(Abridged) We use the statistical tool known as the ``Spectral Correlation Function" SCF to intercompare simulations and observations of the atomic interstellar medium. The simulations considered ...mimic three distinct sets of physical conditions. One of them (run "ISM") is intended to represent a mixture of cool and warm atomic gas, and includes self-gravity and magnetic fields. For each simulation, H I spectral-line maps are synthesized and intercompared, both with each other, and with observations, using the SCF. We find that, when thermal broadening is large in comparison with fine-scale turbulent velocity structure, it masks sub-thermal velocity sub-structure in the synthesized spectra. The H I observations we use here for comparison are of the North Celestial Pole (NCP) Loop. None of the simulations match the NCP Loop data very well. The most realistic sets of line profiles and SCF statistics comes from artifically rescaling the velocity axis of run ISM. Without rescaling, almost all velocity structure is smeared out by thermal broadening. However, if the velocity axis is expanded by a factor of 6, the SCF distributions of run ISM an the NCP Loop match up fairly well. This means that the ratio of thermal to turbulent pressure in run ISM is much too large as it stands, and that the simulation is deficient in turbulent energy. This is a consequence of run ISM not including the effects of supernovae. We conclude that the SCF is a useful tool for understanding and fine-tuning simulations of interstellar gas, and in particular that realistic simulations of the atomic ISM need to include the effects of energetic stellar winds (e.g. supernovae) in order for the ratio of thermal-to-turbulent pressure to give spectra representative of the observed interstellar medium in our Galaxy.
We discuss molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). Initially, a shocked layer forms, and within it, a thin cold layer. An analytical ...model and high-resolution 1D simulations predict the thermodynamic conditions in the cold layer. After \(\sim 1\) Myr of evolution, the layer has column density \(\sim 2.5 \times 10^{19} \psc\), thickness \(\sim 0.03\) pc, temperature \(\sim 25\) K and pressure \(\sim 6650\) K \(\pcc\). These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the 1D simulations, the inflows into the sheets produce line profiles with a central line of width \(\sim 0.5 \kms\) and broad wings of width \(\sim 1 \kms\). 3D numerical simulations show that the cold layer develops turbulent motions and increases its thickness, until it becomes a fully three-dimensional turbulent cloud. Fully developed turbulence arises on times ranging from \(\sim 7.5\) Myr for inflow Mach number \(\Mr = 2.4\) to \(> 80\) Myr for \(\Mr = 1.03\). These numbers should be considered upper limits. The highest-density turbulent gas (HDG, \(n > 100 \pcc\)) is always overpressured with respect to the mean WNM pressure by factors 1.5--4, even though we do not include self-gravity. The intermediate-density gas (IDG, \(10 < n {\rm cm}^ {-3} < 100\)) has a significant pressure scatter that increases with \(\Mr\), so that at \(\Mr = 2.4\), a significant fraction of the IDG is at a higher pressure than the HDG. Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compressive process that forms the clouds themselves, and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic.
We discuss the virial balance of all members of a cloud ensemble in numerical simulations of self-gravitating MHD turbulence. We first discuss the choice of reference frame for evaluating the terms ...entering the virial theorem (VT), concluding that the balance of each cloud should be measured in its own reference frame. We then report preliminary results suggesting that a) the clouds are far from virial equilibrium, with the ``geometric'' (time derivative) terms dominating the VT. b) The surface terms in the VT are as important as the volume ones, and tend to decrease the action of the latter. c) This implies that gravitational binding should be considered including the surface terms in the overall balance.
In this paper we test the results of a recent analytical study by Lazarian and Pogosyan, on the statistics of emissivity in velocity channel maps, in the case of realistic density and velocity fields ...obtained from numerical simulations of MHD turbulence in the interstellar medium (ISM). To compensate for the lack of well-developed inertial ranges in the simulations due to the limited resolution, we apply a procedure for modifying the spectral slopes of the fields while preserving the spatial structures. We find that the density and velocity are moderately correlated in space and prove that the analytical results by Lazarian and Pogosyan hold in the case when these fields obey the fluid conservation equations. Our results imply that the spectra of velocity and density can be safely recovered from the position-position-velocity (PPV) data cubes available through observations, and confirm that the relative contributions of the velocity and density fluctuations to those of the emissivity depend on the velocity resolution used and on the steepness of the density spectral index. Furthermore, this paper supports previous reports that an interpretation of the features in the PPV data cubes as simple density enhancements (i.e., ``clouds'') can be often erroneous, as we observe that changes in the velocity statistics substantially modify the statistics of emissivity within the velocity data cubes.
abridged We investigate the velocity structure of protostellar cores that result from non-magnetic numerical models of the gravoturbulent fragmentation of molecular cloud material. A large fraction ...of the cores analyzed are ``quiescent'', and more than half are identified as ``coherent''. The fact that dynamically evolving cores in highly supersonic turbulent flows can be quiescent may be understood because cores lie at the stagnation points of convergent turbulent flows, where compression is at a maximum, and relative velocity differences are at a minimum. The coherence may be due to an observational effect related to the length and concentration of the material contributing to the line. The velocity dispersion of the our cores often has its local maximum at small offsets from the column density maximum, suggesting that the core is the dense region behind a shock. Such a configuration is often found in observations of molecular cloud cores, and argues in favor of the gravoturbulent scenario of stellar birth as it is not expected in star-formation models based on magnetic mediation. Cores with collapsed objects tend to be near equipartition between their gravitational and kinetic energies, while cores without collapsed objects tend to be gravitationally unbound, suggesting that gravitational collapse occurs immediately after gravity becomes dominant. Finally, cores in simulations driven at large scales are more frequently quiescent and coherent, and have more realistic ratios of \(M_{\rm vir}/M\), supporting the notion that molecular cloud turbulence is driven at large scales.
We suggest that molecular clouds can be formed on short time scales by compressions from large scale streams in the interstellar medium (ISM). In particular, we argue that the Taurus-Auriga complex, ...with filaments of 10-20 pc \(\times\) 2-5 pc, most have been formed by H I flows in \(\lesssim 3\)Myr, explaining the absence of post-T Tauri stars in the region with ages \(\gtrsim 3\) Myr. Observations in the 21 cm line of the H I `halos' around the Taurus molecular gas show many features (broad asymmetric profiles, velocity shifts of H I relative to \(^{12}\)CO) predicted by our MHD numerical simulations, in which large-scale H I streams collide to produce dense filamentary structures. This rapid evolution is possible because the H I flows producing and disrupting the cloud have much higher velocities (5-10 kms) than present in the molecular gas resulting from the colliding flows. The simulations suggest that such flows can occur from the global ISM turbulence without requiring a single triggering event such as a SN explosion.
We examine the idea that diffuse and giant molecular clouds and their substructure form as density fluctuations induced by large scale interstellar turbulence. We do this by investigating the ...topology of various fields in realistic simulations of the ISM. We find that a) the velocity field is continuous across threshold-defined cloud boundaries; b) such cloud boundaries are rather arbitrary, with no correspondence to any actual physical boundary, such as a density discontinuity; c) abrupt velocity jumps are coincident with the density maxima; d) the volume and surface kinetic terms in the Eulerian Virial Theorem for a cloud ensemble are comparable in general; e) the magnetic field exhibits bends and reversals highly correlated with similar density features. These results suggest that clouds are formed by colliding gas streams. Within this framework, we argue that thermal pressure equilibrium is irrelevant for cloud confinement in a turbulent medium, since inertial motions can still distort or disrupt a cloud. Turbulent pressure confinement appears self-defeating, because turbulence contains large-scale motions which necessarily distort cloud boundaries. Density-weighted velocity histograms show similar FWHMs and similar multi-component structure to those of observational line profiles, though the histogram features do not correspond to isolated "clumps", but rather to extended regions throughout a cloud. We argue that the results presented here may be also applicable to small scales with larger densities (molecular clouds and cores) and suggest that quasi- hydrostatic configurations cannot be produced from turbulent fluctuations unless the thermodynamic behavior of the flow becomes nearly adiabatic at late stages of collapse. We expect this to occur only at protostellar densities.
We study the effects of projection of three-dimensional (3D) data onto the plane of the sky by means of numerical simulations of turbulence in the interstellar medium including the magnetic field, ...parameterized cooling and diffuse and stellar heating, self-gravity and rotation. We compare the physical-space density and velocity distributions with their representation in position-position-velocity (PPV) space (``channel maps''), noting that the latter can be interpreted in two ways: either as maps of the column density's spatial distribution (at a given line-of-sight (LOS) velocity), or as maps of the spatial distribution of a given value of the LOS velocity (weighted by density). This ambivalence appears related to the fact that the spatial and PPV representations of the data give significantly different views. First, the morphology in the channel maps more closely resembles that of the spatial distribution of the LOS velocity component than that of the density field, as measured by pixel-to-pixel correlations between images. Second, the channel maps contain more small-scale structure than 3D slices of the density and velocity fields, a fact evident both in subjective appearance and in the power spectra of the images. This effect may be due to a pseudo-random sampling (along the LOS) of the gas contributing to the structure in a channel map: the positions sampled along the LOS (chosen by their LOS velocity) may vary significantly from one position in the channel map to the next.
