Abstracts Elbereth 2016
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Lundi 21 Novembre 10h – 10h20 Météores : à la recherche de ce qui n’existe déjà plus Auriane Egal - IMCCE D’où proviennent les pluies de météores que nous avons la chance d’observer tout au long de l’année ? Pourquoi est-ce si difficile de retrouver les corps parents à l’origine de ces phénomènes ? La réponse à cette question repose en majeure partie sur l’imprécision de la détermination de la vitesse des météores dans l’atmosphère. Depuis la simple utilisation de témoignages visuels jusqu’au développement de réseaux de caméras ultra-sophistiquées, tout est mis en oeuvre pour tenter de contraindre davantage ce paramètre indispensable. Et vous, en seriez-vous capable ? 10h20 – 10h40 Kinematics of Class 0 protostellar envelopes from the CALYPSO survey : the L1527 case study Mathilde Gaudel, Anaëlle Maury and CALYPSO collaboration - SAp/AIM One of the main challenges to the formation of Sun-like stars is the “angular momentum problem” : if the angular momentum of the gas contained in a typical star-forming core is totally transfered to the star during the main accretion phase, called Class 0 phase, the star can not be stable. The gas needs to loose most of its angular momentum. At the end of this accretion phase, a large disk ( 100 AU) of dust, where planets will form later, surrounded the protostar. Can the disk formation is the solution to the angular momentum problem ? Studying the kinematics and understanding the distribution of the angular momentum in Class 0 protostellar envelopes (10 - 10 000 AU) is of uttermost importance to find a solution to this problem. In order to tackle this issue, the CALYPSO (Continuum and Lines in Young Protostellar Objects) IRAM large program provides observations with Plateau de Bure Interferometer and the IRAM 30m telescope of the dust continuum and a dozen molecular lines emission from a large sample of 17 Class 0 protostars. We used the C18O(2- 1) and N2H+(1- 0) molecular lines for tracing envelope’s kinematics. We chose L1527, a borderline Class 0/I object located in the Taurus molecular cloud at 140pc, as a pilot source to ensure our methods are reliably reproducing results from the literature, abundant for this source. Our results are consistent with previous studies and for the first time, we have an estimation of the angular momentum for a single source at different scales in the envelope. We are now ready to do a similar analysis for the other 16 Class 0 protostars from the CALYPSO sample. 10h40 – 11h High-redshift major mergers weakly enhance star formation Jérémy Fensch, Renaud F., Bournaud F., Duc P.-A. et al. - SAp AIM Galaxy mergers are believed to trigger strong starbursts. This is well assessed by observations in the local Universe. However the efficiency of this mechanism has poorly been tested so far for high redshift, actively star forming, galaxies. We present a suite of pc-resolution hydrodynamical numerical simulations to compare the star formation process along a merging sequence of high and low z galaxies, by varying the gas mass fraction between the two models. We show that, for the same orbit, high-redshift gas-rich mergers are less efficient than low-redshift ones at producing starbursts : the star formation rate excess induced by the merger and its duration are both around 10 times lower than in the low gas fraction case. The mechanisms that account for the star formation triggering at low redshift are only mildly, if not at all, enhanced for high gas fraction galaxy encounters. Our results are consistent with the observed increase of the number of major mergers with increasing redshift being faster than the respective increase in the number of starburst galaxies. 11h30 – 11h50 Validation de l’étoile laser allongée Lisa Bardou - LESIA En optique adaptative, pour pouvoir corriger la turbulence atmosphérique, on a besoin de s’aider d’une étoile brillante. Cette étoile guide sert à mesurer le front d’onde turbulent, mais cette mesure est valable uniquement près de ladite étoile. Une large portion du ciel reste alors inaccessible aux observations à haute résolution angulaire. Pour palier ce problème, on utilise des lasers qui créent des étoiles guides artificielles où l’on veut dans le ciel. Aujourd’hui ces étoiles laser sont en service sur les plus grands télescopes (Keck, VLT, Gemini South, etc.). L’Europe a pour projet de construire un nouveau télescope géant de 40m de diamètre. Avec un télescope aussi grand, de nouveaux problèmes apparaissent lors de l’utilisation de ces étoiles artificielles, qui sont pourtant nécessaires pour pouvoir exploiter au maximum le pouvoir de résolution exceptionnel de ce futur télescope. Je vais présenter ici quels sont ces problèmes et l’expérience que l’on a mise en place pour déterminer comment y remédier. 11h50 – 12h10 Decimetre-scaled spectrophotometric properties of the nucleus of comet 67P/Churyumov- Gerasimenko from OSIRIS observations Clément Feller, S. Fornasier, P.H. Hasselmann, A. Barucci, F. Preusker et al. - LESIA We present the results of the photometric and spectrophotometric properties of the 67P/Churyumov- Gerasimenko nucleus derived with the OSIRIS instrument during the closest fly-by over the comet, which took place on 14 th February 2015 at a distance of 6 km from the surface. Several images covering the 0°-33° phase angle range were acquired, and the spatial resolution achieved was 11 cm/pxl. The flown-by region is located on the big lobe of the comet, near the borders of the Ash, Apis and Imhotep regions. Our analysis shows that this region features local heterogeneities at the decimetre scale. We observed difference of reflectance up to 40% between bright spots and sombre regions, and spectral slope variations up to 50%. The spectral reddening effect observed globally on the comet surface by Fornasier et al. (2015) is also observed locally on this region, but with a less steep behaviour. We note that numerous metre-sized boulders, which exhibit a smaller opposition effect, also appear spectrally redder than their surroundings. In this region, we found no evidence linking observed bright spots to exposed water-ice-rich material. We fitted our dataset using the Hapke 2008 photometric model. The region overflown is globally as dark as the whole nucleus (geometric albedo of 6.8%) and it has a high porosity value in the uppermost-layers (86%). These results of the photometric analysis at a decimetre scale indicate that the photometric properties of the flown-by region are similar to those previously found for the whole nucleus. 12h10 – 12h30 Three-dimensional turbulence-resolving modeling of the Venusian cloud layer and induced gravity waves. Maxence Lefèvre - LMD The impact of the cloud convective layer of the atmosphere of Venus on the global circulation remains unclear. The recent observations of gravity waves at the top of the cloud by the Venus Express mission provided some answers. These waves are not resolved at the scale of global circulation models (GCM), therefore we developed an unprecedented 3D turbulence-resolving Large-Eddy Simulations (LES) Venusian model using the Weather Research and Forecast terrestrial model. The forcing consists of three different heating rates : two radiative ones for solar and infrared and one associated with the adiabatic cooling/warming of the global circulation. The rates are extracted from the Laboratoire de Météorlogie Dynamique (LMD) Venus GCM using two different cloud models. Therefore we are able to characterize the convection and associated gravity waves in function of latitude and local time. To assess the impact of the global circulation on the convective layer, we ran simulation using heating rates from a 1D radiative-convective model. 12h30 – 12h50 Can we see neutrino flares ? Exploring the source parameter space for detectability. Claire Guépin, Kumiko Kotera - IAP The new generation of powerful instruments are reaching sensitivities and temporal resolutions that will allow a multi-messengers detection of transient phenomena. In this study, we explore the parameter-space of flaring sources (in particular in terms of luminosity, time-variability or emission energy band) that would enable the detection of transient neutrino signatures. We consider neutrinos produced by photo-hadronic interactions on various photon fields in the source. We give robust necessary conditions on the photon flux from the sources to ensure the detection of neutrinos from current and upcoming experiments. 14h – 14h40 Revue 14h40 – 15h Prédire les ondes gravitationnelles de binaires compactes grâce à la théorie post-newtonienne Tanguy Marchand, Blanchet, Faye - IAP Grâce à la récente détection d’ondes gravitationnelles par l’expérience LIGO, une nouvelle fenêtre sur l’univers s’ouvre en astrophysique. Il est maintenant possible d’observer directement l’empreinte sur l’espace-temps de la fusion de deux trous noirs. Cette nouvelle physique offre d’immenses perspectives aussi bien pour etudier nos modèles astrophysiques que pour tester au niveau fondamental la theorie de la relativité générale. Cependant, il est nécessaire de pouvoir prédire avec précision les signaux emis par les sources attendues afin de pouvoir les détecter et les analyser. Dans ce contexte, la théorie post-newtonienne, consistant à résoudre de façon pertubative les équations d’Einstein, joue un rôle crucial et permet, utilisé de pair avec des simulations numériques, de construire des patrons d’ondes précis des signaux émis par la coalescence de deux objets compacts. Après avoir expliqué les fondements du post-newtonien, nous verrons les récents développements et les nouveaux défis de ce domaine. En particulier, nous verrons comment cette théorie prend en compte les non-linéarités de la relativité générale à un ordre très élevé lors de l’émission et de la propagation des ondes gravitationnelles. 15h – 15h20 Searching for high-redshift galaxy clusters Benjamin Clarenc - IAS Les galaxies sont des objets à la frontière entre astrophysique et cosmologie.