529 research outputs found
Stochastic excitation of non-radial modes I. High-angular-degree p modes
Turbulent motions in stellar convection zones generate acoustic energy, part
of which is then supplied to normal modes of the star. Their amplitudes result
from a balance between the efficiencies of excitation and damping processes in
the convection zones. We develop a formalism that provides the excitation rates
of non-radial global modes excited by turbulent convection. As a first
application, we estimate the impact of non-radial effects on excitation rates
and amplitudes of high-angular-degree modes which are observed on the Sun. A
model of stochastic excitation by turbulent convection has been developed to
compute the excitation rates, and it has been successfully applied to solar
radial modes (Samadi & Goupil 2001, Belkacem et al. 2006b). We generalize this
approach to the case of non-radial global modes. This enables us to estimate
the energy supplied to high-() acoustic modes. Qualitative arguments as
well as numerical calculations are used to illustrate the results. We find that
non-radial effects for modes are non-negligible:
- for high- modes (i.e. typically ) and for high values of ;
the power supplied to the oscillations depends on the mode inertia.
- for low- modes, independent of the value of , the excitation is
dominated by the non-diagonal components of the Reynolds stress term. We
carried out a numerical investigation of high- modes and we find that
the validity of the present formalism is limited to due to the
spatial separation of scale assumption. Thus, a model for very high-
-mode excitation rates calls for further theoretical developments, however
the formalism is valid for solar modes, which will be investigated in a
paper in preparation.Comment: 12 pages, accepted for publication in A&
Solar-like oscillations in massive main-sequence stars. I. Asteroseismic signatures of the driving and damping regions
Motivated by the recent detection of stochastically excited modes in the
massive star V1449 Aql (Belkacem et al., 2009b), already known to be a
Cephei, we theoretically investigate the driving by turbulent convection. By
using a full non-adiabatic computation of the damping rates, together with a
computation of the energy injection rates, we provide an estimate of the
amplitudes of modes excited by both the convective region induced by the iron
opacity bump and the convective core. Despite uncertainties in the dynamical
properties of such convective regions, we demonstrate that both are able to
efficiently excite modes above the CoRoT observational threshold and the
solar amplitudes. In addition, we emphasise the potential asteroseismic
diagnostics provided by each convective region, which we hope will help to
identify the one responsible for solar-like oscillations, and to give
constraints on this convective zone. A forthcoming work will be dedicated to an
extended investigation of the likelihood of solar-like oscillations across the
Hertzsprung-Russell diagram.Comment: 9 pages, 14 figures, accepter in A&
Theoretical power spectra of mixed modes in low mass red giant stars
CoRoT and Kepler observations of red giant stars revealed very rich spectra
of non-radial solar-like oscillations. Of particular interest was the detection
of mixed modes that exhibit significant amplitude, both in the core and at the
surface of the stars. It opens the possibility of probing the internal
structure from their inner-most layers up to their surface along their
evolution on the red giant branch as well as on the red-clump. Our objective is
primarily to provide physical insight into the physical mechanism responsible
for mixed-modes amplitudes and lifetimes. Subsequently, we aim at understanding
the evolution and structure of red giants spectra along with their evolution.
The study of energetic aspects of these oscillations is also of great
importance to predict the mode parameters in the power spectrum. Non-adiabatic
computations, including a time-dependent treatment of convection, are performed
and provide the lifetimes of radial and non-radial mixed modes. We then combine
these mode lifetimes and inertias with a stochastic excitation model that gives
us their heights in the power spectra. For stars representative of CoRoT and
Kepler observations, we show under which circumstances mixed modes have heights
comparable to radial ones. We stress the importance of the radiative damping in
the determination of the height of mixed modes. Finally, we derive an estimate
for the height ratio between a g-type and a p-type mode. This can thus be used
as a first estimate of the detectability of mixed-modes
Second Order Phase Transitions : From Infinite to Finite Systems
We investigate the Equation of State (EOS) of classical systems having 300
and 512 particles confined in a box with periodic boundary conditions. We show
that such a system, independently on the number of particles investigated, has
a critical density of about 1/3 the ground state density and a critical
temperature of about . The mass distribution at the critical point
exhibits a power law with . Making use of the grand partition
function of Fisher's droplet model, we obtain an analytical EOS around the
critical point in good agreement with the one extracted from the numerical
simulations.Comment: RevTex file, 17 pages + 9 figures available upon request from
[email protected]
The underlying physical meaning of the relation
Asteroseismology of stars that exhibit solar-like oscillations are enjoying a
growing interest with the wealth of observational results obtained with the
CoRoT and Kepler missions. In this framework, scaling laws between
asteroseismic quantities and stellar parameters are becoming essential tools to
study a rich variety of stars. However, the physical underlying mechanisms of
those scaling laws are still poorly known. Our objective is to provide a
theoretical basis for the scaling between the frequency of the maximum in the
power spectrum () of solar-like oscillations and the cut-off
frequency (). Using the SoHO GOLF observations together with
theoretical considerations, we first confirm that the maximum of the height in
oscillation power spectrum is determined by the so-called \emph{plateau} of the
damping rates. The physical origin of the plateau can be traced to the
destabilizing effect of the Lagrangian perturbation of entropy in the
upper-most layers which becomes important when the modal period and the local
thermal relaxation time-scale are comparable. Based on this analysis, we then
find a linear relation between and , with a
coefficient that depends on the ratio of the Mach number of the exciting
turbulence to the third power to the mixing-length parameter.Comment: 8 pages, 11 figures. Accepted in A&
Solar-like oscillation amplitudes and line-widths as a probe for turbulent convection in stars
Excitation of solar-like oscillations is attributed to turbulent convection
and takes place at the upper-most part of the outer convective zones.
Amplitudes of these oscillations depend on the efficiency of the excitation
processes as well as on the properties of turbulent convection. We present past
and recent improvements on the modeling of those processes. We show how the
mode amplitudes and mode line-widths can bring information about the turbulence
in the specific cases of the Sun and Alpha Cen A.Comment: 9 pages ; 3 figures ; invited talk given during the Symposium no. 239
"Convection in Astrophysics", International Astronomical Union., held 21-25
August, 2006 in Prague, Czech Republi
Bremsstrahlung from a microscopic model of relativistic heavy ion collisions
We compute bremsstrahlung arising from the acceleration of individual charged baryons and mesons during the time evolution of high-energy Au+Au collisions at the Relativistic Heavy Ion Collider using a microscopic transport model. We elucidate the connection between bremsstrahlung and charge stop- ping by colliding artificial pure proton on pure neutron nuclei. From the inten- sity of low energy bremsstrahlung, the time scale and the degree of stopping could be accurately extracted without measuring any hadronic observables. PACS: 25.75.-q, 13.85.Q
Angular momentum redistribution by mixed modes in evolved low-mass stars. I. Theoretical formalism
Seismic observations by the space-borne mission \emph{Kepler} have shown that
the core of red giant stars slows down while evolving, requiring an efficient
physical mechanism to extract angular momentum from the inner layers. Current
stellar evolution codes fail to reproduce the observed rotation rates by
several orders of magnitude, and predict a drastic spin-up of red giant cores
instead. New efficient mechanisms of angular momentum transport are thus
required.
In this framework, our aim is to investigate the possibility that mixed modes
extract angular momentum from the inner radiative regions of evolved low-mass
stars. To this end, we consider the Transformed Eulerian Mean (TEM) formalism,
introduced by Andrews \& McIntyre (1978), that allows us to consider the
combined effect of both the wave momentum flux in the mean angular momentum
equation and the wave heat flux in the mean entropy equation as well as their
interplay with the meridional circulation.
In radiative layers of evolved low-mass stars, the quasi-adiabatic
approximation, the limit of slow rotation, and the asymptotic regime can be
applied for mixed modes and enable us to establish a prescription for the wave
fluxes in the mean equations. The formalism is finally applied to a benchmark model, representative of observed CoRoT and \emph{Kepler}
oscillating evolved stars.
We show that the influence of the wave heat flux on the mean angular momentum
is not negligible and that the overall effect of mixed modes is to extract
angular momentum from the innermost region of the star. A quantitative and
accurate estimate requires realistic values of mode amplitudes. This is
provided in a companion paper.Comment: Accepted in A&A, 11 pages, and 6 figure
Event-by-event fluctuations of the charged particle ratio from non-equilibrium transport theory
The event by event fluctuations of the ratio of positively to negatively
charged hadrons are predicted within the UrQMD model. Corrections for finite
acceptance and finite net charge are derived. These corrections are relevant to
compare experimental data and transport model results to previous predictions.
The calculated fluctuations at RHIC and SPS energies are shown to be compatible
with a hadron gas. Thus, deviating by a factor of 3 from the predictions for a
thermalized quark-gluon plasma.Comment: This paper clarifies the previous predictions of Jeon and Koch
(hep-ph/0003168) and addresses issues raised in hep-ph/0006023. 2 Figures,
10pp, uses RevTe
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