4,364 research outputs found
Tides and angular momentum redistribution inside low-mass stars hosting planets: a first dynamical model
We introduce a general mathematical framework to model the internal transport
of angular momentum in a star hosting a close-in planetary/stellar companion.
By assuming that the tidal and rotational distortions are small and that the
deposit/extraction of angular momentum induced by stellar winds and tidal
torques are redistributed solely by an effective eddy-viscosity that depends on
the radial coordinate, we can formulate the model in a completely analytic way.
It allows us to compute simultaneously the evolution of the orbit of the
companion and of the spin and the radial differential rotation of the star. An
illustrative application to the case of an F-type main-sequence star hosting a
hot Jupiter is presented. The general relevance of our model to test more
sophisticated numerical dynamical models and to study the internal rotation
profile of exoplanet hosts, submitted to the combined effects of tides and
stellar winds, by means of asteroseismology are discussed.Comment: 32 pages, 10 figures, one table; accepted to Celestial Mechanics and
Dynamical Astronomy, special issue on tide
Understanding tidal dissipation in gaseous giant planets from their core to their surface
Tidal dissipation in planetary interiors is one of the key physical
mechanisms that drive the evolution of star-planet and planet-moon systems.
Tidal dissipation in planets is intrinsically related to their internal
structure. In particular, fluid and solid layers behave differently under tidal
forcing. Therefore, their respective dissipation reservoirs have to be
compared. In this work, we compute separately the contributions of the
potential dense rocky/icy core and of the convective fluid envelope of gaseous
giant planets, as a function of core size and mass. We demonstrate that in
general both mechanisms must be taken into account.Comment: 2 pages, 2 figures, CoRoT Symposium 3 / Kepler KASC-7 joint meeting,
Toulouse, July 2014; To be published by EPJ Web of Conference
The surface signature of the tidal dissipation of the core in a two-layer planet
Tidal dissipation, which is directly linked to internal structure, is one of
the key physical mechanisms that drive systems evolution and govern their
architecture. A robust evaluation of its amplitude is thus needed to predict
evolution time for spins and orbits and their final states. The purpose of this
paper is to refine recent model of the anelastic tidal dissipation in the
central dense region of giant planets, commonly assumed to retain a large
amount of heavy elements, which constitute an important source of dissipation.
The previous paper evaluated the impact of the presence of the static fluid
envelope on the tidal deformation of the core and on the associated anelastic
tidal dissipation, through the tidal quality factor Qc. We examine here its
impact on the corresponding effective anelastic tidal dissipation, through the
effective tidal quality factor Qp. We show that the strength of this mechanism
mainly depends on mass concentration. In the case of Jupiter- and Saturn-like
planets, it can increase their effective tidal dissipation by, around, a factor
2.4 and 2 respectively. In particular, the range of the rheologies compatible
with the observations is enlarged compared to the results issued from previous
formulations. We derive here an improved expression of the tidal effective
factor Qp in terms of the tidal dissipation factor of the core Qc, without
assuming the commonly used assumptions. When applied to giant planets, the
formulation obtained here allows a better match between the an elastic core's
tidal dissipation of a two-layer model and the observations.Comment: 5 pages, 2 figures, Accepted for publication in Astronomy &
Astrophysic
The neurophysiology of biological motion perception in schizophrenia.
IntroductionThe ability to recognize human biological motion is a fundamental aspect of social cognition that is impaired in people with schizophrenia. However, little is known about the neural substrates of impaired biological motion perception in schizophrenia. In the current study, we assessed event-related potentials (ERPs) to human and nonhuman movement in schizophrenia.MethodsTwenty-four subjects with schizophrenia and 18 healthy controls completed a biological motion task while their electroencephalography (EEG) was simultaneously recorded. Subjects watched clips of point-light animations containing 100%, 85%, or 70% biological motion, and were asked to decide whether the clip resembled human or nonhuman movement. Three ERPs were examined: P1, N1, and the late positive potential (LPP).ResultsBehaviorally, schizophrenia subjects identified significantly fewer stimuli as human movement compared to healthy controls in the 100% and 85% conditions. At the neural level, P1 was reduced in the schizophrenia group but did not differ among conditions in either group. There were no group differences in N1 but both groups had the largest N1 in the 70% condition. There was a condition Ă— group interaction for the LPP: Healthy controls had a larger LPP to 100% versus 85% and 70% biological motion; there was no difference among conditions in schizophrenia subjects.ConclusionsConsistent with previous findings, schizophrenia subjects were impaired in their ability to recognize biological motion. The EEG results showed that biological motion did not influence the earliest stage of visual processing (P1). Although schizophrenia subjects showed the same pattern of N1 results relative to healthy controls, they were impaired at a later stage (LPP), reflecting a dysfunction in the identification of human form in biological versus nonbiological motion stimuli
Organic small molecule field-effect transistors with Cytop(TM) gate dielectric: eliminating gate bias stress effects
We report on organic field-effect transistors with unprecedented resistance
against gate bias stress. The single crystal and thin-film transistors employ
the organic gate dielectric Cytop(TM). This fluoropolymer is highly water
repellent and shows a remarkable electrical breakdown strength. The single
crystal transistors are consistently of very high electrical quality: near zero
onset, very steep subthreshold swing (average: 1.3 nF V/(dec cm2)) and
negligible current hysteresis. Furthermore, extended gate bias stress only
leads to marginal changes in the transfer characteristics. It appears that
there is no conceptual limitation for the stability of organic semiconductors
in contrast to hydrogenated amorphous silicon.Comment: 4 pages, 3 figures, to be published in Appl. Phys. Let
CP and related phenomena in the context of Stellar Evolution
We review the interaction in intermediate and high mass stars between their
evolution and magnetic and chemical properties. We describe the theory of
Ap-star `fossil' fields, before touching on the expected secular diffusive
processes which give rise to evolution of the field. We then present recent
results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that
magnetic fields of the kind seen on the main-sequence already exist during the
pre-main sequence phase, in agreement with fossil field theory, and that the
origin of the slow rotation of Ap/Bp stars also lies early in the pre-main
sequence evolution; we also present results confirming a lack of stars with
fields below a few hundred gauss. We then seek which macroscopic motions
compete with atomic diffusion in determining the surface abundances of AmFm
stars. While turbulent transport and mass loss, in competition with atomic
diffusion, are both able to explain observed surface abundances, the interior
abundance distribution is different enough to potentially lead to a test using
asterosismology. Finally we review progress on the turbulence-driving and
mixing processes in stellar radiative zones.Comment: Proceedings of IAU GA in Rio, JD4 on Ap stars; 10 pages, 7 figure
Calculating Cross Sections of Composite Interstellar Grains
Interstellar grains may be composite collections of particles of distinct
materials, including voids, agglomerated together. We determine the various
optical cross sections of such composite grains, given the optical properties
of each constituent, using an approximate model of the composite grain. We
assume it consists of many concentric spherical layers of the various
materials, each with a specified volume fraction. In such a case the usual Mie
theory can be generalized and the extinction, scattering, and other cross
sections determined exactly.
We find that the ordering of the materials in the layering makes some
difference to the derived cross sections, but averaging over the various
permutations of the order of the materials provides rapid convergence as the
number of shells (each of which is filled by all of the materials
proportionately to their volume fractions) is increased. Three shells, each
with one layer of a particular constituent material, give a very satisfactory
estimate of the average cross section produced by larger numbers of shells.
We give the formulae for the Rayleigh limit (small size parameter) for
multi-layered spheres and use it to propose an ``Effective Medium Theory''
(EMT), in which an average optical constant is taken to represent the ensemble
of materials.
Multi-layered models are used to compare the accuracies of several EMTs
already in the literature.Comment: 29 pages, 6 figures, accepted for publication in the Astrophysical
Journal (part 1, scheduled in Vol. 526, #1, Nov. 20
Unravelling tidal dissipation in gaseous giant planets
International audienceContext. Tidal dissipation in planetary interiors is one of the key physical mechanisms that drive the evolution of star-planet and planet-moon systems. New constraints on this dissipation are now obtained both in the solar and exo-planetary systems.Aims. Tidal dissipation in planets is intrinsically related to their internal structure. Indeed, the dissipation behaves very differently when we compare its properties in solid and fluid planetary layers. Since planetary interiors consist of both types of regions, it is necessary to be able to assess and compare the respective intensity of the reservoir of dissipation in each type of layers. Therefore, in the case of giant planets, the respective contribution of the potential central dense rocky/icy core and of the deep convective fluid envelope must be computed as a function of the mass and the radius of the core. This will allow us to obtain their respective strengths.Methods. Using a method that evaluates the reservoir of dissipation associated to each region, which is a frequency-average of complex tidal Love numbers, we compared the respective contributions of the central core and of the fluid envelope.Results. For Jupiter- and Saturn-like planets, we show that the viscoelastic dissipation in the core could dominate the turbulent friction acting on tidal inertial waves in the envelope. However, the fluid dissipation would not be negligible. This demonstrates that it is necessary to build complete models of tidal dissipation in planetary interiors from their deep interior to their surface without any arbitrary assumptions.Conclusions. We demonstrate how important it is to carefully evaluate the respective strength of each type of dissipation mechanism in planetary interiors and to go beyond the usually adopted ad-hoc models. We confirm the significance of tidal dissipation in the potential dense core of gaseous giant planets
Dust and dark Gamma-Ray Bursts: mutual implications
In a cosmological context dust has been always poorly understood. That is
true also for the statistic of GRBs so that we started a program to understand
its role both in relation to GRBs and in function of z. This paper presents a
composite model in this direction. The model considers a rather generic
distribution of dust in a spiral galaxy and considers the effect of changing
some of the parameters characterizing the dust grains, size in particular. We
first simulated 500 GRBs distributed as the host galaxy mass distribution,
using as model the Milky Way. If we consider dust with the same properties as
that we observe in the Milky Way, we find that due to absorption we miss about
10% of the afterglows assuming we observe the event within about 1 hour or even
within 100s. In our second set of simulations we placed GRBs randomly inside
giants molecular clouds, considering different kinds of dust inside and outside
the host cloud and the effect of dust sublimation caused by the GRB inside the
clouds. In this case absorption is mainly due to the host cloud and the
physical properties of dust play a strong role. Computations from this model
agree with the hypothesis of host galaxies with extinction curve similar to
that of the Small Magellanic Cloud, whereas the host cloud could be also
characterized by dust with larger grains. To confirm our findings we need a set
of homogeneous infrared observations. The use of coming dedicated infrared
telescopes, like REM, will provide a wealth of cases of new afterglow
observations.Comment: 16 pages, 8 figures, accepted by A&
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