Observatoire de Paris Institut national de recherche scientifique français Univerité Pierre et Marie Curie Université Paris Diderot - Paris 7

  • Vendredi 13 mai 2022 à 11h00 (Salle de réunion du bâtiment 14 et visioconférence)

    Quelques enjeux et travaux récents de modélisation instrumentale et de traitement du signal à l’ère de SKA

    Julien Girard (LESIA)

  • Mardi 26 avril 2022 à 10h30 (Salle de conférence du bâtiment 17)

    M dwarf stars in the era of PLATO : a thorny problem for Stellar Astrophysics… but also a big opportunity

    Santi Cassisi (INAF, Osservatorio Astronomico d’Abruzzo, Italie)

    M dwarfs are becoming extremely important targets in the search of exo-planets with Earth-like characteristics ; a clear proof is represented by the fact that M dwarf stars represent an important class of objects in the input catalogue of the forthcoming PLATO space mission. As a consequence, nowadays there is a strong, renewed intest toward this peculiar class of stars.

    From the point of view of stellar physics, M dwarfs are extremely intriguing objects due to the peculiar thermal conditions experienced by their interiors and outer layers. For long time, an accurate modelling of these structures has been hampered by the lack of reliable predictions about the input physics to be used in the stellar model computations, as well as about how to account for the various physical processes affecting these stars.

    In this talk, we review the Physics of M dwarf stars, and the improvements obtained in these last years, and we also discuss some open issues related to the modelling of these stars.

  • Jeudi 21 avril 2022 à 16h00 (Salle de réunion du bâtiment 16 et visioconférence)

    Controversies in the heart of NGC 1068

    Pierre Vermot (Czech Academy of Sciences)

    NGC 1068 is the archetype of Seyfert 2 galaxies, and more globally, one of the most observed AGN in the sky. Hence, its observation is of crucial importance for the overall understanding of AGN physics.

    Recently, two observations with the VLTI instruments -GRAVITY and MATISSE- allowed to resolve the bright central infrared source at the heart of the AGN with unprecedented details. However, the interpretation of interferometric data is not straightforward, and the results presented in the three papers published so far are inconsistent with each other, especially regarding the nature and geometry of the source.

    I will introduce the topic, present the different interpretations and their inconsistencies, and suggest possible methods to solve this tension.

  • Vendredi 15 avril 2022 à 11h00 (Salle de conférence du bâtiment 17 et visioconférence)

    La R&D instrumentale PERLS (Plateforme d’Evaluation Radio-Logicielle Spatialisable)

    Moustapha Dekkali (LESIA)

  • Jeudi 31 mars 2022 à 16h00 (Salle de réunion du bâtiment 16 et visioconférence)

    Wavefront tolerances of space-based segmented telescopes at very high contrast

    Iva Laginja (LESIA)

    Exquisite wavefront stability is required for the detection and characterization of Earth-like exoplanets. On segmented telescopes in particular, aberrations from co-phasing errors cause a light leakage through the coronagraph that contaminates the high-contrast images.

    The PASTIS tolerancing model is a framework that allows us to characterize the effect of pupil-plane misalignments on the coronagraph performance. With a model inversion, we can analytically deduce wavefront error tolerances to establish an error budget. In this talk, I will present experimental validations of this WFE tolerancing method performed on the segmented high-contrast testbed HiCAT. We determine wavefront error requirements in the 1e-7 contrast regime for a segmented pupil with a classical Lyot coronagraph and derive per-segment tolerances that correctly yield the targeted contrast levels.

  • Lundi 14 mars 2022 à 15h00 (Salle de conférence du bâtiment 17)

    La grosse comète Bernardinelli-Bernstein

    E. Lellouch, R. Moreno, D. Bockelée-Morvan, N. Biver, P. Santos-Sanz

    Annoncée en juin 2021, la comète C/2014 UN271 (Bernardinelli-Bernstein) est remarquable à plusieurs égards. Découverte à 29 au, avec des images de pré-découverte jusqu’à 34 au, elle se dirige vers un périhélie à 11 au en 2031, et n’a pour l’instant sans doute jamais été à moins de 17 au du Soleil.

    Elle présente de l’activité depuis au moins rh = 24 au. Sa magnitude H pré-activité est d’environ H 8, ce qui suggère une taille exceptionnelle.

    En utilisant ALMA nous avons mesuré son flux thermique, déterminant ainsi sa taille (D 137 km, i.e. presque 2 x plus grande qu’Hale-Bopp), et son albédo (5.3 % dans le rouge), typique des comètes.

  • Jeudi 24 février 2022 à 16h00 (En visioconférence)

    Real-time data acquisition and processing on GPU with applications in radioastronomy and radar systems

    Julien Plante (LESIA)

    Data throughput in modern telescopes has been quickly increasing over the last decade, with bandwidths as high as tens of terabits per second expected with the Square Kilometer Array. A similar trend can be observed with radar systems.

    Moreover, a frequent requirement in these systems is to process the stream of raw data from sensors, sent using transport protocols based on e.g. UDP, in real-time. To be able to process information at high throughput and sometimes low latency, accelerators like GPUs or FPGAs are popular solutions, but are still complex to use.

    During my PhD, I am investigating the use of commercial off-the-shelf network interfaces to perform the data ingestion in a more standard and easy way, with applications on telescopes and on radar systems. I will present my progress with the DPDK library and the GPUDirect technology provided by Nvidia, and how this work is being integrated with the COSMIC framework developed in HRAA. This could provide easier data acquisition for any application in the future.

  • Jeudi 27 janvier 2022 à 15h30 (En visioconférence)

    Gravito-turbulence in accretion disks : instabilities, spirals and locality

    William Béthune (University of Tübingen, Germany)

    Spiral arms have been observed in a number of circumstellar disks. One plausible origin for these structures lies in the self-gravitating nature of the disk, when it is still massive relative to its central star. In this case, they may provide valuable constraints on the conditions of disk dispersal and planet formation, such as the disk mass, accretion rate, etc. However, they are often described as linear waves in razor-thin disks, and their role on the evolution of the disk is commonly reduced to an effective turbulent viscosity. I will look back at these approximations and examine their validity within global simulations of self-gravitating disks. I will discuss the deceptive nature of large-scale spiral arms, and the simple connections that can be made between their observable morphology and the physical conditions in the disk.

  • Jeudi 16 décembre 2021 à 16h00 (Salle de réunion du bâtiment 16 et Visioconférence)

    Disks around evolved binaries : do they form second-generation planets ?

    Jacques Kluska (KU Leuven)

    Most of the planets were formed around young stars. But can they also form around dying stars ? The origin of the diversity and comple : ity of the detected exoplanetary systems stems from how they form in protoplanetary disks. These disks are intensively studied around young stars thanks to the high-angular resolution provided by the VLT or ALMA. However, similar disks are also found around evolved stars, namely post-AGB binaries, raising the exciting but yet unanswered possibility of second-generation planet formation around evolved stars.

    We have now the possibility to probe this possibility using high angular resolution instruments. In this talk, I will show the latest results of an extensive observing campaign of these disks using infrared interferometry at the VLTI (PIONIER, GRAVITY, MATISSE) and an innovative image reconstruction technique to decipher the complex interferometric signal. These observations show that these disks share many similarities with protoplanetary disks around young stars.

    Whether or not planet formation is possible at the end of stellar evolution, these disks enable us to test planet formation processes in a parameter space that is unmet around young stars (e.g., short disk lifetime, high stellar luminosity) and further constrain planet formation processes.

  • Lundi 13 décembre 2021 à 16h00 (En visioconférence)

    Predicted signatures of planet formation and orbital evolution in the emission of protoplanetary discs

    Clément Baruteau (IRAP, Toulouse)

    The classical picture of protoplanetary discs being smooth, continuous structures of gas and dust has been challenged by the growing number of spatially resolved observations. These observations tell us that rings, gaps, large-scale asymmetries like spirals or arcs are common features of the emission of protoplanetary discs, which are often interpreted as signatures of the presence of unseen planets. During this online seminar, I will present some of our recent work on how the formation and orbital evolution of planets can impact the dynamics and the emission of gas and dust in protoplanetary discs. In particular, I will discuss the possibility that a single giant planet may acquire a substantial eccentricity in its protoplanetary disc, and what observational signatures this mechanism predicts.

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