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Séminaires à venir

Vendredi 1 juin 2018, 14h00
Salle de l'atelier, Paris
Protoplanetary disks at high angular resolution
résumé :
With ALMA and high-contrast optical/IR imaging, protoplanetary disks are revealed to be structured objects. They display rings, spirals, vortices and warps. These structures appear to be extremely well-defined and often have high contrast. This poses the question: what processes cause these conspicuous structures? Are these signs of planet formation? Or do they betray the existence of just-born planets in these disks? In this talk I will discuss these observations and some theoretical models that attempt to explain them. I will show that these structures indicate that dust “pebbles” are being moved around and are trapped in so-called “pressure traps”. I will show that planetary/substellar companions perturb the disk, but that also disk-internal processes can explain some of the ringlike dust traps. I will discuss some ideas to explain the strong warps seen in some of these protoplanetary disks. Finally I will give a preview of the results of an ALMA Large Programme on 20 resolved protoplanetary disks.
Lundi 4 juin 2018, 14h00
Salle de l'atelier, Paris
Jour de la semaine exceptionnel
Inefficient jet-induced star formation in Centaurus A: High resolution ALMA observations of the northern filaments
Quentin SALOME
UNAM, Mexico
résumé :
Star formation is one of the key mechanisms driving the evolution of galaxies across cosmic times. The physical properties and the multi-scale dynamics of the molecular gas influence the star formation efficiency. The environment certainly also plays a role in star formation. In particular, recent studies suggest that AGN can regulate the gas accretion and thus slow down star formation. However, evidence of AGN positive feedback is also invoked in a few radio galaxies.

I will present different studies of the northern filaments of Centaurus A at different resolutions. These filaments extend on scales up to 15 kpc, aligned with the radio-jet, and show evidence of recent star formation (Rejkuba et al. 2001). They are the perfect testbed region for positive feedback, here through jet-induced star formation.

At the intersection of the radio jet and one of the HI shells that surround the galaxy (Schiminovich et al. 1994), CO emission in the shell has been detected with SEST (Charmandaris et al. 2000). With APEX, we mapped the CO emission along the FUV filaments that lie at the jet-HI interaction. In particular, we discovered a large amount of molecular gas outside the HI gas (Salomé et al. 2016). However, while the molecular gas reservoir is important, it is very inefficient to form stars compared to star-forming disc galaxies. To understand why star formation is inefficient, we obtained ALMA observations to map the CO emission along the filaments, at a resolution of ~20 pc (Salomé et al. 2017). Such resolution enabled us to separate giant molecular clouds and study their physical properties (mass, size, velocity dispersion).
Vendredi 8 juin 2018, 14h00
Salle de l'atelier, Paris
Planet-Hunting with ALMA
University of Michgan
résumé :
The Atacama Large Millimeter Array is revolutionizing our understanding of planet formation. Today we now have detected numerous disks that exhibit symmetric gaps in the emission distribution from sub-mm/mm sized dust particles present in the disk midplane. These gaps have traditionally between posited as being carved by the presence of hidden planets. I will review the techniques and pitfalls that are used to infer the presence of planets within these systems which rely on difficult to constrain assumptions regarding scaling factors such as the dust to gas ratio or local molecular abundance. More directly, I will present a new technique that relies on measuring velocity residuals in the emission of CO isotopologues that deviate from Keplerian rotation with 2 m/s accuracy. In the HD 163296 disk, these residuals directly trace gradients in the gas pressure across the gaps, without need for a priori knowledge of absolute scaling factors. 2D and 3D hydrodyamical models with planets embedded deep within the gaps beautifully replicate the detected structure in the velocity residuals. This provides the strongest evidence to date that unseen Jupiter mass planets are present in these Myr-old systems. I will discuss the bright future of this technique and outline methods that might be used to further confirm the presence of young planets. One possibility that I will explore is the use of disk chemistry. Here, I will present a 3D physical/chemical model that includes two point sources: star and accreting protoplanet. With generic assumptions based on planet formation theory and observations, we find that the localized heating of an accreting protoplanet can alter the chemistry in its near vicinity by, for example, releasing volatiles that otherwise would be frozen on grain surfaces. Thus, if planets are accreting gas these effects will be present and are predicted to be detectable. In all, we are on the cusp of new era of bringing submm-wave astronomy into the realm of planet detection.

Vendredi 21 septembre 2018, 14h00
Salle de l'atelier, Paris
Understanding the structure of molecular clouds: Multi-line wide-field imaging of Orion B
résumé :
The new generation of wide-bandwidth high-resolution receivers turns
almost any radio observation into a spectral survey. In the case of
wide-field imaging of the interstellar medium, such a wealth of data
provides new diagnostic tools, but also poses new challenges in terms of
data processing and analysis.

The ORION-B project aims at observing 5 square degrees of the Orion B
molecular cloud, or about half of the cloud's surface, over the entire
3mm band. The emission of tens of molecular tracers have been mapped,
including CO isotopologues, HCO+, CN, HNC, N2H+, methanol, SO, CN...
Machine learning techniques have been applied to these maps, in order to
segment the molecular cloud into typical regions based on their
molecular emission, and to idenfify the most meaningful correlations of
different molecular tracers with each other and with physical quantities
such as density or dust temperature.

The spatial coverage, together with the spatial and spectral resolution,
also allow to characterize statistically the kinematics and dynamics of
the gas. The amount of momentum in the compressive and solenoidal
(rotational) modes of turbulence are retrieved, showing that the cloud
is dominated by solenoidal motions, with the compressive modes being
concentrated in two star-forming regions - which is in line with the
overall very low star formation efficiency of the cloud, and highlights
the role of compressive forcing in the star formation process. The
filamentary network of the molecular cloud also proves to have
particluarly low densities, and is very stable against gravitational
collapse and fragmentation, which also points at a young evolutionary
stage of the filaments.
Vendredi 5 octobre 2018, 14h00
Salle de l'atelier, Paris
Astrochemistry in star forming regions : new modeling approaches
Emeric BRON
résumé :
Star-forming regions present rich infrared and millimeter spectra emitted by the gas exposed to the feedback of young stars. This emission is increasingly used to study the star formation cycle in other galaxies, but results from a complex interplay of physical and chemical processes : chemistry in the gas and on grain surfaces, (de)excitation processes of the atoms and molecules, heating and cooling balance,... Its understanding thus requires detailed astrochemical models that include the couplings between these processes. In this talk, I will present several examples where new modeling approaches of specific processes and their couplings proved crucial to solve persistent observational riddles : from the driving role of UV irradiation in the dynamics of photodissociation regions (PDR) to the efficient reformation of molecular hydrogen in these regions.
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