739 research outputs found
Excitation of non-radial stellar oscillations by gravitational waves: a first model
The excitation of solar and solar-like g modes in non-relativistic stars by
arbitrary external gravitational wave fields is studied starting from the full
field equations of general relativity. We develop a formalism that yields the
mean-square amplitudes and surface velocities of global normal modes excited in
such a way. The isotropic elastic sphere model of a star is adopted to
demonstrate this formalism and for calculative simplicity. It is shown that
gravitational waves solely couple to quadrupolar spheroidal eigenmodes and that
normal modes are only sensitive to the spherical component of the gravitational
waves having the same azimuthal order. The mean-square amplitudes in case of
stationary external gravitational waves are given by a simple expression, a
product of a factor depending on the resonant properties of the star and the
power spectral density of the gravitational waves' spherical accelerations.
Both mean-square amplitudes and surface velocities show a characteristic
R^8-dependence (effective R^2-dependence) on the radius of the star. This
finding increases the relevance of this excitation mechanism in case of stars
larger than the Sun.Comment: 8 pages, to be published in MNRAS (in press); corrected typo
Generation of internal gravity waves by penetrative convection
The rich harvest of seismic observations over the past decade provides
evidence of angular momentum redistribution in stellar interiors that is not
reproduced by current evolution codes. In this context, transport by internal
gravity waves can play a role and could explain discrepancies between theory
and observations. The efficiency of the transport of angular momentum by waves
depends on their driving mechanism. While excitation by turbulence throughout
the convective zone has already been investigated, we know that penetrative
convection into the stably stratified radiative zone can also generate internal
gravity waves. Therefore, we aim at developing a semianalytical model to
estimate the generation of IGW by penetrative plumes below an upper convective
envelope. We derive the wave amplitude considering the pressure exerted by an
ensemble of plumes on the interface between the radiative and convective zones
as source term in the equation of momentum. We consider the effect of a thermal
transition from a convective gradient to a radiative one on the transmission of
the wave into the radiative zone. The plume-induced wave energy flux at the top
of the radiative zone is computed for a solar model and is compared to the
turbulence-induced one. We show that, for the solar case, penetrative
convection generates waves more efficiently than turbulence and that
plume-induced waves can modify the internal rotation rate on shorter time
scales. We also show that a smooth thermal transition significatively enhances
the wave transmission compared to the case of a steep transition. We conclude
that driving by penetrative convection must be taken into account as much as
turbulence-induced waves for the transport of internal angular momentum.Comment: Accepted for publication in A&A, 21 page
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&
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&
Period spacings in red giants I. Disentangling rotation and revealing core structure discontinuities
Asteroseismology allows us to probe the physical conditions inside the core
of red giant stars. This relies on the properties of the global oscillations
with a mixed character that are highly sensitive to the physical properties of
the core. However, overlapping rotational splittings and mixed-mode spacings
result in complex structures in the mixed-mode pattern, which severely
complicates its identification and the measurement of the asymptotic period
spacing. This work aims at disentangling the rotational splittings from the
mixed-mode spacings, in order to open the way to a fully automated analysis of
large data sets. An analytical development of the mixed-mode asymptotic
expansion is used to derive the period spacing between two consecutive mixed
modes. The \'echelle diagrams constructed with the appropriately stretched
periods are used to exhibit the structure of the gravity modes and of the
rotational splittings. We propose a new view on the mixed-mode oscillation
pattern based on corrected periods, called stretched periods, that mimic the
evenly spaced gravity-mode pattern. This provides a direct understanding of all
oscillation components, even in the case of rapid rotation. The measurement of
the asymptotic period spacing and the signature of the structural glitches on
mixed modes are then made easy. This work opens the possibility to derive all
seismic global parameters in an automated way, including the identification of
the different rotational multiplets and the measurement of the rotational
splitting, even when this splitting is significantly larger than the period
spacing. Revealing buoyancy glitches provides a detailed view on the radiative
core.Comment: Accepted in A&
Enhanced antiproton production in Pb(160 AGeV)+Pb reactions: evidence for quark gluon matter?
The centrality dependence of the antiproton per participant ratio is studied
in Pb(160 AGeV)+Pb reactions. Antiproton production in collisions of heavy
nuclei at the CERN/SPS seems considerably enhanced as compared to conventional
hadronic physics, given by the antiproton production rates in and
antiproton annihilation in reactions. This enhancement is consistent
with the observation of strong in-medium effects in other hadronic observables
and may be an indication of partial restoration of chiral symmetry
Mode excitation by turbulent convection in rotating stars. I. Effect of uniform rotation
We focus on the influence of the Coriolis acceleration on the stochastic
excitation of oscillation modes in convective regions of rotating stars. Our
aim is to estimate the asymmetry between excitation rates of prograde and
retrograde modes. We extend the formalism derived for obtaining stellar -
and -mode amplitudes (Samadi & Goupil 2001, Belkacem et al. 2008) to include
the effect of the Coriolis acceleration. We then study the special case of
uniform rotation for slowly rotating stars by performing a perturbative
analysis. This allows us to consider the cases of the Sun and the CoRoT target
HD 49933. We find that, in the subsonic regime, the influence of rotation as a
direct contribution to mode driving is negligible in front of the Reynolds
stress contribution. In slow rotators, the indirect effect of the modification
of the eigenfunctions on mode excitation is investigated by performing a
perturbative analysis of the excitation rates. It turns out that the excitation
of solar modes is affected by rotation with excitation rates asymmetries
between prograde and retrograde modes of the order of several percents. Solar
low-order modes are also affected by uniform rotation and their excitation
rates asymmetries are found to reach up to 10 %. The CoRoT target HD 49933 is
rotating faster than the Sun () and we show
that the resulting excitation rates asymmetry is about 10 % for the excitation
rates of modes. We have then demonstrated that and mode excitation
rates are modified by uniform rotation through the Coriolis acceleration. Study
of the effect of differential rotation is dedicated to a forthcoming paper.Comment: 9 pages, 4 figures, accepted in A&
Dileptons and Photons from Coarse-Grained Microscopic Dynamics and Hydrodynamics Compared to Experimental Data
Radiation of dileptons and photons from high energy nuclear collisions
provides information on the space-time evolution of the hot dense matter
produced therein. We compute this radiation using relativistic hydrodynamics
and a coarse-grained version of the microscopic event generator UrQMD, both of
which provide a good description of the hadron spectra. The currently most
accurate dilepton and photon emission rates from perturbative QCD and from
experimentally-based hadronic calculations are used. Comparisons are made to
data on central Pb-Pb and Pb-Au collisions taken at the CERN SPS at a beam
energy of 158 A GeV. Both hydrodynamics and UrQMD provide very good
descriptions of the photon transverse momentum spectrum measured between 1 and
4 GeV, but slightly underestimate the low mass spectrum of e+e- pairs, even
with greatly broadened rho and omega vector mesons. Predictions are given for
the transverse momentum distribution of dileptons.Comment: 35 pages, 17 figure
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
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