments
. In the current paper, we test this hypothesis using a novel paradigm: if the final reconstruction is governed by the initial percept, providing additional, extra-retinal information that modifies the initial percept should predictably alter the final reconstruction. The extra-retinal stimulus was tuned to supplement the information that was under-represented or ambiguous in the optic flow: the subjects were physically displaced or rotated at the onset of the visual stimulus. A highly asymmetric velocity profile (high acceleration, very low deceleration) was used. Subjects were required to guide an input device (in the form of a model vehicle; we measured position and orientation) along the perceived trajectory. We show for the first time that a vestibular stimulus of short duration can influence the perception of a much longer lasting visual stimulus. Perception of the ego-motion translation component in the visual stimulus was improved by a linear physical displacement: perception of the ego-motion rotation component by a physical rotation. This led to a more veridical reconstruction in some conditions, but to a less veridical reconstruction in other conditions
Reconstructing passively travelled manoeuvres: Visuo-vestibular interactions.
We recently published a study of the reconstruction of passively travelled trajectories from optic flow. Perception was prone to illusions in a number of conditions, and not always veridical in the other conditions. Part of the illusionary reconstructed trajectories could be explained if we assume that the subjects based their reconstruction on the ego-motion percept obtained during the stimulus' initial moments. In the current paper, we test this hypothesis using a novel paradigm. If indeed the final reconstruction is governed by the initial percept, then additional, extra-retinal information that modifies the initial percept should predictably alter the final reconstruction. We supplied extra-retinal stimuli tuned to supplement the information that was underrepresented or ambiguous in the optic flow: the subjects were physically displaced or rotated at the onset of the visual stimulus. A highly asymmetric velocity profile (high acceleration, very low deceleration) was used. Subjects were required to guide an input device (in the form of a model vehicle; we measured position and orientation) along the perceived trajectory. We show for the first time that a vestibular stimulus of short duration can influence the perception of a much longer lasting visual stimulus. Perception of the ego-motion translation component in the visual stimulus was improved by a linear physical displacement; perception of the ego-motion rotation component by a physical rotation. This led to a more veridical reconstruction in some conditions, but it could also lead to less veridical reconstructions in other conditions
Visuo-vestibular interaction in the reconstruction of travelled trajectories
We recently published a study of the reconstruction of passively travelled trajectories from optic flow. Perception was prone to illusions in a number of conditions, and not always veridical in the others. Part of the illusionary reconstructed trajectories could be explained by assuming that subjects base their reconstruction on the ego-motion percept built during the stimulus' initial moments
. In the current paper, we test this hypothesis using a novel paradigm: if the final reconstruction is governed by the initial percept, providing additional, extra-retinal information that modifies the initial percept should predictably alter the final reconstruction. The extra-retinal stimulus was tuned to supplement the information that was under-represented or ambiguous in the optic flow: the subjects were physically displaced or rotated at the onset of the visual stimulus. A highly asymmetric velocity profile (high acceleration, very low deceleration) was used. Subjects were required to guide an input device (in the form of a model vehicle; we measured position and orientation) along the perceived trajectory. We show for the first time that a vestibular stimulus of short duration can influence the perception of a much longer lasting visual stimulus. Perception of the ego-motion translation component in the visual stimulus was improved by a linear physical displacement: perception of the ego-motion rotation component by a physical rotation. This led to a more veridical reconstruction in some conditions, but to a less veridical reconstruction in other conditions
Spiral density waves in the outer galactic gaseous discs
Deep HI observations of the outer parts of disc galaxies demonstrate the
frequent presence of extended, well-developed spiral arms far beyond the
optical radius. To understand the nature and the origin of such outer spiral
structure, we investigate the propagation in the outer gaseous disc of
large-scale spiral waves excited in the bright optical disc. Using
hydrodynamical simulations, we show that non-axisymmetric density waves,
penetrating in the gas through the outer Lindblad resonance, can exhibit
relatively regular spiral structures outside the bright optical stellar disc.
For low-amplitude structures, the results of numerical simulations match the
predictions of a simple WKB linear theory. The amplitude of spiral structure
increases rapidly with radius. Beyond optical radii, spirals become
nonlinear (the linear theory becomes quantitatively and qualitatively
inadequate) and unstable to Kelvin-Helmholtz instability. In numerical
simulations, in models for which gas is available very far out, spiral arms can
extend out to 25 disc scale-lengths. A comparison between the properties of the
models we have investigated and the observed properties of individual galaxies
may shed light into the problem of the amount and distribution of dark matter
in the outer halo.Comment: 12 pages, 13 figures (accepted for publication in MNRAS
Properties of quasi-relaxed stellar systems in an external tidal field
In a previous paper, we have constructed a family of self-consistent triaxial
models of quasi-relaxed stellar systems, shaped by the tidal field of the
hosting galaxy, as an extension of the well-known spherical King models. For a
given tidal field, the models are characterized by two physical scales (such as
total mass and central velocity dispersion) and two dimensionless parameters
(the concentration parameter and the tidal strength). The most significant
departure from spherical symmetry occurs when the truncation radius of the
corresponding spherical King model is of the order of the tidal radius, which,
for a given tidal strength, is set by the maximum concentration value admitted.
For such maximally extended (or "critical") models the outer boundary has a
generally triaxial shape, given by the zero-velocity surface of the relevant
Jacobi integral, which is basically independent of the concentration parameter.
In turn, the external tidal field can give rise to significant global
departures from spherical symmetry (as measured, for example, by the quadrupole
of the mass distribution of the stellar system) only for low-concentration
models, for which the allowed maximal value of the tidal strength can be
relatively high. In this paper we describe in detail the intrinsic and the
projected structure and kinematics of the models, covering the entire parameter
space, from the case of sub-critical (characterized by "underfilling" of the
relevant Roche volume) to that of critical models. The intrinsic properties can
be a useful starting point for numerical simulations and other investigations
that require initialization of a stellar system in dynamical equilibrium. The
projected properties are a key step in the direction of a comparison with
observed globular clusters and other candidate stellar systems.Comment: 13 pages, 11 figures, uses emulateapj.cls with apjfonts.sty. Accepted
for publication in The Astrophysical Journa
A dynamical study of Galactic globular clusters under different relaxation conditions
We perform a systematic combined photometric and kinematic analysis of a
sample of globular clusters under different relaxation conditions, based on
their core relaxation time (as listed in available catalogs), by means of two
well-known families of spherical stellar dynamical models. Systems
characterized by shorter relaxation time scales are expected to be better
described by isotropic King models, while less relaxed systems might be
interpreted by means of non-truncated, radially-biased anisotropic f^(\nu)
models, originally designed to represent stellar systems produced by a violent
relaxation formation process and applied here for the first time to the study
of globular clusters. The comparison between dynamical models and observations
is performed by fitting simultaneously surface brightness and velocity
dispersion profiles. For each globular cluster, the best-fit model in each
family is identified, along with a full error analysis on the relevant
parameters. Detailed structural properties and mass-to-light ratios are also
explicitly derived. We find that King models usually offer a good
representation of the observed photometric profiles, but often lead to less
satisfactory fits to the kinematic profiles, independently of the relaxation
condition of the systems. For some less relaxed clusters, f^(\nu) models
provide a good description of both observed profiles. Some derived structural
characteristics, such as the total mass or the half-mass radius, turn out to be
significantly model-dependent. The analysis confirms that, to answer some
important dynamical questions that bear on the formation and evolution of
globular clusters, it would be highly desirable to acquire larger numbers of
accurate kinematic data-points, well distributed over the cluster field.Comment: 18 pages, 7 figures. Accepted for publication in Astronomy &
Astrophysic
Rotating Globular Clusters
Internal rotation is considered to play a major role in the dynamics of some
globular clusters. However, in only few cases it has been studied by
quantitative application of realistic and physically justified global models.
Here we present a dynamical analysis of the photometry and three-dimensional
kinematics of omega Cen, 47 Tuc, and M15, by means of a recently introduced
family of self-consistent axisymmetric rotating models. The three clusters,
characterized by different relaxation conditions, show evidence of differential
rotation and deviations from sphericity. The combination of line-of-sight
velocities and proper motions allows us to determine their internal dynamics,
predict their morphology, and estimate their dynamical distance. The
well-relaxed cluster 47 Tuc is very well interpreted by our model; internal
rotation is found to explain the observed morphology. For M15, we provide a
global model in good agreement with the data, including the central behavior of
the rotation profile and the shape of the ellipticity profile. For the
partially relaxed cluster omega Cen, the selected model reproduces the complex
three-dimensional kinematics; in particular the observed anisotropy profile,
characterized by a transition from isotropy, to weakly-radial anisotropy, and
then to tangential anisotropy in the outer parts. The discrepancy found for the
steep central gradient in the observed line-of-sight velocity dispersion
profile and for the ellipticity profile is ascribed to the condition of only
partial relaxation of this cluster and the interplay between rotation and
radial anisotropy.Comment: 19 pages, 14 figures, accepted for publication in the Astrophysical
Journa
The PROUST radar: First results
Two campaigns took place in 1984 with the PROUST Radar operating in a bistatic mode, the transmitting antenna pointing at the vertical and the receiving one, 1 deg. off the vertical axis. The antenna beam intersection covers an altitude range between 3 and 9 km. The first of these campaigns are analyzed. The results analyzed show the capability of the PROUST Radar to measure the turbulent parameters and study the turbulence-wave interaction. In its present configuration (bistatic mode and 600 m vertical resolution), it has been necessary to make some assumptions that are known not to be truly fulfilled: homogeneous turbulence and constant vertical wind intensity over a 600-m thickness. It is clear that a more detailed study of the interaction between wave and turbulence will be possible with the next version of PROUST Radar (30-m altitude resolution and monostatic mode) that will soon be achieved
Local stability of self-gravitating fluid disks made of two components in relative motion
Context. We consider a simple self-gravitating disk, made of two fluid
components characterized by different effective thermal speeds and interacting
with one another only through gravity; two-component models of this type have
often been considered in order to estimate the impact of the cold interstellar
medium on gravitational instabilities in star-dominated galaxy disks.
Aims. This simple model allows us to produce a unified description of
instabilities in non-viscous self-gravitating disks, some originating from
Jeans collapse, and others from the relative motion between the two components.
In particular, the model suggests that the small streaming velocity between the
two components associated with the so-called asymmetric drift may be the origin
of instability for suitable non-axisymmetric perturbations.
Methods. The result is obtained by examining the properties of a local,
linear dispersion relation for tightly wound density waves in such
two-component model. The parameters characterizing the equilibrium model and
the related dispersion relation allow us to recover as natural limits the
cases, known in the literature, in which the relative drift between the two
components is ignored.
Results. Dynamically, the instability is similar to (although gentler than)
that known to affect counter-rotating disks. However, in contrast to the
instability induced by counter-rotation, which is a relatively rare phenomenon,
the mechanism discussed in this paper is likely to be rather common in nature.
Conclusions. We briefly indicate some consequences of the instability on the
evolution of galaxy disks and possible applications to other astrophysical
systems, in particular to protostellar disks and accretion disks.Comment: 9 pages, 5 figures, Astronomy & Astrophysics, in pres
- …