Physical vs. Observational Properties of Clouds in Turbulent Molecular Cloud Models
Abstract
We examine the question of how well the physical properties of clumps in turbulent molecular clouds can be determined by measurements of observed clump structures. To do this, we compare simulated observations of computational models of isothermal, magnetized, supersonic turbulence to the actual physical structure of the models. We begin by determining how changing the parameters of the turbulence changes the observed structure. Stronger driving produces greater density fluctuations, and longer wavelength driving produces larger structures. Magnetic fields have a less pronounced effect on structure, and one that is not monotonic with field strength. Comparing molecules that trace different densities can help determine the size of the density fluctuations and thus the strength of the driving. Velocity superposition of multiple physical clumps can fully obscure the physical properties of those clumps. Shorter wavelength driving worsens this effect. We examine how Larson's relationships and mass spectra can be interpreted in the presence of superposition. We show that the mean density-size relationship is an observational artifact due to limited dynamical range in column density and the inevitable presence of a lower cutoff in column density. The velocity dispersion-size relationship is reproduced in both physical and observed clumps, although with substantial scatter in the derived slope. Finally, we compute the mass spectra for the simulated observations of the models and the models themselves. We show that, when we look for clumps with high enough resolution, they both converge to the same shape, which appears to be log-normal, rather than a power-law function.Keywords
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