Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models

By E. Ostriker, James M. Stone, Charles F. Gammie. 2000

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We use three-dimensional (3D) numerical magnetohydrodynamic simulations to follow the evolution of cold, turbulent, gaseous systems with parameters chosen to represent conditions in giant molecular clouds (GMCs). We present results of three model cloud simulations in which the mean magnetic field strength is varied (B0 = 1.4-14 μG for GMC parameters), but an identical initial turbulent velocity field is introduced. We describe the energy evolution, showing that (1) turbulence decays rapidly, with the turbulent energy reduced by a factor 2 after 0.4-0.8 flow crossing times (~2-4 Myr for GMC parameters), and (2) the magnetically supercritical cloud models gravitationally collapse after time ≈6 Myr, while the magnetically subcritical cloud does not collapse. We compare density, velocity, and magnetic field structure in three sets of model "snapshots" with matched values of the Mach number ℳ ≈ 9,7,5. We show that the distributions of volume density and column density are both approximately log-normal, with mean mass-weighted volume density a factor 3-6 times the unperturbed value, but mean mass-weighted column density only a factor 1.1-1.4 times the unperturbed value. We introduce a spatial binning algorithm to investigate the dependence of kinetic quantities on spatial scale for regions of column density contrast (ROCs) on the plane of the sky. We show that the average velocity dispersion for the distribution of ROCs is only weakly correlated with scale, similar to mean size-line width distributions for clumps within GMCs. We find that ROCs are often superpositions of spatially unconnected regions that cannot easily be separated using velocity information; we argue that the same difficulty may affect observed GMC clumps. We suggest that it may be possible to deduce the mean 3D size-line width relation using the lower envelope of the 2D size-line width distribution. We analyze magnetic field structure and show that in the high-density regime n ≳ 103 cm-3, total magnetic field strengths increase with density with logarithmic slope ~1/3-2/3. We find that mean line-of-sight magnetic field strengths may vary widely across a projected cloud and are not positively correlated with column density. We compute simulated interstellar polarization maps at varying observer orientations and determine that the Chandrasekhar-Fermi formula multiplied by a factor ~0.5 yields a good estimate of the plane-of sky magnetic field strength, provided the dispersion in polarization angles is ≲25°.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/17f8e30a369311ae9b2bf6227fb7a5c7ef32070e

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Citations in the corpus

THE FRACTAL DENSITY STRUCTURE IN SUPERSONIC ISOTHERMAL TURBULENCE: SOLENOIDAL VERSUS COMPRESSIVE ENERGY INJECTION (2007, background): “…(e.g., Porter et al. 1992; Vázquez-Semadeni 1994; Padoan et al. 1997; Passot & Vázquez-Semadeni 1998; Stone et al. 1998; Mac Low 1999; Klessen 2000; Ostriker et al. 2001; Boldyrev et al. 2002; Li et al. 2003; Padoan et al. 2004b; Jappsen et al. 2005; Ballesteros-Paredes et al. 2006; Kritsuk et…”
SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores (2003, background): “Hence, the unsmoothed simulations—particularly the higher resolution (2563) ones of Ostriker et al. (2001)—best represent the data reported here.”
Physical versus Observational Properties of Clouds in Turbulent Molecular Cloud Models (2001, background): “…as the present paper, they addressed them only partially, studying, for example, only objects in physical space (VBR97) or in observational space (Ostriker et al. 2001), or giving physical arguments to explain the observed Larson relationships (see, e.g., Larson 1981; Kegel 1989; Scalo 1990).”
AN IMPRINT OF MOLECULAR CLOUD MAGNETIZATION IN THE MORPHOLOGY OF THE DUST POLARIZED EMISSION (2013, methodology): “…& Pudritz 2000; Nakamura & Li 2008), elongated structures along the magnetic field lines when the medium is strongly compressed supersonically (Ostriker et al. 2001; Heitsch et al. 2001; Falceta-Gonçalves et al. 2008), and linear correlation of the magnetic field strength with column density…”
THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD (2010, background): “Numerical simulations have shown that the probability density function (PDF) of volume densities in molecular clouds can be fitted by a log-normal distribution (e.g. Ostriker et al. 2001; Nordlund & Padoan 1999; Li et al. 2004; Klessen 2000).”
How Stellar Feedback Simultaneously Regulates Star Formation and Drives Outflows (2015, background): “For supersonic isothermal turbulence, the probability distribution functions (PDFs) of both the volume and surface densities are approximately lognormal (e.g. Ostriker et al. 2001; Vázquez-Semadeni & García 2001; Federrath et al. 2010; Konstandin et al. 2012a,b, 2015).”
The density variance–Mach number relation in supersonic turbulence – I. Isothermal, magnetized gas (2012, background): “Soc., 34, 333 Klessen R. S., 2000, ApJ, 535, 869 Klessen R. S., Burkert A., 2000, ApJS, 128, 287 Kritsuk A. G., Norman M. L., Padoan P., Wagner R., 2007, ApJ, 665, 416 Kritsuk A. G., Norman M. L., Wagner R., 2011, ApJ, 727, L20 Krumholz M. R., McKee C. F., 2005, ApJ, 630, 250 Landau L. D., Lifshitz ”
MODELING MOLECULAR HYDROGEN AND STAR FORMATION IN COSMOLOGICAL SIMULATIONS (2008, background): “Numerical simulations of turbulent molecular clouds typically find a lognormal density distribution inside the clouds with the width that depends on the average Mach number of the flows: σ2ln ρ ≈ 1 − 2 (Padoan et al. 1997; Ostriker et al. 2001; McKee & Ostriker 2007).”
The structure of the interstellar medium of star‐forming galaxies (2011, result): “Our predictions are also, interestingly, similar to the predictions in higher resolution simulations of individual GMCs with quite different physics included (e.g. ignoring radiation pressure but including magnetic fields; see Ostriker et al. 2001; Tasker 2011).”
THE STAR FORMATION RATE OF SUPERSONIC MAGNETOHYDRODYNAMIC TURBULENCE (2009, background): “…on the distribution of local shock velocity and preshock density and is known to follow a LogNormal pdf (Vazquez-Semadeni 1994; Padoan et al. 1997; Nordlund & Padoan 1999; Ostriker et al. 2001; Li et al. 2004; Kritsuk et al. 2007; Beetz et al. 2008; Lemaster & Stone 2008; Federrath et al. 2008).”
2MASS wide field extinction maps. I. The Pipe nebula (2006, background): “Recently, this argument has been challenged by Ostriker et al. (2001) by showing a good agreement with a lognormal distribution in the molecular cloud IC5146 (Lada et al. 1999).”
Theory of Star Formation (2007, background): “As a consequence, the width of the shell increases only as the cube root of the distance from the head of the jet (Masson & Chernin, 1993; Ostriker et al., 2001).”
Rapid Formation of Molecular Clouds and Stars in the Solar Neighborhood (2001, methodology): “However, the numerical simulations of PVP, Ostriker et al. (1999, 2001), and Heitsch et al. (2000) show that the geometry indicated in Figure 4 and expressed in equation (18) generally is not relevant for understanding cloud formation when the magnetic field is weak or of intermediate strength with…”