24 research outputs found
Probing the mid-layer structure of red giants
No full text27 pages, accepted in A\&AInternational audienceContext. The space-borne missions CoRoT and Kepler have already brought stringent constraints on the internal structure of low-mass evolved stars, a large part of which results from the detection of mixed modes. However, all the potential of these oscillation modes as a diagnosis of the stellar interior has not been fully exploited yet. In particular, the coupling factor or the gravity-offset of mixed modes, q and ɛg, are expected to provide additional constraints on the mid-layers of red giants, which are located between the hydrogen-burning shell and the neighborhood of the base of the convective zone. The link between these parameters and the properties of this region, nevertheless, still remains to be precisely established.Aims: In the present paper, we investigate the potential of the coupling factor in probing the mid-layer structure of evolved stars.Methods: Guided by typical stellar models and general physical considerations, we modeled the coupling region along with evolution. We subsequently obtained an analytical expression of q based on the asymptotic theory of mixed modes and compared it to observations.Results: We show that the value of q is degenerate with respect to the thickness of the coupling evanescent region and the local density scale height. On the subgiant branch and the beginning of the red giant branch (RGB), the model predicts that the peak in the observed value of q is necessarily associated with the important shrinking and the subsequent thickening of the coupling region, which is located in the radiative zone at these stages. The large spread in the measurement is interpreted as the result of the high sensitivity of q to the structure properties when the coupling region becomes very thin. Nevertheless, the important degeneracy of q in this regime prevents us from unambiguously concluding on the precise structural origin of the observed values. In later stages, the progressive migration of the coupling region toward the convective zone is expected to result in a slight and smooth decrease in q, which is in agreement with observations. At one point just before the end of the first-dredge up and the luminosity bump, the coupling region becomes entirely located in the convective region and its continuous thickening is shown to be responsible for the observed decrease in q. We demonstrate that q has the promising potential to probe the migration of the base of the convective region as well as convective extra-mixing during this stage. We also show that the frequency-dependence of q cannot be neglected in the oscillation spectra of such evolved RGB stars, which is in contrast with what is assumed in the current measurement methods. This fact can have an influence on the physical interpretation of the observed values. In red clump stars, in which the coupling regions are very thin and located in the radiative zone, the small variations and spread observed in q suggest that their mid-layer structure is very stable.Conclusions: A structural interpretation of the global observed variations in q was obtained and the potential of this parameter in probing the dynamics of the mid-layer properties of red giants is highlighted. This analytical study paves the way for a more quantitative exploration of the link of q with the internal properties of evolved stars using stellar models for a proper interpretation of the observations. This will be undertaken in the following papers of this series
Implications of the generation of internal gravity waves by penetrative convection for the internal rotation evolution of low-mass stars
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Influence of plume-induced internal gravity waves on the rotation profile of low-mass stars
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Influence of plume-induced internal gravity waves on the rotation profile of low-mass stars
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Harmonisation et diffusion des ressources numériques 3D des grottes ornées
Get PDFNational audienceThe national centre for prehistory (CNP, Centre national de la Préhistoire) serves as a depository for all the digital resources concerning prehistoric caves and shelters with parietal wall paintings. The centre tries to carry out a campaign of 3D digitisation every year in order to enrich its holdings. These resources are at one and the same time archival records of parietal painting and material for their future conservation, study and interpretation. Because they have not undergone any treatment or manipulation likely to bias their interpretation, this raw data represents for the centre and its partners a precious resource for documenting and studying prehistoric sites. So today it has become very important to better document and formalise these resources in order to guarantee their preservation and availability, both for the general public and for experts. Consequently, the centre has recently joined forces with the UMR MAP team, specialised in models and simulations for architecture and the heritage, in the framework of a general agreements between the Ministry of Culture and the CNRS and aiming at a better characterisation of the data so far acquired, and its better diffusion. The first point involves the production of pertinent descriptors in order to accompany the 3D data with clear cataloguing information about provenance, nature and quality. The second aspect of the project involves harmonising the data and ensuring its perpetuation and the development of prototype web platforms allowing for visualisation and manipulation via an adaptive interface. The project is part of a broader programme aimed at making a contribution to the traceability of 3D digital imagery produced for scientific research. In this article we propose a progress report on the work far carried out on digital data for parietal art, with particular focus on the recent developments in the visualisation of 3D data.Dépositaire des ressources numériques des grottes et abris ornés, le Centre national de la préhistoire (CNP) s’attache à encourager et à réaliser en moyenne une campagne de numérisation 3D par an, afin d’enrichir ce fonds de données. Ces ressources constituent à la fois des archives des sites d’art pariétal et des supports pour leur conservation, leur étude et leur médiation. Du fait qu’elles n’ont subi aucun traitement ou manipulation susceptibles de biaiser leur exploitation, les données brutes constituent pour le CNP et ses partenaires une ressource précieuse pour l’étude et la documentation des sites ornés. Ainsi, il devient aujourd’hui impératif de s’intéresser à la documentation et la formalisation de ces ressources, afin de les pérenniser et garantir leur utilisation au bénéfice du plus grand nombre, qu’il s’agisse d’experts ou du grand public. À ce titre, le CNP a sollicité l’UMR MAP (Modèles et simulations pour l’architecture et le patrimoine) pour initier ensemble dans le cadre de l’accord-cadre CNRS-ministère de la Culture un travail d’une part sur la description de ces données et d’autre part sur leur diffusion. Le premier point implique de produire des descripteurs pertinents permettant d’accompagner les données 3D d’indications claires quant à leur provenance, leur nature et leur qualité. Le second aspect implique l’harmonisation et la pérennisation des données, ainsi que le développement d’un prototype de plateforme web permettant leur visualisation et manipulation à travers une interface adaptative. Ce travail s’inscrit dans une réflexion plus globale dont l’objectif est de contribuer à la traçabilité des données numériques 3D destinées à des utilisations scientifiques. Nous proposons dans cet article de détailler l’ensemble du travail réalisé dans le cadre de ce projet sur les ressources numériques des grottes ornées, en s’intéressant particulièrement aux développements réalisés pour la visualisation des données 3D
Period spacings in red giants: III. Coupling factors of mixed modes
No full textInternational audienceContext. The power of asteroseismology relies on the capability of global oscillations to infer the stellar structure. For evolved stars, we benefit from unique information directly carried out by mixed modes that probe their radiative cores. This third article of the series devoted to mixed modes in red giants focuses on their coupling factors, which have remained largely unexploited up to now.Aims. With the measurement of coupling factors, we intend to give physical constraints on the regions surrounding the radiative core and the hydrogen-burning shell of subgiants and red giants.Methods. A new method for measuring the coupling factor of mixed modes was implemented, which was derived from the method recently implemented for measuring period spacings. This new method was automated so that it could be applied to a large sample of stars. Results. Coupling factors of mixed modes were measured for thousands of red giants. They show specific variation with mass and evolutionary stage. Weak coupling is observed for the most evolved stars on the red giant branch only; large coupling factors are measured at the transition between subgiants and red giants as well as in the red clump.Conclusions. The measurement of coupling factors in dipole mixed modes provides a new insight into the inner interior structure of evolved stars. While the large frequency separation and the asymptotic period spacings probe the envelope and core, respectively, the coupling factor is directly sensitive to the intermediate region in between and helps determine its extent. Observationally, the determination of the coupling factor is a prior to precise fits of the mixed-mode pattern and can now be used to address further properties of the mixed-mode pattern, such as the signature of buoyancy glitches and core rotation
