540 research outputs found
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&
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
Time-Resolved Ultrafast Transient Polarization Spectroscopy to Investigate Nonlinear Processes and Dynamics in Electronically Excited Molecules on the Femtosecond Time Scale
We report a novel experimental technique to investigate ultrafast dynamics in
photoexcited molecules by probing the third-order nonlinear optical
susceptibility. A non-colinear 3-pulse scheme is developed to probe the
ultrafast dynamics of excited electronic states using the optical Kerr effect
by time-resolved polarization spectroscopy. Optical heterodyne and optical
homodyne detection are demonstrated to measure the third-order nonlinear
optical response for the S1 excited state of liquid nitrobenzene, which is
populated by 2-photon absorption of a 780 nm 35 fs excitation pulse.Comment: 12 pages, 4 figures. Changes from previous version: added panel
labels to figures 3-
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
Relativistic Hadron-Hadron Collisions in the Ultra-Relativistic Quantum Molecular Dynamics Model (UrQMD)
Hadron-hadron collisions at high energies are investigated in the
Ultra-relativistic-Quantum-Molecular-Dynamics approach (UrQMD). This
microscopic transport model is designed to study pp, pA and A+A collisions. It
describes the phenomenology of hadronic interactions at low and intermediate
energies ( GeV) in terms of interactions between known hadrons and
their resonances. At high energies, GeV, the excitation of color
strings and their subsequent fragmentation into hadrons dominates the multiple
production of particles in the UrQMD model. The model shows a fair overall
agreement with a large body of experimental h-h data over a wide range of h-h
center-of-mass energies. Hadronic reaction data with higher precision would be
useful to support the use of the UrQMD model for relativistic heavy ion
collisions.Comment: 66 pages, Download the UrQMD model from
http://www.th.physik.uni-frankfurt.de/~urqmd/urqmd.htm
Period-luminosity relations in evolved red giants explained by solar-like oscillations
Solar-like oscillations in red giants have been investigated with CoRoT and
Kepler, while pulsations in more evolved M giants have been studied with
ground-based microlensing surveys. After 3.1 years of observation with Kepler,
it is now possible to make a link between these different observations of
semi-regular variables. We aim to identify period-luminosity sequences in
evolved red giants identified as semi-regular variables. Then, we investigate
the consequences of the comparison of ground-based and space-borne
observations. We have first measured global oscillation parameters of evolved
red giants observed with Kepler with the envelope autocorrelation function
method. We then used an extended form of the universal red giant oscillation
pattern, extrapolated to very low frequency, to fully identify their
oscillations. From the link between red giant oscillations observed by Kepler
and period-luminosity sequences, we have identified these relations in evolved
red giants as radial and non-radial solar-like oscillations. We were able to
expand scaling relations at very low frequency. This helped us to identify the
different sequences of period-luminosity relations, and allowed us to propose a
calibration of the K magnitude with the observed frequency large separation.
Interpreting period-luminosity relations in red giants in terms of solar-like
oscillations allows us to investigate, with a firm physical basis, the time
series obtained from ground-based microlensing surveys. This can be done with
an analytical expression that describes the low-frequency oscillation spectra.
The different behavior of oscillations at low frequency, with frequency
separations scaling only approximately with the square root of the mean stellar
density, can be used to address precisely the physics of the semi-regular
variables.Comment: Accepted in A&
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
Angular momentum redistribution by mixed modes in evolved low-mass stars. II. Spin-down of the core of red giants induced by mixed modes
The detection of mixed modes in subgiants and red giants by the CoRoT and
\emph{Kepler} space-borne missions allows us to investigate the internal
structure of evolved low-mass stars. In particular, the measurement of the mean
core rotation rate as a function of the evolution places stringent constraints
on the physical mechanisms responsible for the angular momentum redistribution
in stars. It showed that the current stellar evolution codes including the
modelling of rotation fail to reproduce the observations. An additional
physical process that efficiently extracts angular momentum from the core is
thus necessary.
Our aim is to assess the ability of mixed modes to do this. To this end, we
developed a formalism that provides a modelling of the wave fluxes in both the
mean angular momentum and the mean energy equations in a companion paper. In
this article, mode amplitudes are modelled based on recent asteroseismic
observations, and a quantitative estimate of the angular momentum transfer is
obtained. This is performed for a benchmark model of 1.3 at three
evolutionary stages, representative of the evolved pulsating stars observed by
CoRoT and Kepler.
We show that mixed modes extract angular momentum from the innermost regions
of subgiants and red giants. However, this transport of angular momentum from
the core is unlikely to counterbalance the effect of the core contraction in
subgiants and early red giants. In contrast, for more evolved red giants, mixed
modes are found efficient enough to balance and exceed the effect of the core
contraction, in particular in the hydrogen-burning shell. Our results thus
indicate that mixed modes are a promising candidate to explain the observed
spin-down of the core of evolved red giants, but that an other mechanism is to
be invoked for subgiants and early red giants.Comment: Accepted in A&A, 7 pages, 8 figure
Microscopic Analysis of Thermodynamic Parameters from 160 MeV/n - 160 GeV/n
Microscopic calculations of central collisions between heavy nuclei are used
to study fragment production and the creation of collective flow. It is shown
that the final phase space distributions are compatible with the expectations
from a thermally equilibrated source, which in addition exhibits a collective
transverse expansion. However, the microscopic analyses of the transient states
in the reaction stages of highest density and during the expansion show that
the system does not reach global equilibrium. Even if a considerable amount of
equilibration is assumed, the connection of the measurable final state to the
macroscopic parameters, e.g. the temperature, of the transient ''equilibrium''
state remains ambiguous.Comment: 13 pages, Latex, 8 postscript figures, Proceedings of the Winter
Meeting in Nuclear Physics (1997), Bormio (Italy
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