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E. Vazquez-Semadeni (Instituto de Astronomia, UNAM), J. Ballesteros-Paredes (Inst. de Astronomia, UNAM, and Harvard-Smithsonian CFA), J. M. Scalo (U. Texas at Austin)
We examine the scenario that diffuse HI and giant molecular clouds and their substructure form as density fluctuations induced by large scale interstellar turbulence. We discuss the topology of the velocity, density and magnetic fields within and at the boundaries of the clouds emerging in high-resolution two-dimensional simulations of the ISM. We find that the velocity field is continuous across cloud boundaries for a hierarchy of clouds of progressively smaller sizes. Abrupt velocity jumps are coincident with the density maxima, indicating that the clouds are formed by colliding gas streams, the magnetic field exhibiting bends and reversals at the collision sites. Both sub- and super-Alfvenic motions are observed within the clouds.
We argue that thermal pressure equilibrium is irrelevant for cloud confinement in a turbulent medium, since inertial motions can still distort or disrupt a cloud, unless it is strongly gravitationally bound. Turbulent pressure confinement appears self-defeating, because turbulence contains large-scale motions which necessarily distort Lagrangian cloud boundaries.
We discuss the compatibility of the present scenario with observational data. We find that density-weighted velocity histograms are consistent with observational line profiles and that estimated lifetimes for the simulated clouds are ~107 yr. Finally, we argue that the scenario presented here may be also applicable to small scales with larger densities (molecular clouds and their substructure), and conjecture that quasi-hydrostatic configurations cannot be produced from turbulent fluctuations unless the thermodynamic behavior of the flow becomes nearly adiabatic.
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The author(s) of this abstract have provided an email address for comments about the abstract: enro@astroscu.unam.mx