LESIA - Observatoire de Paris

Les missions spatiales habitées

Filippo Pantellini — 2012-06-25T09:52:35Z

Une rétrospective des missions spatiales habitées sera présentée par Michel Tognini, directeur, jusqu'en Janvier 2012, des astronautes de l'Agence Spatiale Européenne, ancien astronaute, spécialiste de missions sur le vol Antarés avec la station orbitale MIR en 1999 et sur la navette américaine Columbia de la NASA pour le déploiement du télescope X-CHANDRA.

- Saison 2011-2012

Stereoscopic analysis of Jovian DAM by using STEREO/WAVES Experiment

Filippo Pantellini — 2012-04-30T12:26:39Z

- Saison 2011-2012

Planets Twice the Size of Earth: Orbits, Radii, and Occurrence

Filippo Pantellini — 2011-10-17T15:52:17Z

The talk will report the observed distribution of planet radii, orbital distances, and occurrence frequency for planets with orbital periods less than 50 days around Solar-type stars. We draw from extensive Kepler measurements that offer good completeness for planets with radii as small as 2.0 Earth-radii.

- Saison 2011-2012

Stirring Saturn's magnetosphere with vortical winds in the upper atmosphere

Filippo Pantellini — 2011-09-27T15:55:46Z

Electromagnetic phenomena in Saturn's magnetosphere and ionosphere vary periodically at a period close to that of planetary rotation. Arguments relating to the slow drift of the modulation period and the inertia of different regions of Saturn's environment require that the system be driven from regions remote from the equatorial surface, plausibly the high altitude atmosphere. The assumption that the system is driven from the ionospheric ends of the flux tubes is supported by characteristics of the magnetic perturbations observed near the magnetic equator. This presentation will describe results from a magnetohydrodynamic simulation in which the atmosphere drives vortical winds in the southern ionosphere. The wind pattern rotates about the spin axis at the modulation period of Saturn Kilometric Radiation (SKR). The simulation reproduces many features reported for magnetospheric perturbations. It also exhibits field-aligned current (FAC) patterns that correspond to properties reported for SKR emissions. In a second run of the simulation, an additional vortical flow structure is introduced into the northern high-altitude atmosphere. The northern structure rotates about the spin axis at a slightly shorter period. This second simulation provides insight into the way in which two separate frequencies affect the magnetosphere and the FACs.

Co-authors: Xianzhe Jia and Tamas Gombosi

Etude dynamique des disques autour d'étoiles Be

Filippo Pantellini — 2011-09-22T15:47:41Z

La formation d'un disque autour d'une étoile Be est un mécanisme encore assez peu connu. Depuis quelques années, le modèle de décrétion s'est démarqué des autres car lui seul parvient à reproduire l'ensemble des propriétés observées de ces étoiles. Cependant ce modèle reste encore difficile à utiliser pour les étoiles Be actives car les effets d'éjection de masse stellaire dans le disque de décrétion sont encore assez peu explorés. Des simulations SPH (Smoothed Particle Hydrodynamics) couplées à de puissants codes de transfert radiatif permettent désormais de modéliser le comportement des disques de décrétion sur des longues périodes de temps.

Au cours de ce séminaire, je présenterai d'abord le modèle de décrétion par une étude de quelques exemples illustratifs. Je montrerai ensuite ses implications pour différents observables (photométriques, polarimétriques, etc). Enfin le séminaire se terminera par un présentation d'un travail de modélisation mené pour l'étude de l'étoile Be binaire Delta Sco, système qui a récemment fait l'objet d'une attention particulière de la communauté en raison du passage au périastre (et éventuellement dans le disque) de son compagnon.

Coronal heating in coupled photosphere-chromosphere-coronal systems: turbulence and leakage

Filippo Pantellini — 2011-09-20T15:01:04Z

Coronal loops act as a resonant cavity for low frequency fluctuations that are transmitted from the deeper layers of the solar atmosphere. Such fluctuations are amplified in the corona and lead to the development of turbulence that in turn is able to dissipate the accumulated energy, thus heating the corona. However trapping is not perfect, some energy leaks down to the chromosphere on a long timescale, thus limiting the turbulent heating.

We consider the combined effects of turbulence and energy leakage from the corona to the photosphere in determining the turbulent energy level and associated heating rate in models of coronal loops which include the chromosphere and transition region. We use a piece-wise constant model for the Alfvén speed in loops and a Reduced MHD - Shell model to describe the interplay between turbulent dynamics in the direction perpendicular to the mean field and propagation along the field. Turbulence is sustained by incoming fluctuations which are equivalent, in the line-tied case, to forcing by the photospheric shear flows. We find that: (i) Leakage always plays a role, whatever the intensity of the photospheric forcing; the dissipation time never becomes much lower than the leakage time, at least in the three-layer model. Hence, the energy as well as the dissipation levels are systematically lower than in the line-tied model. (ii) In all models, the energy level is close to the resonant prediction, i.e., assuming effective turbulent correlation time longer than the Alfvén coronal crossing time. (iii) The heating rate is approximately given by the ratio of photospheric energy divided by the Alfvén crossing time. (iv) The coronal spectral range is divided in two, an inertial range with 5/3 spectral slope, and a large scale peak where nonlinear couplings are inhibited by trapped resonant modes. (v) In the realistic 3-layer model, the two-component spectrum leads to a global decrease of damping equal to Kolmogorov damping reduced by a factor u_rms/B₀.

Seminar in french

Atmospheric characterisation of exoplanets : towards hot neptunes, super-earths, and Earth-like planets

Filippo Pantellini — 2011-09-14T10:26:56Z

More than 600 extrasolar planets have been detected so far and an intense characterisation effort has been undertaken to unveil the atmospheric properties of some of these distant worlds seen in transit accross their stars. A large number of transiting exoplanets are found in extreme irradiation environments, very close to their stars, and the question arise of whether the atmospheres of these planets remain stable or get blown away. Atmospheric evaporation was observed in some hot giant exoplanets or "hot jupiters", but does not significantly alter the fate of these massive objects ( 300 Earth masses). Hot neptunes, on the other hand, are a class of exoplanets with typical masses around 20x Earth. They are the link between hot jupiters and super-earths (1 to10 Earth masses). It is surmised that the latters can be evaporation remnants, with atmospheres completely eroded by the extreme stellar irradiation. In this case, could hot neptunes be the progenitors of the hot rocky planets detected by the Corot and Kepler missions ? Detecting their extended atmospheres and measuring their mass loss rates and atmospheric heating efficiencies are key steps towards the understanding of the atmospheric dynamics and properties of low-mass exoplanets. After an introduction about planetary transits, I will review the results we have obtained with HST on the atmospheric evaporation of transiting exoplanets, on both observational and theoretical sides. I will finally discuss the prospects about atmospheric characterisation for Earth-size planets in more temperate - habitable - environments, and how the upcoming transit of Venus in June 2012 and the PLATO mission proposed to ESA could lead to a new era where atmospheric characterisation is common for hundreds of Earth-like exoplanets.

Seminar in french

Nonlinear dynamics of the stellar-planetary environment: Langmuir waves & turbulence

Filippo Pantellini — 2011-09-09T14:04:08Z

Lecture 3/4 of a series of open lectures on “Nonlinear dynamics of the stellar-planetary environment". Reference : ApJL 330, L77 (1988); PSS 48, 9 (2000); A&A 390, 351 (2002); ASR 46, 472 (2010)

- Saison 2011-2012

Nonlinear dynamics of the stellar-planetary environment: Drift waves & turbulence

Filippo Pantellini — 2011-09-09T13:58:14Z

Lecture 4/4 of a series of open lectures on “Nonlinear dynamics of the stellar-planetary environment". Reference : PRL 91, 034102 (2003); PRL 98, 014101 (2007); PRL 104, 254102 (2010)

- Saison 2011-2012

Nonlinear dynamics of the stellar-planetary environment: Alfven waves & turbulence

Filippo Pantellini — 2011-09-09T13:55:02Z

Lecture 2/4 of a series of open lectures on “Nonlinear dynamics of the stellar-planetary environment". Reference : JGR 111, A07S03 (2006); NPG 14, 17 (2007); AG 27, 1789 (2009)

- Saison 2011-2012