Models for molecular clouds:
by
Doris Folini and
Rolf Walder


Page contentsIntroductionDriven supersonic turbulence in shock bound slabs MHDwaves as structuring agents References 

IntroductionToday, molecular clouds are regarded as a part of a larger, turbulent environment. In this new picture, they are dynamical, supersonically turbulent, transient entities resulting from the collision of large scale flows. Picking up this idea of molecular clouds being related to the collision of large scale flows, we use our work on driven supersonic turbulence in shock bound slabs to learn more about the structuring of molecular clouds.Observations also show the existence of magnetic fields in at least some molecular clouds. Ordered magnetic fields have been observed on scales of about 0.05 pc in prestellar cores and on somewhat larger scales in starforming regions. Coherent velocities in prestellar cores are observed on scales of about 0.01 pc. On larger scales, observed line widths indicate supersonic motions. Taken together, these observations suggest highdensity condensates, threaded by magnetic fields, to be embedded in a supersonically turbulent environment. Under such conditions, the generation of magentic waves is to be expected. In view of this background, we use high resolution numerical simulations to investigate how Alfvénwaves affect a selfgravitating slab when injected at the slab center. For all the simulations we use the AMRCART codes from our AMAZE package. Driven supersonic turbulence in shock bound slabsColliding flows, and the resulting, shock bound interaction zones, are ubiquiteous in astrophysics. We use high resolution numerical simulations to investigate the basic properties of such collision zones. The results are of interest for a wide variety of astrophysical objects. Here we concentrate on some aspects which are probably relevant with regard to molecular clouds. Further results on shock bound slabs we give on a seperate page.The model we useWe consider a 2D, plane parallel situation where two flows, whose momentum fluxes are balanced, collide head on. In all simulations we use Euler equations together with a polytropic equation of state. In the isothermal simulations we solve the adiabatic Euler equations with gamma = 1.000001. The resulting collision zone accumulates matter as time evolves. The temperature of this collision zone is, in essence, constant and equal to the temperature of the colliding flows. Consequently, the zone becomes more and more spatially extended as it accumulates matter.Some of our findings
MHDwaves as structuring agentsThe model we useWe consider a 1D (xdirection), planeparallel, selfgravitating slab which we assume to be symmetric with respect to a central plane at x=0 (yzplane, infinitely extended). In this geometry, all variables are functions of distance x to the central plane and time t only. Velocities and magnetic fields perpendicular to the xdirection are allowed, but gradients can occur only in xdirection. As initial conditions we use the stationary WKB solution by Martin et al. A & A 326, 1997. To describe the time evolution of this slab, we use ideal, isothermal MHD equations, including a source term to account for selfgravity. The use of isothermal equations implies that we assume instantaneous, optically thin radiative cooling. We consider only one sort of particles and consequently have no wave damping due to ionneutral friction in our model. At x=0, the inner boundary of our model problem, we assume that a purely monochromatic, circularly polarized, lefthanded Alfvénwave is generated, traveling in positive xdirection.Some of our findings
ReferencesR. Walder and D. Folini 1999The formation of knots and filaments in shocks Astrophysics and Space Science 260, 215224 (Available as a 761 KB gzipped psfile) R. Walder and D. Folini 2000 On the stability of colliding flows: radiative shocks, thin shells, and supersonic turbulence Astrophysics and Space Science, 274, 343352 (Available as a 0.5 MB gzipped psfile) R. Walder and D. Folini 1999 Radiative Shocks, Supersonic Turbulence and Structure Formation in Space Proceedings of the Seventh International Conference on Hyperbolic Problems: Theory, Numerics, Applications (Available as a 2.7 MB gzipped psfile) 
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Last Update: October 14, 2002 