Calendar

Juin
6
mer
Niels Ligterink: Prebiotic building blocks: From interstellar observations to laboratory synthesis @ Atmosphere
Juin 6 @ 11 h 00 min – 12 h 00 min
Séminaire

Prebiotic molecules show chemical similarities with biologically relevant molecules, such as amino acids, nucleobases, sugars and peptide chains, and are thought to be involved in their formation.  The interstellar presence of prebiotic molecules has led to the idea that biotic molecules on Earth may have derived from these interstellar molecules.

In this seminar, observations of prebiotics towards the young sun-like protobinary IRAS 16293-2422 are presented, showing that prebiotic molecules likely were present at the earliest formational stages of our Solar system. Solid-state formation pathways of these molecules are investigated in the laboratory and show that much of the prebiotic inventory can derive from reactions on icy dust grains.

Juin
20
mer
Jean-Loup Baudino: Are we ready to characterise exoplanet atmospheres with the James Webb Space Telescope observations ? @ Univers 21
Juin 20 @ 11 h 00 min – 12 h 00 min
Séminaire

The James Webb Space Telescope (JWST )will open a
new area in the domain of exoplanet atmosphere
characterizations  and  will  require  accurate  
models  to  interpret the observations.  In this context,
we propose a protocol to compare various atmospheric
codes,  to identify and discuss the significant
differences in the results and to help the codes evolve
to become as consistent as possible.  We applied this
protocol on 3 forward models and one retrieval.  We updated
them to account for the major differences and we are now
able to identify the remaining differences observable with
the JWST.

Juin
27
mer
Vincent Hue — Juno at Jupiter: Juno-UVS overview and observations of the Io footprint @ Univers 21
Juin 27 @ 11 h 00 min – 12 h 00 min
Séminaire
Juno is currently on an elliptical and polar orbit around Jupiter, since July 2016, and has successfully gathered data during 12 perijoves sequences. Juno offers a fantastic opportunity to study Jupiter’s magnetosphere and its spectacular auroral emissions, which can be seen as a window screen of the entire Jovian magnetosphere. The Juno Ultraviolet Spectrograph (UVS) is a UV spectrograph with a bandpass spanning 70 to 205 nm and is designed to characterize Jupiter’s UV emissions. We present a brief overview on Juno-UVS operations. We then present observations of the Io footprints as seen by UVS. The Io footprints are a characterization of the complex magnetodynamic interaction at Io and modulated by the inner magnetosphere of Jupiter. Previous observations taken with the Hubble Space Telescope allowed the characterization of the footprints as a function of Io’s centrifugal latitude, despite observational bias that Earth-based observers are subject to. Juno’s unique vantage point in the Jovian system removes these biases allowing UVS access to the full range of Io’s varying centrifugal latitude and all possible local time geometries. 
Sep
12
mer
Marcelo Tucci Maia — Solar Twins: planet-host star chemical connection and chemical evolution of the Galaxy @ Univers 21
Sep 12 @ 11 h 00 min – 12 h 00 min
Séminaire
Solar twins are stars that have spectra very similar to the Sun, with effective temperature, surface gravity and metallicity around solar values. This similarity allow us to determine very precise stellar parameters and chemical abundances (~0.01 dex), that makes possible the investigation of effects that can imprint subtle changes in the chemical pattern of a star, for example planet engulfments. 
Also, the high precision atmospheric parameters that can be derived for these objects permit us a reliable determination of their ages using a traditional isochrone method which, in association with the abundances determination, can bring many benefits to studies of the chemical evolution of the Galaxy.
I will talk about the differential method, and discuss about past and recent works on the planet-host star chemical connection and the chemical evolution of the Galaxy.
Sep
14
ven
Jack Lissauer: Kepler’s Multiple Planet Systems @ Univers 21
Sep 14 @ 11 h 00 min – 12 h 00 min
Séminaire

More than one-third of the 4000+ planet candidates found by NASA’s Kepler spacecraft are associated with target stars that have more than one planet candidate, and such “multis” account for the vast majority of candidates that have been verified as true planets.The large number of multis tells us that flat multiplanet systems like our Solar System are common. Virtually all of the candidate planetary systems are stable, as tested by numerical integrations that assume a physically motivated mass-radius relationship. Statistical studies performed on these candidate systems reveal a great deal about the architecture of planetary systems, including the typical spacing of orbits and flatness. The characteristics of several of the most interesting confirmed Kepler & K2 multi-planet systems will also be discussed.

Sep
19
mer
Matt Clement : Planet formation in the inner solar system and the early instability model @ Salle 261
Sep 19 @ 11 h 00 min – 12 h 00 min
Séminaire

The leading evolutionary model for the outer solar system, an orbital instability between the solar system’s giant planets, has been shown to greatly disturb the orbits of the young terrestrial planets. Undesirable outcomes such as over-excited orbits, ejections and collisions can be avoided if the instability occurs before the inner planets are fully formed. Such a scenario also has the advantage of limiting the mass and formation time of Mars when it occurs within several million years (Myr) of gas disk dissipation. The dynamical effects of the instability cause many small embryos and planetesimals to scatter away from the forming Mars, and lead to heavy mass depletion in the Asteroid Belt. We present new simulations of this scenario that demonstrate its ability to accurately reproduce the eccentricity, inclination and resonant structures of the Asteroid Belt. Furthermore, we perform simulations using an integration scheme which accounts for the fragmentation of colliding bodies. The final terrestrial systems formed in these simulations provide a better match to the actual planets’ compact mass distribution and dynamically cold orbits. An early instability scenario is thus very successful at simultaneously replicating the dynamical state of both the inner and outer solar system.

Sep
26
mer
Clément Baruteau — Observational predictions of the presence of giant planets in the dust’s radio emission of protoplanetary discs @ Univers 21
Sep 26 @ 11 h 00 min – 12 h 00 min
Séminaire

The classical picture of protoplanetary discs forming smooth, continuous structures of gas and dust has been challenged by the growing number of spatially resolved observations. These observations indicate that radial discontinuities and large-scale asymmetries may be common features of the emission of protoplanetary discs, and they are often interpreted as signatures of the presence of (hidden) planets. They stress the need to better understand how disc-planets interactions generally, and planetary migration more specifically, impact the dust’s thermal emission in protoplanetary discs. In this talk, I will report our recent and ongoing efforts in predicting the dust’s radio emission in protoplanetary discs due to the presence and migration of massive gap-opening planets, via two-fluid (gas+dust) hydrodynamical simulations post-processed with radiative transfer calculations. I will show how these predictions apply to the discs around AB Aurigae and MWC 758.

Oct
3
mer
David Smith — Faint Gamma-ray Pulsars @ Univers 21
Oct 3 @ 11 h 00 min – 12 h 00 min
Séminaire

In its 11th year on orbit, the Fermi LAT continues to discover GeV gamma-rays from
about 24 pulsars per year. The most sensitive way to find them is to use rotation ephemerides
to « phase fold » the gamma photons, to then look whether the resulting
phase histogram is flat or not. We have done this for over a thousand radio pulsars,
using ephemerides provided by the Nancay, Jodrell Bank, and Parkes radio telescopes.
I will present sixteen gamma-ray pulsars we found, and explain how they enhance the sample
of over 220 pulsars we already had. I will discuss possible causes of the gamma-ray
« deathline » near a spindown power of Edot ~ few 1E33 erg/s.
Finally, I will say some words about extrapolating the observed gamma-ray pulsar
population to estimate the contribution of unresolved pulsars to the diffuse background.

Oct
10
mer
Didier Fraix-Burnet : Unsupervised classification: exploring the high dimension @ Univers 21
Oct 10 @ 11 h 00 min – 12 h 00 min
Séminaire

Dealing with large amount of data  is  a new problematic task in astrophysics. One may distinguish the management of these data (astroinformatics) and their scientific use (astrostatistics) even if the border is rather fuzzy. Dimensionality reduction  in both the number of observations and the number of variables (observables) is necessary for an easier physical understanding. This is the purpose of classification which has been traditionally eye-based and essentially still is but this becomes not possible anymore. In this talk, I present a general overview of machine learning approaches for unsupervised classification, with applications to stars (chemical abundances) and galaxies (spectra).

Nov
7
mer
César Gattano — It is shaking in the radio extragalactic celestial reference frame! @ Univers 21
Nov 7 @ 11 h 00 min – 12 h 00 min
Séminaire

During the 90s, the keystones of the celestial reference system took distance since the community left a stellar realisation for an extragalactic realisation. Very long baseline interferometry [VLBI] is used in this purpose because it determines the absolute astrometric positions of thousands of active galactic nuclei with an accuracy of tens of microsecond of arc. The realisation of the extragalactic celestial reference frame by a well-chosen set of sources is at the basis of modern geodesy for wich scientific and societal challenges are regularly highlighted.

The VLBI astrometric accuracy stayed unrivaled for the 40 last years. Only the Gaia space mission competes VLBI nowadays. By skiping technical and technological challenges that allowed this feat from the ground, I will explain that this precision makes us sensitive to perturbations linked to the complex and animated physical structure of the active galactic nuclei. Until now, the adopted strategy for the realisation of a hyper-stable celestial reference frame is to put aside the seemingly most turbulent sources. I will give some elements that let us think this strategy will not be good enough at medium term. The challenges of the future for ever more accurate celestial frame will require the study of those sources and their regular monitoring in collaboration with the astrophysical community in order to understand (i) on which sources can we rely on to realize our celestial reference frame and (ii) given a source, can it be sufficiently stable on a finite time to be useful for the realisation of a celestial reference frame.