Physical and Statistical Properties of Clouds in Numerical Simulations of the Interstellar Medium

Details Physical and Statistical Properties of Clouds in Numerical Simulations of the Interstellar Medium Physical and Statistical Properties of Clouds in Numerical Simulations of the Interstellar Medium

Nov 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........21b&link_type=abstract

PhD Thesis, Instituto de Astronomía, Universidad Nacional Autónoma de México, México, 1999.

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The present work focuses on determining and understanding the physical and statistical properties of interstellar clouds, primarily employing numerical simulations of the turbulent ISM by Vázquez-Semadeni, Passot, & Pouquet (1995a, 1996); Passot, Vázquez-Semadeni y Pouquet (1995), and comparing the properties of the clouds arising in those simulations with observational data. The most important results are: The clouds in the numerical simulations have comparable kinetic, magnetic and gravitational energies (Ballesteros-Paredes, & Vázquez-Semadeni 1995) in spite of not being in equilibrium. Thus, energy equipartition does not necessarily imply virial equilibrium. The clouds in the numerical simulations are not ``virialized'' (i.e., are not in virial equilibrium) (Ballesteros-Paredes & Vázquez-Semadeni 1997), but have in general non-zero values of the second time-derivative of their moment of inertia. In the simulations, the clouds are dynamical entities, changing their shapes end exchanging mass, energy and momentum with their surrounding medium. The clouds in the numerical simulations reproduce the velocity dispersion-size relationship observed in real clouds (Larson 1981), but do not follow the mean density-size relationship. This result supports the suggestion that the former relationship may originate directly as a consequence of the energy spectrum of the turbulence, while the observational density-size relationship may be an artifact of sensitivity limitations of observational surveys (Vázquez-Semadeni, Ballesteros-Paredes, & Rodríguez 1997). The clouds in the numerical simulations also seem to reproduce the mass spectrum of the clouds (Vázquez-Semadeni, Ballesteros-Paredes & Rodríguez 1997). Both super-Alfvénic and sub-Alfvénic velocities are found in the clouds in the numerical simulations (Ballesteros-Paredes, Vázquez-Semadeni, & Scalo 1999), contrary to the standard assumption that motions within molecular clouds are sub-Alfvénic. The clouds in the simulations do not have sharp boundaries. Instead, the density field is continuous and the velocity field does not show shocks at the edges of the clouds (Ballesteros-Paredes, Vázquez-Semadeni, & Scalo 1999). The clouds in the simulations are turbulent density fluctuations, i.e., the density peaks are due to the advection of mass produced by the velocity field (Ballesteros-Paredes, Vázquez-Semadeni, & Scalo 1999). It is also suggested that the apparently sharp boundaries of the interstellar clouds are due to a phase transition between the atomic and molecular regimes, as has been suggested by other authors (e.g., Blitz 1991). It is also predicted that the magnetic field must have reversals within clouds as a consequence of the advection of the magnetic field (Ballesteros-Paredes, Vázquez-Semadeni, & Scalo 1999). For an effectively polytropic gas, gravitational collapse triggered by the external turbulence cannot be stopped unless the effective polytropic index γeff changes during the process. This may not occur until proto-stellar densities are reached (Ballesteros-Paredes, Vázquez-Semadeni, & Scalo 1999). Cloud formation by larger-scale stream collisions may cause simultaneous compression along elongated regions, which, based only on their internal velocity dispersion, might appear to be causally disconnected. This process has been invoked as a possible solution of the so-called Post-T-Tauri problem in Ballesteros-Paredes, Hartmann, & Vázquez-Semadeni (1999). Thesis available through http://www.astroscu.unam.mx/turbulence

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