LESIA - Observatoire de Paris

Studies regarding/using red supergiants : the Great Dimming of Betelgeuse, HR diagram, and the Milky Way

2022-06-27T15:57:46Z

Red supergiants are a class of massive stars soon ending their lives as supernovae. Their properties, such as the mass-loss rates and effective temperatures, are important research targets in the fields of massive-star evolution and subsequent supernovae. In addition, the high luminosity of red supergiants makes them a good probe to investigate the stellar metallicities of the Milky Way and nearby galaxies.

In this talk, I will introduce some of our recent results regarding/using red supergiants : (1) the Great Dimming event of Betelgeuse investigated with the optical/infrared photometry obtained with the Himawari-8 meteorological satellite, (2) accurate determination of the effective temperature of red supergiants for the comparison to the stellar evolution model on the HR diagram, and (3) near-infrared high-resolution spectroscopy of red supergiants in the Milky Way to probe the metallicity distribution in it, especially in the inner Galactic disk.

Hayabusa 2 and Microsatellites

2022-06-21T12:30:20Z

The speaker, Professor Yoshida, has been working on a variety of robotics research topics for the dynamics and control of space robotic systems, ranging from orbital free-flying robots to planetary exploration rovers. The applications are extended to the development of university-based micro-satellites and also the terrestrial applications of space technology, such as robotic remote exploration for disaster response applications. His technical contributions are evidenced by many space flight and robotic missions, such as ETS-VII (orbital experiments of a free-flying space robot,) HAYABUSA / HAYABUSA-2 (Japanese asteroid sample return probes,) RISING / RISING-2 (50kg-class microsatellites for science missions,) and QUINCE (a mobile robot to aid in the Fukushima power plant incident.)

In the seminar talk, a highlight will be made on his contribution to HAYABUSA-2 (Japanese asteroid sample return probes) and micro-satellites development at Space Robotics Lab of Tohoku University, Japan.

Progress on Scalar Vortex Coronagraphs : an alternative for direct imaging

2022-06-17T07:57:12Z

The direct imaging and characterization of Earth-like exoplanets around Sun-like stars to be carried out by future flagship missions such as HabEx and LUVOIR, depends on the ability of coronagraphs to achieve contrasts on the order of 10^-10 at close angular separations and over large bandwidths ( 20%). Vector Vortex Coronagraphs (VVCs) have recently demonstrated 2e-9 raw contrast in broadband light but have several limitations due to their polarization dependent properties. In this seminar I will present progress thus far on exploring the potential for scalar vortex coronagraphs (SVCs) as a viable solution.

The most massive stars

2022-05-30T08:09:46Z

Massive stars may have been the first sources of light after the Big Bang. They are potential contributors to the re-ionization of the Universe and have likely played a crucial role in galaxy formation. The most massive stars today easily outshine the sun by a factor of a million or more, hence provide strong radiative feedback on their host environment. Through powerful stellar winds and supernova ejecta they enrich their surroundings with newly processed chemical elements, which constitute the building block of terrestrial planets and life. The recent detection of gravitational waves revealed surprisingly high black hole masses, pointing to very massive progenitor stars in binary systems.

In this talk I will first sketch the role of massive stars in the grand scheme of things. Then, I will focus on aspects of the outcome of the formation of the most massive stars, including new insights into the formation of massive close binaries and the maximum formation mass, a.k.a., the upper mass limit.

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

2022-05-09T15:38:08Z

- Saison 2021-2022

Controversies in the heart of NGC 1068

2022-04-19T15:50:05Z

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.

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

2022-04-12T12:42:00Z

- Saison 2021-2022

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

2022-04-12T07:51:20Z

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.

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

Agnès Fave — 2022-03-29T14:34:52Z

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.

La grosse comète Bernardinelli-Bernstein

2022-03-07T14:41:01Z

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.