Titre: The polar regions of Jupiter
Par : Vincent Hue (LAM)
Abstract: The polar atmosphere of Jupiter is a complex region were (photo-)chemistry, dynamics and magnetospheric-coupling are intertwined. Several decades of ground-based observations and spacecraft measurements (Cassini, Juno) are progressively revealing how rich and complex these regions are. Species such as HCN, CO, H2O were brought during the Shoemaker-Levy 9 impact in 1994 and provide important dynamical tracers, as they have spread across Jupiter’s atmosphere over the last ~30 years. Hydrocarbons originate from methane-photolysis and are affected by auroral precipitations, leading to the formation of aerosols across the polar cap. Magnetosphere-ionosphere coupling generates powerful electrojets that drag the neutral stratosphere underneath. I will review our current understanding of these regions, based on observations from Cassini, Juno, ALMA, Gemini, and IRTF.
Titre: Streaming instabilities in modern protoplanetary disks
Par : Min-Kai Lin (ASIAA, Taiwan)
Abstract: The formation of km-sized planetesimals is a key step in providing the building blocks of planets. The streaming instability (SI) that arises from the mutual interaction between pebbles and gas is one of the leading mechanisms for planetesimal formation. Under appropriate conditions, the SI can drive the rapid growth of pebble enhancements to the point of gravitational collapse. However, the efficiency of the SI under more realistic conditions expected in protoplanetary disks has not been tested. To this end, I present extended models of the classic SI, including turbulence, stratification, magnetic fields, and non-isothermodynamics. I describe new challenges and pathways for planetesimal formation via the SI when these effects are considered.
Titre: Présentation de la résidence artistique de Benjamin Ducroq
Par : Benjamin Ducroq
Abstract:
Nous accueillons cet automne Benjamin Ducroq, metteur en scène, comédien et musicien en résidence artistique au LAB dans le cadre de la préparation d’un spectacle dont la création est prévue en novembre 2024.
Il nous présentera son projet puis nous aurons un temps d’échange où vous pourrez prendre contact avec lui si vous souhaitez participer à la résidence.
Origine(s)
Théâtre – musique – conférence immersive
Pour cette nouvelle création, la compagnie propose un voyage… un voyage à travers le temps, à travers l’infini sidéral, à travers le monde des idées, et ce pays fascinant et méconnu : les mathématiques. Elles sont fondamentales en sciences, et régissent notre monde. Elles sont le point de départ de notre travail, nous permettant d’ouvrir le champ d’étude aux lois de la physique et au cosmos.
Intitulé Origine(s), ce spectacle porté par le metteur en scène, comédien et musicien Benjamin Ducroq, prendra la forme d’un concert, d’une conférence et d’un dialogue avec les spectateurs sous un planétarium.
Pour réaliser ce spectacle qui verra le jour en novembre 2024, Benjamin Ducroq choisi de l’écrire avec les publics, et au plus près du terrain. Afin de s’inspirer, et de partager une réelle aventure artistique et scientifique, s’organiseront des résidences en laboratoire de recherche mais aussi en milieu scolaire.
Titre: Constraining the X-ray reflection in low accretion rate AGN using XMM-Newton, NuSTAR and Swift
Par : Yaherlyn Diaz
Abstract:
An interesting feature in AGN accreting at low rate is the weakness of the reflection features in their X-ray spectra, which can result from the gradual disappearance of the torus with decreasing accretion rates. It has been suggested that low luminosity AGN (LLAGN) would have a different reflector configuration compared with high luminosity AGN. Our purpose is to constrain the geometry and column density of the reflector in a sample of LLAGN covering a broad X-ray range of energy combining data from XMM-Newton + NuSTAR+Swift. We use XMM-Newton + NuSTAR + Swift observations of a hard X-ray-flux limited sample of 17 LLAGN from BASS/DR2 with accretion rates λ_Edd<10^−3. We fit all spectra using a reflection model for torus and accretion disk reflectors. We found a tentative correlation between the torus column density and the accretion rate, LLAGN shows a lower column density compared with the high-luminosity objects. We also confirm the relation between Γ and λ_Edd, with a smaller scatter than previously reported, thanks to the inclusion of high-energy data and the reflection models. Our results are consistent with a break at λ_Edd ~10^−3, suggestive of a different accretion mechanism compared with higher accretion AGN.
Titre: The structure and history of the Milky Way disc, as told by its star clusters
Par : Tristan Cantat (MPIA)
Abstract:
The Gaia mission has had a huge impact on our ability to detect, classify, and characterise stellar aggregates in the Milky Way. It has greatly improved the census of star clusters of all ages, from the Solar neighbourhood to the edge of the Galaxy. Gaia has revealed the complex spatial structure of young associations, and shown that their overall distribution only weakly correlates with the expected spiral pattern of the Milky Way. At the other end of the timeline, we are now able to visualise the process of cluster disruption, and to map the spatial distribution of clusters up to several Gyr with unprecedented precision, unraveling the evolution of the Milky. Despite the recent advances, we still do not know what are the main parameters that allow some clusters to survive to old ages, or even if the Milky Way is still forming gravitationally bound clusters. The upcoming ground-based campaigns WEAVE and 4MOST as well as future space-based astrometric missions such as JASMINE and GaiaNIR hold the keys to better understanding the cluster population in the Milky Way.
Titre: Le transport des éléments dans les naines blanches tel que révélé par Gaia
Par : Antoine Bédard (University of Warwick, UK)
Abstract: Les naines blanches représentent le stade final de l’évolution de plus de 95% des étoiles. Ces cadavres stellaires sont pratiquement dépourvus de source d’énergie interne et sont donc condamnés à se refroidir continuellement avec le temps. Grâce à cette propriété, les naines blanches agissent comme des « horloges cosmiques » et renferment donc une mine d’informations sur le passé de notre Galaxie. Dans les dernières années, les données du satellite Gaia ont décuplé le nombre de naines blanches connues et fourni un portrait exceptionnellement détaillé de la population locale. En particulier, des structures inattendues ont été découvertes dans le diagramme HR des naines blanches, révélant ainsi des lacunes importantes dans notre compréhension de ces objets. Dans ce séminaire, je présenterai les plus récents efforts de modélisation visant à combler ces lacunes, et je montrerai que le diagramme HR Gaia peut être élégamment expliqué par le transport des éléments à l’intérieur des naines blanches. D’une part, la bifurcation de la séquence des naines blanches en deux branches principales peut être attribuée au dragage convectif (« convective dredge-up » en anglais) du carbone dans l’enveloppe stellaire. D’autre part, l’accumulation de naines blanches massives à un endroit bien précis du diagramme HR est le résultat d’un processus de distillation déclenché par la cristallisation du noyau. La distillation occasionne un transport très efficace des isotopes lourds (notamment le néon-22) vers le centre de l’étoile, ce qui a pour effet de libérer une grande quantité d’énergie gravitationnelle et ainsi d’interrompre le refroidissement pour plusieurs milliards d’années. La découverte de ce phénomène, jamais observé auparavant dans aucun autre type d’étoiles, défie notre conception même des naines blanches comme cadavres stellaires.
Titre: Rotation and dynamics of main-sequence solar-type stars in the PLATO era
Par : Sylvain Breton (INAF – Osservatorio Astrofisico di Catania)
Abstract:
Setting constraints on the rotation profile, both radial and latitudinal, of main-sequence solar-type stars, from the surface to the core, is a fundamental problem if we want to improve our understanding of stellar evolution (especially in order to get better estimates of stellar ages) and of the interactions of stars with their environment, especially exoplanets. As of today, the core rotation profile of the Sun and other main-sequence solar-type stars remains an enigma. This is due to the absence of detection of gravity modes (g modes) driven by buoyancy, which are evanescent in convective regions, and therefore have very low-amplitude at the surface of main-sequence solar-type stars. Nevertheless, large-scale photometric surveys of these past years have been able to provide us valuable insights on the rotational dynamics of the surface and the upper regions of the stars, thanks to acoustic mode (p modes) asteroseismology and/or the characterisation of photospheric active regions. The upcoming PLATO mission will bring new observational perspectives to constrain these different aspects.
In this seminar, I will first present the possibilities offered by space-based photometry in order to measure surface rotation in large samples of solar-type stars and constrain the dynamics of their convective envelope. In particular, I will describe the machine learning methodology ROOSTER that I developed and the way it will be implemented in the PLATO standard pipeline to monitor stellar rotation and activity. After having presented which yield we can expect from PLATO in terms of stellar rotation and activity characterisation, I will explain how the physically-motivated modelling of stellar active regions (spot modelling) may be connected to low-frequency magneto-inertial waves propagating in the convective envelope. In order to probe stellar internal dynamics and related deep transport processes, I will then discuss the perspective of detecting g modes excited by convective motions in late F-type stars, which are solar-type pulsators with a shallow convective envelope. To this purpose, I used the ASH code to perform several deep-shell 3D hydrodynamic simulations of a 1.3 Msun late F-type star. When describing these simulations, I will highlight the influence of rotation on the increased excitation of the g modes. I will finally present possible evidence of g-mode signature detected in the periodogram of late F-type stars observed by the Kepler satellite, which could allow us to calibrate stellar evolution models in order to reach the accuracy levels required by the PLATO mission.
Titre: Defining Initial Conditions of High-Mass Star Formation from the ASHES Project
Par : Kaho Morii (University of Tokyo/NAOJ)
Abstract: Physical properties in infrared dark clouds (IRDCs) provide insights into the initial conditions for the formation of high-mass stars and clusters. We have performed the ALMA Survey of 70 µm Dark High-mass Clumps in Early Stages (ASHES) on thirty-nine high-mass clumps, the dense parts of IRDCs. These targets are dark at 24 µm and 70 µm and have low temperatures, high masses, and high densities, making them the best candidates to study the earliest stages of high-mass star formation. Our ALMA observations reveal their internal structure and have identified an unprecedented number of 839 cores from dust continuum emission. We find that less than 1% (7/839) of the cores have masses of more than 27 Msun. All of these cores are gravitationally bound and associated with molecular outflows. No high-mass prestellar cores were observed. Furthermore, 90% (35 out of 39) of our sample host only low- to intermediate-mass cores, implying that additional mass input is required for high-mass star formation. The observed core properties generally follow the thermal Jeans fragmentation of the clumps. In this talk, I will present the first results of the statistical study of the 839 cores, as well as a summary of the pilot survey, and discuss the properties of the early stages of high-mass star formation.
Titre: The nuclear stellar disc of the Milky Way
Par : Rainer Schoedel (Instituto de Astrofisica de Andaluzia)
Abstract: The Galactic Centre region is dominated by the nuclear stellar disc (NSD), a rotating structure with a radius of ~200 pc that is kinematically and chemically different from the surrounding bulge. The NSD overlaps with the central molecular zone (CMZ), where ~10% of the Galaxy’s molecular gas is concentrated. The extreme conditions in the NSD and CMZ closely match those in high-redshift star forming regions. I will summarise our current knowledge about the NSD, obtained mostly through high angular resolution near-infrared studies, and how the NSD relates to the larger structure of the Milky Way. I will describe how my group are currently working on obtaining proper motions for millions of stars in the NSD, a region inaccessible to Gaia, and how we use the new data to better understand structure, formation history and recent star formation in the NSD.