The Column Density Variance in Turbulent Interstellar Media: A Fractal Model Approach
Abstract
Fractional Brownian motion structures are used to investigate the dependency of column density variance (σ^{2} _{ln N }) in the turbulent interstellar medium on the variance of threedimensional density (σ^{2} _{ln ρ}) and the powerlaw slope of the density power spectrum. We provide quantitative expressions to infer the threedimensional density variance, which is not directly observable, from the observable column density variance and spectral slope. We also investigate the relationship between the column density variance and sonic Mach number (M _{s}) in the hydrodynamic (HD) regime by assuming the spectral slope and density variance to be functions of sonic Mach number, as obtained from the HD turbulence simulations. They are related by the expression σ^{2} _{ln N } = Aσ_{ln ρ} ^{2} = Aln (1 + b ^{2} M ^{2} _{s}), suggested by Burkhart & Lazarian for the magnetohydrodynamic case. The proportional constant A varies from ≈0.2 to ≈0.4 in the HD regime as the turbulence forcing parameter b increases from 1/3 (purely solenoidal forcing) to 1 (purely compressive forcing). It is also discussed that the parameter A is lowered in the presence of a magnetic field.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2012
 DOI:
 10.1088/20418205/761/2/L17
 arXiv:
 arXiv:1211.2860
 Bibcode:
 2012ApJ...761L..17S
 Keywords:

 ISM: clouds;
 ISM: structure;
 turbulence;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 5 pages, 4 figures, 1 table, accepted for publication in ApJL