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.
Titre: L’astronomie dans les tables de l’an mil : l’exemple du registre capitulaire de Saint-Emilion
Par : Jean-Paul Parisot
Abstract:
Très récemment, dans le cadre d’une exposition aux Archives Départementales, les historiens de l’Université Bordeaux Montaigne ont découvert, dans un manuscrit du 12e siècle (la plus ancienne archive écrite de St-Emilion) une table astronomique reliée (accidentellement ?) au XVIe, sans rapport avec celui-ci. Constituée de 59 lignes et 19 colonnes, elle présente sans aucun doute une signature lunaire. Elle est constitué de lettres de 3 alphabets latins, composés de lettres standards et de lettres ponctuées (avec un point à droite ou à gauche); il s’avère que c’est la seule table médiévale portant ces lettres, ce qui va faciliter son identification.
En réalité, elle complète les calendriers liturgiques chrétiens; apparue en 725 dans les oeuvres de Bède Le Vénérable, elle est accompagnée de dizaines d’autres pages astronomiques (techniques et pédagogiques). Les nombreux manuscrits qui naissent à partir de cette date (appelés calendriers carolingiens) deviennent le support de l’enseignement de Charlemagne, pour disparaître complètement au XIIIe siècle en raison de 2 évènements indépendants : l’arrivée d’une astronomie de très haut niveau transmise par le monde musulman et la création des universités.
En me basant sur quelques manuscrits (dont ceux d’une personnalité locale, Abbon de Fleury), je présenterai l’usage que l’on peut faire de cette table, qui constitue un calendrier lunaire perpétuel d’une précision d’environ 2 jours sur la lune astronomique. En m’écartant du contenu liturgique, je mettrai l’accent sur quelques points inattendus de cet enseignement (les marées, les mouvements planétaires « fantaisistes », la tentative de numération à base 10 et le cryptage des messages)
Titre: The Far-Infrared Spectroscopy Space Telescope (FIRSST) mission proposal
Par : Martina Wiedner
Abstract:
After the Herschel satellite, ISO and a generation of smaller satellites as well as the grounding of the SOFIA airplane observatory the far-IR sky is currently hidden from our view, with the exception of occasional short glimpses by balloon missions. NASA has therefore published a call for a far-IR (or X-ray) mission to be launched around 2032. Five X-ray proposals and three far-IR proposals, PRIMA, SALTUS and FIRSST have been submitted to NASA last November. In the talk I will present the FIRSST mission its science goals, the satellite and its instruments, in particular the heterodyne receiver. FIRSST aims to identify the physical processes that form super-Earth and mini-Neptune sized planets, traces the flow of water from star-forming cores to rocky planets and pierce the dusty shrouds to witness galaxy growth. FIRSST has a 1.8m cryogenically cooled primary mirror and is equipped with two instruments: a US direct detection spectroscopy instrument (DDSI) and a mostly European heterodyne spectroscopy instrument (HSI), which we have designed within an international consortium.
Titre: Thales, activités spatiales & ingénierie scientifique
Par : M. Marseille & L. Mercuri
Abstract:
Après une introduction des activités spatiales de Thales (observations de la Terre et de l’Univers), notamment des multiples mission scientifiques spatiales CNES sur lesquelles nous contribuons, nous présenterons ce qui est réalisé en termes d’ingéniérie logicielle scientifique au sein de notre département « Augmented Data » de Toulouse, les méthodologies employées, et ce que nous qualifions de bonnes pratiques logicielles. L’IA sera aussi abordée, tant sur son développement que son industrialisation. Enfin, nous vous exposerons les perspectives de carrière et d’évolution au sein de notre groupe, sur des profils proches de vos activités scientifiques. Nous aborderons également nos processus de recrutements, en particulier les attendus pour les candidats qui souhaiteraient se tourner vers notre groupe.
Ce séminaire sera suivi d’une discussion ouverte à tous à 14H00.
Titre: Highlights of the SOFIA Legacy Program FEEDBACK and Future Perspectives on Orion A
Par : Slawa Kabanovic (U. Cologne)
Abstract:
During the FEEDBACK legacy survey, 11 high-mass star-forming regions were observed using the [CII] 158 and [OI] 63 micron lines with the upGREAT receiver on SOFIA. We here present the scientific highlights of the program and give a status report on our ongoing work in Orion A.
One significant focus of FEEDBACK was the exploration of galactic HII region bubbles. Expanding [CII] shells were identified in all sources, and potential driving mechanisms were assessed. Our findings indicate that, in the majority of cases (e.g. RCW120, RCW36, NGC7538), the high-velocity expansion is attributed to the stellar wind from massive stars. Recently, we uncovered signs of cloud destruction due to stellar winds and radiation, quantifying this phenomenon using [CII] in the RCW79 region.
Utilizing a multi-layer radiative transfer model for CO and [CII] emission, coupled with HI self-absorption (HISA) studies, we investigated the RCW 120 bubble. Our study detected significant amounts of cold C+ linked to an extensive HI envelope and revealed that the related molecular cloud exhibits a flattened structure likely originating from a residual filament.
In Cygnus X, we unveiled the highly dynamic interaction of CO-dark gas in the form of atomic flows with a small molecular fraction), using [CII], CO emission and HISA. The cloud formation timescale is only ~1 Myr, consistent with recent simulations of fast, colliding HI flows.
As part of the Orion Legacy project (PI S. Kabanovic), multiple long-time integrations across Orion A were performed, successfully detecting weak [13CII] transition lines. These observations enable the first determination of the local [12CII]/[13CII] abundance ratio and possible fractionation effects directly in different physical conditions in Orion A, such as NGC 1977, M43, and M42.
Titre: On the demographics and structure of protoplanetary disks
Par : Gaspard Duchêne (IPAG)
Abstract: Protoplanetary disks represent the birth environment of the mature exoplanets now known by the thousands. Over the past decade, several major instrumentation breakthroughs (such as HST, « extreme » adaptive optics, ALMA and now JWST) have led to a slew of high-resolution observations that shed crucial light on the structure of these disks and on the processes at play in forming these planets. In this talk, I will illustrate how a unique observing point of view, when disks are seen almost exactly edge-on, provides both a much less biased view of the demographics of protoplanetary disks, and a unique opportunity to probe their radial and vertical structure. In particular, I will demonstrate how we have used high-resolution observations to quantify the degree of settling of planet-forming pebbles and smaller dust grains, to directly image photoevaporative winds emanating from the surface of some disks, and to find suggestive evidence for the presence of dust-entraining winds far above the disk surface.
Titre: Data reduction and inverse problem approaches – An efficient (and cheap) way to push the instrumental limits
Par : Anthony Berdeu, LESIA / Observatoire de Paris – PSL
Abstract: Study of astrophysical objects requires always more complex models (black hole surrounding, exoplanet atmosphere, accretion disk, galaxy formation and evolution, …) whose inputs imply always more precise measurements. On the other side, the astronomical instruments start to reach their fundamental limits (negligible sensor readout noise, precise mirror polishing, technologies harder to scale up to extremely large telescopes, …). During my talk, I will discuss how data science applied for optimal data reduction and processing via inverse problem approaches can bridge this gap, by pushing the experimental limits without the need of further instrumental developments. I will introduce different applications of my research: integral field spectroscopy (SPHERE/IFS), blind deconvolution and PSF reconstruction (SPHERE/ZIMPOL) and extreme adaptative optics (GRAVITY+).