563 research outputs found
Hydrodynamical stellar models including rotation, internal gravity waves and atomic diffusion. I. Formalism and tests on Pop I dwarfs
In this paper, we develop a formalism in order to incorporate the
contribution of internal gravity waves to the transport of angular momentum and
chemicals over long time-scales in stars. We show that the development of a
double peaked shear layer acts as a filter for waves, and how the asymmetry of
this filter produces momentum extraction from the core when it is rotating
faster than the surface. Using only this filtered flux, it is possible to
follow the contribution of internal waves over long (evolutionary) time-scales.
We then present the evolution of the internal rotation profile using this
formalism for stars which are spun down via magnetic torquing. We show that
waves tend to slow down the core, creating a "slow" front that may then
propagate from the core to the surface. Further spin down of the surface leads
to the formation of a new front. Finally we show how this momentum transport
reduces rotational mixing in a 1.2Msun, Z=0.02 model, leading to a surface
lithium abundance in agreement with observations in the Hyades.Comment: 14 pages, accepted for publication in A&
Angular momentum transport by internal gravity waves III - Wave excitation by core convection and the Coriolis effect
This is the third in a series of papers that deal with angular momentum
transport by internal gravity waves. We concentrate on the waves excited by
core convection in a 3Msun, Pop I main sequence star. Here, we want to examine
the role of the Coriolis acceleration in the equations of motion that describe
the behavior of waves and to evaluate its impact on angular momentum transport.
We use the so-called traditional approximation of geophysics, which allows
variable separation in radial and horizontal components. In the presence of
rotation, the horizontal structure is described by Hough functions instead of
spherical harmonics. The Coriolis acceleration has two main effects on waves.
It transforms pure gravity waves into gravito-inertial waves that have a larger
amplitude closer to the equator, and it introduces new waves whose restoring
force is mainly the conservation of vorticity. Taking the Coriolis acceleration
into account changes the subtle balance between prograde and retrograde waves
in non-rotating stars. It also introduces new types of waves that are either
purely prograde or retrograde. We show in this paper where the local deposition
of angular momentum by such waves is important.Comment: 9 pages, 10 figures, accepted for publication by A&
Angular momentum transport by internal gravity waves. IV - Wave generation by surface convection zone, from the pre-main sequence to the early-AGB in intermediate mass stars
This is the fourth in a series of papers that deal with angular momentum
transport by internal gravity waves in stellar interiors. Here, we want to
examine the potential role of waves in other evolutionary phases than the main
sequence. We study the evolution of a 3Msun Population I model from the
pre-main sequence to the early-AGB phase and examine whether waves can lead to
angular momentum redistribution and/or element diffusion at the external
convection zone boundary. We find that, although waves produced by the surface
convection zone can be ignored safely for such a star during the main sequence,
it is not the case for later evolutionary stages. In particular, angular
momentum transport by internal waves could be quite important at the end of the
sub-giant branch and during the early-AGB phase. Wave-induced mixing of
chemicals is expected during the early-AGB phase.Comment: A&A in press; 11 figure
Impact of internal gravity waves on the rotation profile inside pre-main sequence low-mass stars
We study the impact of internal gravity waves (IGW), meridional circulation,
shear turbulence, and stellar contraction on the internal rotation profile and
surface velocity evolution of solar metallicity low-mass pre-main sequence
stars. We compute a grid of rotating stellar evolution models with masses
between 0.6 and 2.0Msun taking these processes into account for the transport
of angular momentum, as soon as the radiative core appears and assuming no more
disk-locking from that moment on.IGW generation along the PMS is computed
taking Reynolds-stress and buoyancy into account in the bulk of the stellar
convective envelope and convective core (when present). Redistribution of
angular momentum within the radiative layers accounts for damping of prograde
and retrograde IGW by thermal diffusivity and viscosity in corotation
resonance. Over the whole mass range considered, IGW are found to be
efficiently generated by the convective envelope and to slow down the stellar
core early on the PMS. In stars more massive than ~ 1.6Msun, IGW produced by
the convective core also contribute to angular momentum redistribution close to
the ZAMS. Overall, IGW are found to significantly change the internal rotation
profile of PMS low-mass stars.Comment: Accepted for publication in A&A (15 pages
Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars
(Abridged) We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species in the
radiative regions of rotating stars. We focus on cases where meridional
circulation and shear-induced turbulence only are included in the simulations.
Our analysis is based on a 2-D representation of the secular hydrodynamics,
which is treated using expansions in spherical harmonics. We present a full
reconstruction of the meridional circulation and of the associated fluctuations
of temperature and mean molecular weight along with diagnosis for the transport
of angular momentum, heat and chemicals. In the present paper these tools are
used to validate the analysis of two main sequence stellar models of 1.5 and 20
Msun for which the hydrodynamics has been previously extensively studied in the
literature. We obtain a clear visualization and a precise estimation of the
different terms entering the angular momentum and heat transport equations in
radiative zones. This enables us to corroborate the main results obtained over
the past decade by Zahn, Maeder, and collaborators concerning the secular
hydrodynamics of such objects. We focus on the meridional circulation driven by
angular momentum losses and structural readjustements. We confirm
quantitatively for the first time through detailed computations and separation
of the various components that the advection of entropy by this circulation is
very well balanced by the barotropic effects and the thermal relaxation during
most of the main sequence evolution. This enables us to derive simplifications
for the thermal relaxation on this phase. The meridional currents in turn
advect heat and generate temperature fluctuations that induce differential
rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&
Analyse et contrôle de la qualité des données utilisées lors des expertises de barrages
National audienceLes données utilisées lors de l'expertise de barrages ont été structurées sous la forme d'indicateurs. Ces indicateurs sont fréquemment " imparfaits " car ils contiennent incertitude, imprécision ou incomplétude. Nous proposons une voie d'analyse et de contrôle de ces imperfections basée sur (i) l'identification et l'explicitation des sources d'imperfections sous forme de critères, (ii) la quantification des critères sur une échelle de notation, (iii) l'agrégation de ces notes par la méthode d'analyse multicritère ELECTRE TRI. Au final est obtenu un score de qualité associé à chaque indicateur. Cette représentation des imperfections a notamment pour objectif la définition d'actions correctives pour le système d'évaluation afin d'améliorer la qualité des indicateurs en réduisant les imperfections qui leur sont associées. La démarche est appliquée à un cas réel au cours d'une revue décennale / Data used during dam review were structured as indicators. These indicators are frequently imperfect as they contain uncertainty, imprecision, incompleteness. In this paper, we propose an analysis and control of these imperfections based on (i) the identification of the various sources of imperfection and their definition as criteria, (ii) the providing of a reliable way to assess these criteria and (iii) the aggregation of the values resulting from the assessment of the criteria using the multi-criteria analysis ELECTRE TRI. A quality score associated to each data is obtained at the end of the process. This imperfection representation notably aims at proposing corrective actions to improve the quality of indicators by lowering the linked imperfections. This approach was applied to a real-case study
Effects of rotational mixing on the asteroseismic properties of solar-type stars
The influence of rotational mixing on the evolution and asteroseismic
properties of solar-type stars is studied. Rotational mixing changes the global
properties of a solar-type star with a significant increase of the effective
temperature resulting in a shift of the evolutionary track to the blue part of
the HR diagram. These differences are related to changes of the chemical
composition, because rotational mixing counteracts the effects of atomic
diffusion leading to larger helium surface abundances for rotating models than
for non-rotating ones. Higher values of the large frequency separation are then
found for rotating models than for non-rotating ones at the same evolutionary
stage, because the increase of the effective temperature leads to a smaller
radius and hence to an increase of the stellar mean density. Rotational mixing
also has a considerable impact on the structure and chemical composition of the
central stellar layers by bringing fresh hydrogen fuel to the core, thereby
enhancing the main-sequence lifetime. The increase of the central hydrogen
abundance together with the change of the chemical profiles in the central
layers result in a significant increase of the values of the small frequency
separations and of the ratio of the small to large separations for models
including shellular rotation. This increase is clearly seen for models with the
same age sharing the same initial parameters except for the inclusion of
rotation as well as for models with the same global stellar parameters and in
particular the same location in the HR diagram. By computing rotating models of
solar-type stars including the effects of a dynamo that possibly occurs in the
radiative zone, we find that the efficiency of rotational mixing is strongly
reduced when the effects of magnetic fields are taken into account, in contrast
to what happens in massive stars.Comment: 11 pages, 15 figures, accepted for publication in A&
Angular Momentum Transport by Internal Gravity Waves. I - Pop I Main Sequence Stars
We examine the generation of gravity waves by the surface convection zone of
low-mass main sequence stars with solar metallicity. It is found that the total
momentum luminosity in waves rises with stellar mass, up to the
quasi-disappearance of the convection zone around 6500K (corresponding to a
mass of about 1.4 Msun for solar metallicity) where the luminosity drastically
drops. We calculate the net momentum extraction associated with these waves and
explain how the calculated mass dependence helps resolve the enigma of the Li
dip in terms of rotational mixing, forming a coherent picture of mixing in all
main sequence stars.Comment: Accepted for publication in A&
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