LERMA UMR8112

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



Accueil > en > Research > Interstellar medium and Plasmas > Turbulence & magnetic field

Turbulence & magnetic field

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The interstellar medium is the main ingredient, actor, and tracer of the formation of new stars and planetary systems in galaxies. One of the most important query in the field is to understand how the dynamical, magnetic, chemical, and thermal properties of the medium, which are tightly coupled to each other, drive its collapse from its most diffuse states down to environments with stellar densities ; and how they control the chemical evolution of the matter, from the most simple molecules to the production of complex organic and prebiotic species.

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How can we explain the presence of molecules in diffuse irradiated environments, where UV photons are suppose to efficiently break molecular bounds ? What combination of microphysical and macroscopic processes is responsible for the formation of complex structures, from the galactic scales down to the mean free path of atoms and molecules ? What is the nature of turbulence in the interstellar medium ? What are its main energy sources and how is it dissipated ? At last, what impact does the magnetic field have on the formation of density structures and on the dynamics of chemical compounds ? Such are the questions which interest our team.

The understanding of all these processes raise, however, several modeling difficulties. Following the time-dependent evolution of hundreds of species in a three dimensional space cannot be achieved numerically, even with the most advanced computer cluster, because the scales involved range over several orders of magnitude. Our team thus applies two complementary approaches : the development of numerical simulations which describe the dynamical evolution of interstellar matter in three dimensions ; and the conception of modeling tools which treat hundreds of physical processes and their coupling in systems at lower dimensions. Our work also includes the reduction and the analysis of observations obtained in the framework of international collaboration with (most recently) the Herschel, Planck, SOFIA, and ALMA telescopes.


Find below a few of our most recent results.


Filaments of matter and magnetic field

Recent observations with the Herschel space telescope have revealed the presence of filaments of matter in molecular clouds. In addition, studies of the polarization of the dust thermal emission has given access to the orientation of the magnetic field in interstellar environments. All these observations show that the magnetic field is aligned with the most diffuse filaments and orthogonal to those with the largest density, a trend in accordance with the results of many numerical simulations. Those links between density structures and the magnetic field open new perspectives regarding the formation of interstellar filaments and their evolution towards gravitationally-unstable entities which potentially lead to the birth of new stars.

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Dissipative scales of turbulence

The dissipation of turbulent energy play a decisive role in the chemical enrichment of the diffuse interstellar gas. In order to assess the dynamical and statistical properties of the dissipation, we have developed and run MHD simulations of decaying turbulence with viscous, ohmic, and ambipolar diffusion. This work shows that 60% of the dissipation occur in 10% of the entire volume and that the viscous diffusion is dominated by incompressible motions. The dissipation takes place in coherent structures characterized by their fractal dimension, that couple different scales of the flow and which have important effects on many observable quantities (e.g. velocity increments).

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Modeling the coupling between dynamics and chemistry

The TDR model developed by our team is a state-of-the-art numerical code designed to treat the chemical evolution of the dissipative structures of interstellar turbulence. The code was used to interpret the observations of many molecules recently observed in the diffuse medium. The comparison with the model predictions shows that turbulent dissipation is sufficient to explain the chemical richness of the gas, in particular the large abundances of CO, HCO+, CH+, and SH+. In addition, simultaneous analysis of couple of species lead to fundamental properties of the dissipation including the energy transfer rate, the timescale of dissipation, and the degree of fragmentation of interstellar matter.

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Observation of the diffuse galactic matter

In the framework of the international key program PRISMAS of the Herschel space telescope, we have performed an exhaustive spectroscopic analysis of the diffuse interstellar gas seen in absorption. Our most recent work was dedicated to the fine structure lines of ionized carbon and nitrogen. The detection of the [NII] line confirms the large filling factor of the warm ionized medium (WIM) and indicates that the WIM contributes to about 5% of [CII] absorption. The study of absorption lines of HF and p-H2O corroborates the idea that these species can be used as tracers of H2. At last, the analysis of OH+, H2O+, and H3O+ confirms previous data which show that the cosmic ray ionization rate is far larger in the diffuse gas than in the dense structures.


Recent or significant publications

Gerin, M. ;, Ruaud, M. ; Goicoechea, J. ; et al., 2015, A&A, 573, A30
Indriolo, N. ; Neufeld, D.A. ; Gerin, M. ; et al. 2015, ApJ, 800, 40
Montier, L. ; Plaszczynski, S. ; Levrier, F. ; et al., 2015, A&A, 574, 135
Montier, L. ; Plaszczynski, S. ; Levrier, F. ; et al., 2015, A&A, 574, 136
Planck intermediate results. XIX. Planck Collaboration, A&A, 2015, 576, 104
Planck intermediate results. XX. Planck Collaboration, A&A, 576, 2015,105
Planck intermediate results. XXXV. Planck Collaboration, A&A in press
Neufeld, D. A. ; Black, J. ; Gerin, M. ; et al., 2015 ApJ 807, 54
Neufeld, D.A. ; Godard, B. ; Gerin, M. ; et al., 2015, A&A 577, A49
Godard, B. ; Falgarone, E. ; Pineau des Forêts, G., 2014, A&A, 570, A27
Hennebelle, P. ; Falgarone, E., 2012, ARAA, 20, 55
Lesaffre, P. ; Pineau des Forêts, G. ; Godard, B. ; et al., 2013, A&A, 550, 106
Levrier, F. ; Le Petit, F. ; Hennebelle, P. ; et al., 2012, A&A, 544, 22
Momferratos, G. ; Lesaffre, P. ; Falgarone, E. ; et al, 2014, MNRAS, 443, 86
Persson, C. ; Gerin, M. ; Mookerjea, B. ; et al., 2014, A&A 568, A37
Zaroubi, S. ; Jelić, V. ; de Bruyn, A. G. ; et al., 2015, MNRAS, 454, L46


Team members

Berthet Manuel

Falgarone Edith

Gerin Maryvonne

Godard Benjamin

Gusdorf Antoine

Lesaffre Pierre

Levrier François

Ngoc Le Tram

Pérault Michel

Orkisz Jan

Rabasse Jean-François

Séminaires à venir

Vendredi 28 juin 2019, 14h00
Salle de l'atelier, Paris
Is accretion-driven turbulence a key process for galaxy growth ?
Pierre GUILLARD
IAP
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
Andrew LEHMANN
ENS
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
Xavier HERNANDEZ
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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