41 research outputs found
Shear mixing in stellar radiative zones I. Effect of thermal diffusion and chemical stratification
Turbulent transport of chemical elements in radiative zones of stars is
considered in current stellar evolution codes thanks to phenomenologically
derived diffusion coefficients. Recent local numerical simulations (Prat &
Ligni\`eres 2013, A&A, 551, L3) suggest that the coefficient for radial
turbulent diffusion due to radial differential rotation satisfies , in qualitative agreement with Zahn's model. However,
this model does not apply when differential rotation is strong with respect to
stable thermal stratification or when chemical stratification has a significant
dynamical effect, a situation encountered at the outer boundary of
nuclear-burning convective cores. We extend our numerical study to consider the
effects of chemical stratification and of strong shear, and compare the results
with prescriptions used in stellar evolution codes. We performed local, direct
numerical simulations of stably stratified, homogeneous, sheared turbulence in
the Boussinesq approximation. The regime of high thermal diffusivities, typical
of stellar radiative zones, is reached thanks to the so-called
small-P\'eclet-number approximation, which is an asymptotic development of the
Boussinesq equations in this regime. The dependence of the diffusion
coefficient on chemical stratification was explored in this approximation.
Maeder's extension of Zahn's model in the strong-shear regime is not supported
by our results, which are better described by a model found in the geophysical
literature. As regards the effect of chemical stratification, our quantitative
estimate of the diffusion coefficient as a function of the mean gradient of
mean molecular weight leads to the formula , which is compatible in the weak-shear regime with
the model of Maeder & Meynet (1996, A&A, 313, 140).Comment: 10 pages, 9 figures, accepted in A&
Seismic diagnosis from gravity modes strongly affected by rotation
Most of the information we have about the internal rotation of stars comes
from modes that are weakly affected by rotation, for example by using
rotational splittings. In contrast, we present here a method, based on the
asymptotic theory of Prat et al. (2016), which allows us to analyse the
signature of rotation where its effect is the most important, that is in
low-frequency gravity modes that are strongly affected by rotation. For such
modes, we predict two spectral patterns that could be confronted to observed
spectra and those computed using fully two-dimensional oscillation codes.Comment: 3 pages, 1 figure, to appear in the proceedings of the Joint TASC2 &
KASC9 Workshop SPACEINN & HELAS8 Conference "Seismology of the Sun and the
Distant Stars 2016
Perturbative analysis of the effect of a magnetic field on gravito-inertial modes
Magnetic fields have been measured recently in the core of red giant stars
thanks to their effects on stellar oscillation frequencies. The search for
magnetic signatures in pulsating stars, such as Doradus or Slowly
Pulsation B stars, requires to adapt the formalism developed for the slowly
rotating red giants to rapidly rotating stars. We perform a theoretical
analysis of the effects of an arbitrary magnetic field on high radial order
gravity and Rossby modes in a rapidly rotating star. The magnetic effects are
treated as a perturbation. For high radial order modes, the contribution of the
radial component of the magnetic field is likely to dominate over the azimuthal
and latitudinal components. The rotation is taken into account through the
traditional approximation of rotation. General expressions of the frequency
shift induced by an arbitrary radial magnetic field are derived. Approximate
analytical forms are obtained in the high-order high-spin-parameter limits for
the modes most frequently observed in Dor stars. We propose simple
methods to detect seismic magnetic signatures and measure possible magnetic
fields in such stars. These methods offer new possibilities to look for
internal magnetic fields in future observations, such as the ones of the PLATO
mission, or to revisit existing Kepler or TESS data.Comment: Accepted for publication in Astronomy & Astrophysics (18 pages, 18
figures
Internal magnetic fields in 13 red giants detected by asteroseismology
While surface fields have been measured for stars across the HR diagram,
internal magnetic fields remain largely unknown. The recent seismic detection
of magnetic fields in the cores of several Kepler red giants has opened a new
avenue to understand better the origin of magnetic fields and their impact on
stellar structure and evolution. We aim to use asteroseismology to
systematically search for internal magnetic fields in red giant stars and to
determine the strengths and geometries of these fields. Magnetic fields are
known to break the symmetry of rotational multiplets. In red giants,
oscillation modes are mixed, behaving as pressure modes in the envelope and as
gravity modes in the core. Magnetism-induced asymmetries are expected to be
stronger for g-dominated modes than for p-dominated modes and to decrease with
frequency. After collecting a sample of 2500 Kepler red giant stars with clear
mixed-mode patterns, we specifically searched for targets among 1200 stars with
dipole triplets. We identified 13 stars exhibiting clear asymmetric multiplets
and measured their parameters, especially the asymmetry parameter and the
magnetic frequency shift. By combining these estimates with best-fitting
stellar models, we measured average core magnetic fields ranging from 20 to
150kG, corresponding to 5% to 30% of the critical field strengths. We showed
that the detected core fields have various horizontal geometries, some of which
significantly differ from a dipolar configuration. We found that the field
strengths decrease with stellar evolution, despite the fact that the cores of
these stars are contracting. Even though these stars have strong internal
magnetic fields, they display normal core rotation rates, suggesting no
significantly different histories of angular momentum transport compared to
other red giant stars. We also discuss the possible origin of the detected
fields.Comment: Accepted for publication in A&A. Long appendi
Wave chaos in rapidly rotating stars
Effects of rapid stellar rotation on acoustic oscillation modes are poorly
understood. We study the dynamics of acoustic rays in rotating polytropic stars
and show using quantum chaos concepts that the eigenfrequency spectrum is a
superposition of regular frequency patterns and an irregular frequency subset
respectively associated with near-integrable and chaotic phase space regions.
This opens new perspectives for rapidly rotating star seismology and also
provides a new and potentially observable manifestation of wave chaos in a
large scale natural system.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev.
Regular modes in rotating stars
Despite more and more observational data, stellar acoustic oscillation modes
are not well understood as soon as rotation cannot be treated perturbatively.
In a way similar to semiclassical theory in quantum physics, we use acoustic
ray dynamics to build an asymptotic theory for the subset of regular modes
which are the easiest to observe and identify. Comparisons with 2D numerical
simulations of oscillations in polytropic stars show that both the frequency
and amplitude distributions of these modes can accurately be described by an
asymptotic theory for almost all rotation rates. The spectra are mainly
characterized by two quantum numbers; their extraction from observed spectra
should enable one to obtain information about stellar interiors.Comment: 5 pages, 4 figures, discussion adde
Rotation, magnétisme et turbulence dans les étoiles
Ce travail de thèse concerne à la fois la modélisation des processus magnétohydrodynamiques dans les étoiles et l'obtention de contraintes observationnelles sur ces processus. Une partie importante du mémoire est consacrée à l'étude des effets de la rotation sur les modes propres d'oscillation des étoiles. La méconnaissance de ces effets est depuis longtemps identifiée comme l'un des principaux obstacles à l'interprétation des fréquences d'oscillations observées dans les étoiles massives et de masse intermédiaire. Je décris ici la mise au point d'un code d'oscillation tenant compte des effets de la rotation, l'exploration des nouvelles propriétés des modes au moyen de ce code, l'interprétation de ces propriétés dans le cadre d'une théorie asymptotique et comment elles peuvent être utilisées pour interpréter les spectres de fréquences observés. Pour construire la théorie asymptotique, j'ai été amené à m'intéresser au chaos quantique (ou chaos d'onde), une thématique qui est issue de l'étude des propriétés semi-classiques des systèmes quantiques mais qui concerne en fait la physique des ondes en général. La deuxième thématique concerne le magnétisme des étoiles de masse intermédiaire (de type B tardif et de type A) de la séquence principale. C'est un sujet relié mais distinct de celui de ma thèse de doctorat qui était consacré à la modélisation de l'évolution du moment cinétique de ces étoiles dans leur phase pré-séquence-principale. Il s'agit ici d'observations spectropolarimétriques qui renouvellent assez largement notre vision du magnétisme de ces étoiles. D'abord, la mise en évidence d'une limite inférieure à l'intensité du champ des étoiles Ap/Bp a permis de proposer un nouveau scénario pour expliquer l'origine de cette classe d'étoiles, la seule classe d'étoiles magnétiques connue jusqu'alors dans ce domaine du diagramme HR. Puis, la découverte d'un champ magnétique de très faible amplitude à la surface de Véga a montré l'existence d'un nouveau type de magnétisme dans ce domaine. La troisième thématique concerne l'étude des mouvements turbulents dans les intérieurs stellaires, d'une part la modélisation de leur contribution au transport des éléments chimiques dans les zones radiatives d'étoile et d'autre part l'origine des structures cohérentes de la convection turbulente observées à la surface du Soleil
Numerical Exploration of Oscillation Modes in Rapidly Rotating Stars
peer reviewedIn this chapter, we show that two-dimensional oscillation codes help us to improve our knowledge of the effects of rapid rotation on acoustic (p) and gravity (g) modes. We first discuss how to solve the full problem of stellar oscillations in rapidly rotating stars by including the effects of the Coriolis force as well as those of the centrifugal distortion. We illustrate the development and the validation of a 2-D code using the Two-dimensional Oscillation Program (TOP) as an example. We then describe what strategies are being developed to explore the p- and g-mode spectra, how effective these methods are, and what intrinsic difficulties they face. In the last part of this chapter, we present results obtained with these techniques