Laboratoire d’Études du Rayonnement et de la Matière en Astrophysique et Atmosphères

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Stages Master 1

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Stages Master 2

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Sujet : Modélisation 3D d’un nuage interstellaire (des molécules en phase gazeuse)
Encadrant : Laurent Pagani - laurent.pagani@obspm.fr (...)

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Séminaires à venir

Vendredi 28 juin 2019, 14h00
Salle de l'atelier, Paris
Is accretion-driven turbulence a key process for galaxy growth ?
résumé :
Spitzer and Herschel infrared spectroscopy has revealed a population of nearby galaxies with weak star formation and unusually bright emission lines (e.g. [CII], H2), with very broad linewidths. The line luminosities are greatly in excess of that expected by photoelectric heating of the gas, suggesting that they are powered by the dissipation of turbulent kinetic energy. This discovery of large masses of gas not associated with star formation reveal the potentially important, but largely unexplored, role that turbulence plays in the energetics and formation of multiphase gas on galactic scales. Is this relevant for filamentary gas accretion onto halos of galaxies? I will discuss a toy model in which some of the gravitational potential energy is transferred into gas accretion streams as they penetrate deeper into halos of young galaxies, and part of that energy is dissipated through a turbulent cascade in the warm infalling gas. We have modeled the excitation of the [CII] line as gas is cooling isobarically during its transition from the warm ionized to cold neutral medium. We find that the contribution of [CII] to the total gas cooling rate is increased to 30% and that this [CII] luminosity fraction is largely independent of metallicity. This may explain the recent ALMA detections of [CII] line emission from very high-redshift galaxies, that is not co-spatial with their UV-continuum and have ratios of [CII] to infrared luminosity that are higher than that expected from star formation.
Vendredi 5 juillet 2019, 14h00
Salle de l'atelier, Paris
Distributions of shock waves: probing extra-galactic turbulence
résumé :
Galactic super-winds driven by stars or supermassive black holes are an important feedback mechanism impacting the formation and evolution of galaxies as well as the enrichment of the intergalactic medium. These multiphase winds are observed at velocities (~1000 km/s) that would completely destroy molecules and ionise atoms if their energy dissipated in simple large scale shocks. An emerging picture instead considers a turbulent cascade mediating the transfer of energy from the large scale to the small, dissipating in myriad lower velocity shocks.

In this context I will present my work on low and intermediate velocity (2-50 km/s) molecular shocks. At low velocities in the dense interstellar medium, the rich complexity of magnetohydrodynamics allows for different kinds of shocks at speeds around the Alfven velocity. Counter intuitively, warm J-type shocks re-emerge at very low velocities which may be important for molecule production in turbulent molecular clouds. At higher velocities, shocks are hot enough to produce significant UV radiation that propagates ahead of the shock to generate a radiative precursor. Such a shock requires a careful treatment of the radiative transfer, and a self-consistent iterative method. I will present my implementation of such methods in the Paris-Durham shock code.
Vendredi 20 septembre 2019, 14h00
----------, Paris
Challenging a Newtonian prediction through Gaia wide binaries
UNAM, Mexico
résumé :
Under Newtonian dynamics, the relative motion of the components of a binary star should follow a Keplerian scaling with separation. Once orientation effects and a distribution of ellipticities are accounted for, dynamical evolution can be modelled to include the effects of Galactic tides and stellar mass perturbers. This furnishes a prediction for the relative velocity between the components of a binary and their projected separation. After reviewing recent work evidencing the existence of a critical acceleration scale in Elliptical Galaxies and Globular Clusters, I will show new results showing such a phenomenology in Gaia wide binaries using the latest and most accurate astrometry available. The results are consistent with the Newtonian prediction for projected separations below 7000 AU, but inconsistent with it at larger separations, where accelerations are expected to be lower than the critical a0 value of MONDian gravity. This result challenges Newtonian gravity at low accelerations and shows clearly the appearance of gravitational anomalies of the type usually attributed to dark matter at galactic scales, now at much smaller stellar scales.

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