358 research outputs found
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
Noise and thermal stability of vibrating micro-gyrometers preamplifiers
The preamplifier is a critical component of gyrometer's electronics. Indeed
the resolution of the sensor is limited by its signal to noise ratio, and the
gyrometer's thermal stability is limited by its gain drift. In this paper, five
different kinds of preamplifiers are presented and compared. Finally, the
design of an integrated preamplifier is shown in order to increase the gain
stability while reducing its noise and size.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
A new seismic analysis of Alpha Centauri
Models of alpha Cen A & B have been computed using the masses determined by
Pourbaix et al. (2002) and the data derived from the spectroscopic analysis of
Neuforge and Magain (1997). The seismological data obtained by Bouchy and
Carrier (2001, 2002) do help improve our knowledge of the evolutionary status
of the system. All the constraints are satisfied with a model which gives an
age of about 6 Gyr for the binary.Comment: to be published in Astronomy and Astrophysic
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&
Pratiques du titrement dans les villes en développement : trois cas d'étude (Inde, Ethiopie, Mauritanie) : rapport de recherche présenté dans le cadre de l'appel à projets "La sécurisation du droit de propriété dans les pays en voie de développement"
Les trois cas d’études en Inde, en Ethiopie et en Mauritanie ont permis de voir comment est appréhendé un même cadre référentiel qui pose la sécurisation foncière au coeur de la lutte contre la pauvreté. Plusieurs types de villes (petites, secondaires, capitales) et formes d’urbain (zone centrale, quartiers périurbains, urbain diffus) ont permis de souligner quelques uns des enjeux que représente la titrisation dans les Suds. Le contexte urbain joue fortement. L’impact des politiques de titrement foncier dépend réellement de la taille de la ville et des formes urbaines. L’urbanisation généralisée et rapide dans les trois pays étudiés, longtemps considérés comme étant à dominante rurale, implique l’adaptation et la création de nouveaux régimes fonciers. Dans nos différents cas d’étude, l’accès au sol urbain fait émerger de nouveaux besoins en termes de réglementation foncière, de nouveaux types de reconnaissance, mais aussi et surtout de nouvelles pratiques de sécurisation se tissent entre les habitants et les autorités et entre habitants. La présente étude a fait ressortir l’importance de la circulation à l’échelle internationale de politiques urbaines et « bonnes pratiques », pensées depuis Washington par les Institutions internationales, et réappropriées de façon tout à fait originale localement. La comparaison des trois cas a mis en lumière l’importance des pratiques habitantes, qui, en Ethiopie, en Inde et en Mauritanie, exploitent et construisent des opportunités offertes par les normes juridiques relatives à l’accès à la propriété, plus qu’elles arrivent à en bénéficier pleinement et simplement. Bien souvent, les réformes appuyées par les bailleurs de fond ne font que se surimposer à des juridictions foncières déjà complexes, souvent coûteuses à respecter pour les habitants qui optent pour des pratiques plus informelles. La réforme sur le papier n’est que rarement suivie par la réforme en action
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
The CoRoT target HD 49933: 2- Comparison of theoretical mode amplitudes with observations
From the seismic data obtained by CoRoT for the star HD 49933 it is possible,
as for the Sun, to constrain models of the excitation of acoustic modes by
turbulent convection. We compare a stochastic excitation model described in
Paper I (arXiv:0910.4027) with the asteroseismology data for HD 49933, a star
that is rather metal poor and significantly hotter than the Sun. Using the mode
linewidths measured by CoRoT for HD 49933 and the theoretical mode excitation
rates computed in Paper I, we derive the expected surface velocity amplitudes
of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic
approximation relating the mode amplitudes in intensity to those in velocity,
we derive the expected values of the mode amplitude in intensity. Our amplitude
calculations are within 1-sigma error bars of the mode surface velocity
spectrum derived with the HARPS spectrograph. The same is found with the mode
amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other
hand, at high frequency, our calculations significantly depart from the CoRoT
and HARPS measurements. We show that assuming a solar metal abundance rather
than the actual metal abundance of the star would result in a larger
discrepancy with the seismic data. Furthermore, calculations that assume the
``new'' solar chemical mixture are in better agreement with the seismic data
than those that assume the ``old'' solar chemical mixture. These results
validate, in the case of a star significantly hotter than the Sun and Alpha Cen
A, the main assumptions in the model of stochastic excitation. However, the
discrepancies seen at high frequency highlight some deficiencies of the
modelling, whose origin remains to be understood.Comment: 8 pages, 3 figures (B-W and color), accepted for publication in
Astronomy & Astrophysics. Corrected typo in Eq. (4). Updated references.
Language improvement
Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars
We find that low-degree low-order g-modes become unstable in metal-poor
low-mass stars due to the -mechanism of the pp-chain. Since the
outer convection zone of these stars is limited only to the very outer layers,
the uncertainty in the treatment of convection does not affect the result
significantly. The decrease in metallicity leads to decrease in opacity and
hence increase in luminosity of a star. This makes the star compact and results
in decrease in the density contrast, which is favorable to the
-mechanism instability. We find also instability for high order
g-modes of metal-poor low-mass stars by the convective blocking mechanism.
Since the effective temperature and the luminosity of metal-poor stars are
significantly higher than those of Pop I stars, the stars showing
Dor-type pulsation are substantially less massive than in the case of Pop I
stars. We demonstrate that those modes are unstable for about
stars in the metal-poor case.Comment: 4 pages, 4 figures, To be published in Astrophysics and Space Science
Proceedings series (ASSP). Proceedings of the "20th Stellar Pulsation
Conference Series: Impact of new instrumentation and new insights in stellar
pulsations", 5-9 September 2011, Granada, Spai
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