110 research outputs found
Procyon-A and Eta-Bootis: Observational Frequencies Analyzed by the Local-Wave Formalism
In the present analysis of Procyon-A and Eta-Bootis, we use the local-wave
formalism which, despite its lack of precision inherent to any semi-analytical
method, uses directly the model profile without any modification when
calculating the acoustic mode eigenfrequencies. These two solar-like stars
present steep variations toward the center due to the convective core
stratification, and toward the surface due to the very thin convective zone.
Based on different boundary conditions, the frequencies obtained with this
formalism are different from that of the classical numerical calculation. We
point out that (1) the frequencies calculated with the local-wave formalism
seem to agree better with observational ones. All the frequencies detected with
a good confident level including those classified as 'noise' find an
identification, (2) some frequencies can be clearly identified here as
indications of the core limit.Comment: SOHO 18 / GONG 2006 / HELAS I Meetin
Standard Solar models in the Light of New Helioseismic Constraints II. Mixing Below the Convective Zone
In previous work, we have shown that recent updated standard solar models
cannot reproduce the radial profile of the sound speed at the base of the
convective zone (CZ) and fail to predict the Li7 depletion. In parallel,
helioseismology has shown that the transition from differential rotation in the
CZ to almost uniform rotation in the radiative solar interior occurs in a
shallow layer called the tachocline. This layer is presumably the seat of large
scale circulation and of turbulent motions. Here, we introduce a macroscopic
transport term in the structure equations, which is based on a hydrodynamical
description of the tachocline proposed by Spiegel and Zahn, and we calculate
the mixing induced within this layer. We discuss the influence of different
parameters that represent the tachocline thickness, the Brunt-Vaissala
frequency at the base of the CZ, and the time dependence of this mixing process
along the Sun's evolution. We show that the introduction of such a process
inhibits the microscopic diffusion by about 25%. Starting from models including
a pre-main sequence evolution, we obtain: a) a good agreement with the observed
photospheric chemical abundance of light elements such as He3, He4, Li7 and
Be9, b) a smooth composition gradient at the base of the CZ, and c) a
significant improvement of the sound speed square difference between the
seismic sun and the models in this transition region, when we allow the
phostospheric heavy element abundance to adjust, within the observational
incertitude, due to the action of this mixing process. The impact on neutrino
predictions is also discussed.Comment: 15 pages, 7 figures, to be published in ApJ (used emulateapj style
for latex2e). New email for A. S. Brun: [email protected]
Cooling Flows of Self-Gravitating, Rotating, Viscous Systems
We obtain self-similar solutions that describe the dynamics of a
self-gravitating, rotating, viscous system. We use simplifying assumptions; but
explicitly include viscosity and the cooling due to the dissipation of energy.
By assuming that the turbulent dissipation of energy is as power law of the
density and the speed v_{rms} and for a power-law dependence of viscosity on
the density, pressure, and rotational velocity, we investigate turbulent
cooling flows. It has been shown that for the cylindrically and the spherically
cooling flows the similarity indices are the same, and they depend only on the
exponents of the dissipation rate and the viscosity model. Depending on the
values of the exponents, which the mechanisms of the dissipation and viscosity
determine them, we may have solutions with different general physical
properties. The conservation of the total mass and the angular momentum of the
system strongly depends on the mechanisms of energy dissipation and the
viscosity model.Comment: 19 pages, 5 figures, To appear in ApJ (scheduled for the v574, July
20, 2002
Detection of periodic signatures in the solar power spectrum. On the track of l=1 gravity modes
In the present work we show robust indications of the existence of g modes in
the Sun using 10 years of GOLF data. The present analysis is based on the
exploitation of the collective properties of the predicted low-frequency (25 to
140 microHz) g modes: their asymptotic nature, which implies a quasi
equidistant separation of their periods for a given angular degree (l). The
Power Spectrum (PS) of the Power Spectrum Density (PSD), reveals a significant
structure indicating the presence of features (peaks) in the PSD with near
equidistant periods corresponding to l=1 modes in the range n=-4 to n=-26. The
study of its statistical significance of this feature was fully undertaken and
complemented with Monte Carlo simulations. This structure has a confidence
level better than 99.86% not to be due to pure noise. Furthermore, a detailed
study of this structure suggests that the gravity modes have a much more
complex structure than the one initially expected (line-widths, magnetic
splittings...). Compared to the latest solar models, the obtained results tend
to favor a solar core rotating significantly faster than the rest of the
radiative zone. In the framework of the Phoebus group, we have also applied the
same methodology to other helioseismology instruments on board SoHO and ground
based networks.Comment: Proceedings of the SOHO-18/GONG2006/HELAS I: Beyond the spherical Su
Simulations of turbulent convection in rotating young solar-like stars: Differential rotation and meridional circulation
We present the results of three-dimensional simulations of the deep
convective envelope of a young (10 Myr) one-solar-mass star, obtained with the
Anelastic Spherical Harmonic code. Since young stars are known to be faster
rotators than their main sequence counterparts, we have systematically studied
the impact of the stellar rotation speed, by considering stars spinning up to
five times as fast as the Sun. The aim of these nonlinear models is to
understand the complex interactions between convection and rotation. We discuss
the influence of the turbulence level and of the rotation rate on the intensity
and the topology of the mean flows. For all of the computed models, we find a
solar-type superficial differential rotation, with an equatorial acceleration,
and meridional circulation that exhibits a multicellular structure. Even if the
differential rotation contrast decreases only marginally for high rotation
rates, the meridional circulation intensity clearly weakens according to our
simulations. We have also shown that, for Taylor numbers above a certain
threshold (Ta>10^9), the convection can develop a vacillating behavior. Since
simulations with high turbulence levels and rotation rates exhibit strongly
cylindrical internal rotation profiles, we have considered the influence of
baroclinic effects at the base of the convective envelope of these young Suns,
to see whether such effect can modify the otherwise near cylindrical profiles
to produce more conical, solar-like profiles.Comment: 32 pages, 18 figures, 2 tables, to appear in Ap
Non equilibrium thermodynamics and cosmological pancakes formation
We investigate the influence of non equilibrium thermodynamics on
cosmological structure formation. In this paper, we consider the collapse of
planar perturbations usually called "Zel'dovich pancakes". We have developed
for that purpose a new two fluids (gas and dark matter) hydrodynamical code,
with three different thermodynamical species: electrons, ions and neutral
particles (T_e\ne T_i \ne T_n). We describe in details the complex structure of
accretion shock waves. We include several relevant processes for a low density,
high temperature, collisional plasma such as non-equilibrium chemical
reactions, cooling, shock heating, thermal energy equipartition between
electrons, ions and neutral particles and electronic conduction. We find two
different regions in the pancake structure: a thermal precursor ahead of the
compression front and an equipartition wave after the compression front where
electrons and ions temperatures differ significantly. This complex structure
may have two interesting consequences: pre-heating of unshocked regions in the
vicinity of massive X-ray clusters and ions and electrons temperatures
differences in the outer regions of X-rays clusters.Comment: 30 pages, including 8 figures, accepted for publication in The
Astrophysical Journa
Young stellar object jet models: From theory to synthetic observations
Astronomical observations, analytical solutions and numerical simulations
have provided the building blocks to formulate the current theory of young
stellar object jets. Although each approach has made great progress
independently, it is only during the last decade that significant efforts are
being made to bring the separate pieces together. Building on previous work
that combined analytical solutions and numerical simulations, we apply a
sophisticated cooling function to incorporate optically thin energy losses in
the dynamics. On the one hand, this allows a self-consistent treatment of the
jet evolution and on the other, it provides the necessary data to generate
synthetic emission maps. Firstly, analytical disk and stellar outflow solutions
are properly combined to initialize numerical two-component jet models inside
the computational box. Secondly, magneto-hydrodynamical simulations are
performed in 2.5D, following properly the ionization and recombination of a
maximum of ions. Finally, the outputs are post-processed to produce
artificial observational data. The first two-component jet simulations, based
on analytical models, that include ionization and optically thin radiation
losses demonstrate promising results for modeling specific young stellar object
outflows. The generation of synthetic emission maps provides the link to
observations, as well as the necessary feedback for the further improvement of
the available models.Comment: accepted for publication A&A, 20 pages, 11 figure
Advances in secular magnetohydrodynamics of stellar interiors dedicated to asteroseismic spatial missions
With the first light of COROT, the preparation of KEPLER and the future
helioseismology spatial projects such as GOLF-NG, a coherent picture of the
evolution of rotating stars from their birth to their death is needed. We
describe here the modelling of the macroscopic transport of angular momentum
and matter in stellar interiors that we have undertaken to reach this goal.
First, we recall in detail the dynamical processes that are driving these
mechanisms in rotating stars and the theoretical advances we have achieved.
Then, we present our new results of numerical simulations which allow us to
follow in 2D the secular hydrodynamics of rotating stars, assuming that
anisotropic turbulence enforces a shellular rotation law. Finally, we show how
this work is leading to a dynamical vision of the Hertzsprung-Russel diagram
with the support of asteroseismology and helioseismology, seismic observables
giving constraints on the modelling of the internal transport and mixing
processes. In conclusion, we present the different processes that should be
studied in the next future to improve our description of stellar radiation
zones.Comment: 14 pages, 3 figures, Proceeding of the Joint HELAS and CoRoT/ESTA
Workshop (20-23 November 2006, CAUP, Porto - Portugal
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