3,914 research outputs found
Characterizing the propagation of gravity waves in 3D nonlinear simulations of solar-like stars
The revolution of helio- and asteroseismology provides access to the detailed
properties of stellar interiors by studying the star's oscillation modes. Among
them, gravity (g) modes are formed by constructive interferences between
progressive internal gravity waves (IGWs), propagating in stellar radiative
zones. Our new 3D nonlinear simulations of the interior of a solar-like star
allows us to study the excitation, propagation, and dissipation of these waves.
The aim of this article is to clarify our understanding of the behavior of IGWs
in a 3D radiative zone and to provide a clear overview of their properties. We
use a method of frequency filtering that reveals the path of {individual}
gravity waves of different frequencies in the radiative zone. We are able to
identify the region of propagation of different waves in 2D and 3D, to compare
them to the linear raytracing theory and to distinguish between propagative and
standing waves (g modes). We also show that the energy carried by waves is
distributed in different planes in the sphere, depending on their azimuthal
wave number. We are able to isolate individual IGWs from a complex spectrum and
to study their propagation in space and time. In particular, we highlight in
this paper the necessity of studying the propagation of waves in 3D spherical
geometry, since the distribution of their energy is not equipartitioned in the
sphere.Comment: 14 pages, 12 figues, accepted by Astronomy & Astrophysic
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
Experimental demonstration of the supersonic-subsonic bifurcation in the circular jump: A hydrodynamic white hole
We provide an experimental demonstration that the circular hydraulic jump
represents a hydrodynamic white hole or gravitational fountain (the
time-reverse of a black hole) by measuring the angle of the Mach cone created
by an object in the "supersonic" inner flow region. We emphasise the general
character of this gravitational analogy by showing theoretically that the white
hole horizon constitutes a stationary and spatial saddle-node bifurcation
within dynamical-systems theory. We also demonstrate that the inner region has
a "superluminal" dispersion relation, i.e., that the group velocity of the
surface waves increases with frequency, and discuss some possible consequences
with respect to the robustness of Hawking radiation. Finally, we point out that
our experiment shows a concrete example of a possible "transplanckian
distortion" of black/white holes.Comment: 5 pages, 5 figures. New "transplanckian effect" described. Several
clarifications, additional figures and references. Published versio
Sending femtosecond pulses in circles: highly non-paraxial accelerating beams
We use caustic beam shaping on 100 fs pulses to experimentally generate
non-paraxial accelerating beams along a 60 degree circular arc, moving
laterally by 14 \mum over a 28 \mum propagation length. This is the highest
degree of transverse acceleration reported to our knowledge. Using diffraction
integral theory and numerical beam propagation simulations, we show that
circular acceleration trajectories represent a unique class of non-paraxial
diffraction-free beam profile which also preserves the femtosecond temporal
structure in the vicinity of the caustic
Physical conditions in the ISM towards HD185418
We have developed a complete model of the hydrogen molecule as part of the
spectral simulation code Cloudy. Our goal is to apply this to spectra of
high-redshift star-forming regions where H2 absorption is seen, but where few
other details are known, to understand its implication for star formation. The
microphysics of H2 is intricate, and it is important to validate these
numerical simulations in better-understood environments. This paper studies a
well-defined line-of-sight through the Galactic interstellar medium (ISM) as a
test of the microphysics and methods we use. We present a self-consistent
calculation of the observed absorption-line spectrum to derive the physical
conditions in the ISM towards HD185418, a line-of-sight with many observables.
We deduce density, temperature, local radiation field, cosmic ray ionization
rate, chemical composition and compare these conclusions with conditions
deduced from analytical calculations. We find a higher density, similar
abundances, and require a cosmic ray flux enhanced over the Galactic background
value, consistent with enhancements predicted by MHD simulations.Comment: 31 pages, accepted for publication in Ap
Arbitrary non-paraxial accelerating periodic beams and spherical shaping of light
We report the observation of arbitrary accelerating beams designed using a
non-paraxial description of optical caustics. We use a spatial light
modulator-based setup and techniques of Fourier optics to generate circular and
Weber beams subtending over 95 degrees of arc. Applying a complementary binary
mask also allows the generation of periodic accelerating beams taking the forms
of snake-like trajectories, and the application of a rotation to the caustic
allows the first experimental synthesis of optical accelerating beams upon the
surface of a sphere in three dimensions.Comment: 4 pages, 4 figures articl
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