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

  • 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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  • Vendredi 10 décembre 2021 à 11h00 (Salle de conférence du bâtiment 17 et Visioconférence)

    Théorie de la diffusion cohérente et incohérente en fréquence

    Véronique Bommier (LESIA)

  • Vendredi 26 novembre 2021 à 11h00 (Salle de réunion du bâtiment 14)

    Turbulence dans le vent solaire : multifractalité et ondelettes

    André Mangeney

  • Jeudi 25 novembre 2021 à 16h00 (Salle de réunion du bâtiment 16 et Visioconférence)

    Probing Unique Regimes of Exoplanet Science with Interferometry at CHARA and VLTI

    Tyler Gardner (University of Michigan)

    Current detection limits struggle to find exoplanets around hot (A/B-type) stars. Searching for planets in this regime provides crucial information on how planet occurrence rate scales with stellar mass. We are carrying out an interferometric survey to discover au-regime giant planets via differential astrometry orbiting individual stars of sub-arcsecond binary systems. The combination of milli-arcsecond resolution with stable wavelength calibration provides precision at the few tens of micro-arcsecond level in short observations at CHARA/MIRCX and VLTI/GRAVITY. This allows us to detect the wobble of a star from orbiting companions down to a few Jupiter masses. I will present the status of our survey, including newly detected tertiary stellar companions. We show that we are beginning to probe down to planetary masses. I am also involved with efforts to measure the orbit-dependent spectra of "hot Jupiter"-type exoplanets with interferometry, which can place useful constraints on atmosphere circulation models. I will present our first promising candidate detection, along with prospects for the future of characterizing close-in exoplanets with this technique.

  • Lundi 22 novembre 2021 à 16h00 (En visioconférence)

    The signature of planet migration in transit-timing variation

    Jean Teyssandier (Université de Namur)

    Planetary systems containing two or more planets sometimes appear to exhibit resonant configurations, where the period ratio between the planets is close to the period ratio of two small integers. These configurations enhance dynamical interactions between the planets, which can be detected by variations in the time of transits of individual planets (the so-called TTVs). In addition, resonant configurations are commonly thought to be the result of convergent migration, a process by which young planets migrate in a gaseous discs at different speeds so that the outer planet "catches up" with the inner one and captures it in a resonance. In this talk, we will demonstrate what is the imprint of disc migration on the TTV signals that are observed today. We will focus on two systems, K2-24 and Trappist-1, and link their current architecture to their past history.

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  • Jeudi 18 novembre 2021 à 14h00 (Amphithéatre Evry Schatzmann batiment 18)

    Transport of chemical elements in stars : Lithium abundance dispersion in metal-poor stars

    Morgan Deal (Instituto de Astrofísica e Ciências do Espaço (IA))

    The formation and evolution of light elements in the Universe act as important cosmological constraints. The oldest stars of the Galaxy have long been assumed to display in their outer layers the primordial lithium abundance, although all studies of stellar physics proved that this abundance must have decreased with time. The primordial Li abundance deduced from the observations of the Cosmological Background is indeed larger than the maximum one observed in these stars. Recent observations gave evidence of a large Li abundance dispersion in very metal poor stars.

    During this presentation, we show that the amount of depletion needed to solve the lithium problem, can be accounted for a large part by the transport processes of chemical elements in stars. We show that stellar models including non-standard transport processes such as the rotation-induced mixing and penetrative convection are able to explain the lithium surface abundances of Population II stars, when using a lithium initial abundance in accordance with the primordial lithium abundance obtained from latest BBN results.

    We also address the general question of the lithium abundance dispersion obtained from observations of carbon enhanced metal-poor stars, and how the interplay of atomic diffusion and accretion of matter modifies the element abundances in these metal-poor stars. In particular, we focus on the hydrodynamic processes that could take place after accretion. We show that the observations of lithium dispersion, associated with carbon enrichment, are well accounted for in terms of accretion onto the metal-poor stars, with accreted masses smaller than a few Jupiter masses, when using a primordial initial lithium abundance.

  • Jeudi 28 octobre 2021 à 16h00

    The Keck Planet Imager and Characterizer capabilities : Imaging Exoplanets and the Galactic Center

    Jacques-Robert Delorme (AO Scientist - Keck)

    The Keck Planet Imager and Characterizer (KPIC) is a purpose-built instrument to demonstrate technological and instrumental concepts initially developed for the exoplanet direct imaging field. Located downstream of the current Keck II adaptive optic (AO) system, KPIC consists of multiple upgrades : new coronagraphs for NIRC2, an infrared Pyramid wavefront sensor and a fiber injection unit (FIU) capable of combining the high-contrast imaging capability of the AOs system with the high dispersion spectroscopy capability of the current Keck high resolution infrared spectrograph (NIRSPEC). Deployed at Keck in September 2018, this instrument has already been used to image and acquire high-resolution spectra (R > 30,000) of multiple targets of interest. In the near term, it will be used to spectrally characterize known directly imaged exoplanets and low-mass brown dwarf companions visible in the northern hemisphere with a spectral resolution high enough to enable spin and planetary radial velocity measurements as well as Doppler imaging of atmospheric weather phenomena. In this presentation, I will present the new capabilities offered by KPIC as well as some of its first science results.

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