New Ways for Studying Interstellar Turbulence

By Blakesley Burkhart (Uni. Wisconsin Madison)

Room D18 (2nd floor) at ENS, 24 rue Lhomond, 13:00 to 13:45

We study which statistical tools are promising for obtaining information on turbulence in molecular clouds and diffuse medium. For our tests we make use of three-dimensional 5123 compressible MHD isothermal simulations performed for different sonic and Alfv´enic Mach numbers (Ms and Ma). We introduce the bispectrum, a new tool for statistical studies of the interstellar medium which, unlike an ordinary power spectrum of turbulence, preserves the phase information of the stochastic field. We show that the bispectra of the 3D stochastic density field and of column densities, available from observations, are similar. This opens good prospects for studies of molecular clouds and diffuse media with the new tool. We use the bispectrum technique to define the role of non-linear wave-wave interactions in the turbulent energy cascade. We also obtained the bispectrum function for density and column densities with varying magnetic field strength. As expected, a strong correlation is obtained for wave modes k1 = k2 for all models. Larger values of Ms result in increased correlations for modes with k1 =/= k2. This effect gets more evident with increasing magnetic field intensity. We believe that the different MHD wave modes, e.g. Alfven and magneto-acoustic, which arise in strongly magnetized turbulence, may be responsible for the increased correlations compared to purely hydrodynamical perturbations. In addition to the bispectrum, we calculated the 3rd and 4th statistical moments of density and column density, namely, skewness and kurtosis, respectively. We found a strong dependence of skewness and kurtosis with Ms. In particular, as Ms increases, so does the Gaussian asymmetry of the density distribution. We also studied the correlations of column density with dispersion of velocities and magnetic field, as well as the correlations of density with magentic and kinetic energy and MA for comparison. Our results show that column density correlates with magnetic field for high Ms is proportional to density for low Ms. This trend is independent of the turbulent kinetic energy andcan be used to characterize inhomogeneities of physical properties in low density clumps in the ISM.

Energy cascades in MHD

Par Alexandros Alexakis (ENS)

Salle D18 (2e étage) à l'ENS, 24 rue Lhomond, 13:00 à 13:45

Most astrophysical and planetary systems—e.g., solar and stellar winds, accretion disks, and interstellar medium—are in a turbulent state and coupled to magnetic fields. Understanding and quantifying the statistical properties of magneto-hydro-dynamic (MHD) turbulence is crucial to explain many physical processes in the cosmos. Although the phenomenological theory of hydro-dynamic (HD) turbulence has been verified up to small corrections, a similar statement cannot be made for MHD turbulence. Since the phenomenological description of Hydrodynamic turbulence by Kolmogorov in 1941 there have been many attempts to derive a similar description for turbulence in conducting fluids (i.e Magneto-Hydrodynamic turbulence). However such a description is going to be based inevitably on strong assumptions (that are typically borrowed from hydrodynamics) that do not however necessarily apply to the MHD case. In this talk I will discuss some of the properties and differences of the energy and helicity cascades in turbulent MHD and HD flows. The investigation is going to be based on the analysis of direct numerical simulations. The cascades in MHD turbulence appear to be a more non-local process (in scale space) than in Hydrodynamics. Some implications of these results to turbulent cascade models will be discussed.

A square degree of the galactic plane: a multi-wavelength look at the Vela Molecular Cloud star formation

Par Massimo de Luca (LRA, ENS Paris)

Salle D18 (2e étage) à l'ENS, 24 rue Lhomond, 13:00 à 13:45

In this talk I will review the present day knowledge of the star formation occurring in a molecular cloud located on the Galactic plane, known with the name of Vela Molecular Ridge - Cloud D.

A multi-wavelength study of this region has pointed out a high efficiency in producing both low- and intermediate-mass stars, both in isolation and in clusters. In order to characterize the cloud's young population, I will present answers to some important questions concerning: the correlation between young star distribution and large scale cloud properties, the classification in mass and age of the proto-stars, the individuation of the earliest objects and the description of their mass loss phenomena.

A particular attention will be paid to the description of the methods used to deal with the relatively large database and to root out and connect information spread over wide wavelength and spatial ranges.

Meridional circulation in protoplanetary disks: issues and preliminary results

Par Sebastien Fromang (CEA/DSM/IRFU)

Salle D18 (2e étage) à l'ENS, 24 rue Lhomond, 13:00 à 13:45

Saturation and time dependence of geodynamo models

Par Martin Schrinner

Salle D18 (2e étage) à l'ENS, 24 rue Lhomond, 11:00 à 11:45

Large-scale magnetic fields in planets, stars and galaxies are maintained by hydromagnetic dynamo action. A magnetic field builds up due to an appropriate motion of a conducting fluid and saturates with increasing field strength owing to the back reaction of the Lorentz force on the flow. In this study we focus on the latter and address the question under which conditions a saturated velocity field stemming from geodynamo simulations leads to an exponential growth of the magnetic field in a corresponding kinematic calculation. In order to settle this question, we perform global self-consistent geodynamo simulations and calculate the evolution of a kinematically advanced tracer field. The self-consistent velocity field enters the induction equation for the tracer field in each time step, but the tracer field does not contribute to the Lorentz force. This experiment has been established by Cattaneo and Tobias (2008) and is closely related to the test-field method (Schrinner et al. 2007). We find two dynamo regimes in which the tracer field either grows exponentially or approaches a state aligned with the actual self-consistent field after a transition period. Both regimes can be distinguished by the Rossby number and coincide with the dipolar and multipolar dynamo regimes identified by Christensen and Aubert (2006). Dipolar dynamos with low Rossby number are kinematically stable whereas the tracer field grows exponentially in the multipolar dynamo regime. This difference in the saturation process for dynamos in both regimes comes along with differences in their time variability. Within our sample of 20 models, solely kinematically unstable dynamos show dipole reversals and large excursions. The complicated time behaviour of these dynamos presumably relates to the alternating growth of several competing dynamo modes. On the other hand, an eigenmode computation suggests that dynamos with low Rossby number are dominated by only one fundamental mode, which is repeatedly quenched and rebuilt. All other modes in this case are clearly subcritical. In this sense, dynamo models in the low Rossby number regime, i.e. fast rotators, exhibit a simple time dependence and their saturation merely results in a fluctuation about their critical state.

Extragalactic star formation activity

By Estelle Bayet (UCL)

Room D18 (2nd floor) at ENS, 24 rue Lhomond, 13:00 to 13:45

I aim to better determine the physical and chemical properties of the molecular gas contained in extragalactic star-forming regions, to better infer the galaxy formation and evolution. Indeed, studying the star formation activity in a large range of galaxy types is an indirect way to better describe their stellar content, being one of the main galaxy component regulating its history.

I will first present in my talk the observational and modelling results I have obtained looking at the carbon monoxide and neutral carbon lines in nearby and more distant (z>=6) galaxies. Going deeper inside the extragalactic clouds where massive stars are forming, I will then present the observational clues and the modelling predictions we have performed observing the CS molecule. Then, I will present the comparison I have performed between these two emissions, completing it by the dust radiation, very important too in such star-forming regions. Finally, I will show how this work is particularly interesting under the context of the future observations of Herschel and ALMA.