185 research outputs found
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
Simplified open repair for anterior chest wall deformities. Analysis of results in 205 patients
SummaryIntroductionPectus deformities are the most frequently seen congenital thoracic wall anomalies. The cause of these conditions is thought to be abnormal elongation of the rib cartilages. We here report our clinical experience and the results of a sternochondroplasty procedure based on the subperichondrial resection of the elongated cartilages.HypothesisThis technique is a valuable surgical strategy to treat the wide variety of pectus deformities.Patients and methodsDuring the period from October 2001 through September 2009, 205 adult patients (171 men and 34 women) underwent pectus excavatum (181), carinatum (19) or arcuatum (5) repair. The patients’ pre and postoperative data were collected using a computerized database, and the results were assessed with a minimum 2-year follow-up.ResultsThe postoperative morbidity rate was minimal and the mortality was nil. The surgeon graded cosmetic results as excellent (72.5%), good (25%) or fair (2.5%), while patients reported better results. Patients with pectus excavatum were found to have much more patent foramen ovale (PFO) than the normal adult population, which occluded after the procedure in 61% of patients, and significant improvement was found in exercise cardiopulmonary function and exercise tolerance at the 1-year follow-up.DiscussionOur sternochondroplasty technique based on the subperichondrial resection of the elongated cartilages allows satisfactory repair of both pectus excavatum and sternal prominence. It is a safe procedure that might improve the effectiveness of surgical therapy in patients with pectus deformities.Level of evidenceLevel IV. Retrospective study
Coriolis darkening in late-type stars II. Effect of self-sustained magnetic fields in stratified convective envelope
Modeling the surface brightness distribution of stars is of prime importance
to interpret observations. Nevertheless, this remains quite challenging for
cool stars as it requires one to model the MHD turbulence that develops in
their convective envelope. In Paper I, the effect of the Coriolis acceleration
on the surface heat flux has been studied by means of hydrodynamic simulations.
In this paper, we aim to investigate the additional effect of dynamo magnetic
fields. We focus on an envelope thickness that is representative of either a
M dwarf, a young red giant star or a pre-main sequence star.
We performed a parametric study using numerical MHD simulations of anelastic
convection in thick rotating spherical shells. For each model, we computed the
mean surface distribution of the heat flux, and examined the leading-order
effect of the magnetic field on the obtained latitudinal luminosity profile. We
identify three different regimes. Close to the onset of convection, while the
first unstable modes tend to convey heat more efficiently near the equator,
magnetic fields are shown to generally enhance the mean heat flux close to the
polar regions (and the tangent cylinder). By progressively increasing the
Rayleigh number, the development of a prograde equatorial jet was previously
shown to make the equator darker when no magnetic field is taken into account.
For moderate Rayleigh numbers, magnetic fields can instead inverse the mean
pole-equator brightness contrast (which means going from a darker to a brighter
equator when a dynamo sets in) and finally induce a similar regime to that
found close to the onset of convection. For more turbulent models with larger
Rayleigh numbers, magnetic fields alternatively tend to smooth out the
brightness contrast. This general behavior is shown to be related to the
quenching of the surface differential rotation by magnetic fields.Comment: Accepted in A&A - Acceptance date: 04/03/202
A comparison of wave mode identification techniques
The four point measurements available from the Cluster mission enable spatiotemporal effects in data sets to be resolved. One application of these multipoint measurements is the determination of the wave vectors and hence the identification of wave modes that exist within the plasma. Prior to multi-satellite missions, wave identification techniques were based upon the interpretation of observational data using theoretically defined relations. However, such techniques are limited by the quality of the data and the type of plasma model employed. With multipoint measurements, wave modes can be identified and their wave directions determined purely from the available observations. This paper takes two such methods, a phase differencing technique and k-filtering and compares their results. It is shown that both methods can resolve the k vector for the dominant mirror mode present in the data. The phase differencing method shows that the nature of the wave environment is constantly changing and as such both methods result in an average picture of the wave environment in the period analysed. The k-filtering method is able to identify other modes that are present
Thorough characterisation of the 16 Cygni system
Context. The advent of space-based photometry observations provided high-quality asteroseismic data for a large number of stars. These observations enabled the adaptation of advanced analyses techniques, until then restricted to the field of helioseismology, to study the best asteroseismic targets. Amongst these, the 16Cyg binary system holds a special place, as they are the brightest solar twins observed by the Kepler mission. For this specific system, modellers have access to high-quality asteroseismic, spectroscopic and interferometric data, making it the perfect testbed for the limitations of stellar models.
Aims. We aim to further constrain the internal structure and fundamental parameters of 16CygA&B using linear seismic inversion techniques of both global indicators and localised corrections of the hydrostatic structure.
Methods. We start from the models defined by detailed asteroseismic modelling in our previous paper and extend our analysis by applying variational inversions to our evolutionary models. We carried out inversions of so-called seismic indicators and attempted to provide local corrections of the internal structure of the two stars.
Results. Our results indicate that linear seismic inversions alone are not able to discriminate between standard and non-standard models for 16CygA&B. We confirm the results of our previous studies that used linear inversion techniques, but consider that the observed differences could be linked to small fundamental parameters variations rather than to a missing process in the models.
Conclusions. We confirm the robustness and reliability of the results of the modelling we performed in our previous paper. We conclude that non-linear inversions are likely required to further investigate the properties of 16CygA&B from a seismic point of view, but that these inversions have to be coupled to analyses of the depletion of light elements such as lithium and beryllium to constrain the macroscopic transport of chemicals in these stars and also to constrain potential non-standard evolutionary paths
Attribution and contestation: Relations between elites and other social groups
In this article we explore the often ambiguous relations between elites and other social groups, both subordinate and of relatively equal standing. The article draws on two distinctive ethnographic cases: the white Franco-Mauritian elite, and the expert elite of management consultants in a Western European context. Our analysis of the two cases provides insights into how the power and status of elites is both contested and attributed by the people they interact with and relate to in concrete, yet substantially different contexts and situations. The aim is to show how the position and power of different kinds of elites is relationally negotiated and achieved. As we argue, a better understanding of the role of other social groups in the attribution, maintenance and contestation of status is relevant for understanding both more traditional economic elites and expert elites without tight networks
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