Turbulent Formation of Interstellar Structures and the Connection Between Simulations and Observations

I review recent results derived from numerical simulations of the turbulent interstellar medium (ISM), in particular concerning the nature and formation of turbulent clouds, methods for comparing the structure in simulations and observations, and the effects of projection of three-dimensional structures onto two dimensions. Clouds formed as turbulent density fluctuations are probably not confined by thermal pressure, but rather their morphology may be determined by the large-scale velocity field. Also, they may have shorter lifetimes than normally believed, as the large-scale turbulent modes have larger associated velocities than the clouds' internal velocity dispersions. Structural characterization algorithms have started to distinguish the best fitting simulations to a particular observation, and have opened several new questions, such as the nature of the observed line width-size relation and of the relation between the structures seen in channel maps and the true spatial distribution of the density and velocity fields. The velocity field apparently dominates the morphology seen in intensity channel maps, at least in cases when the density field exhibits power spectra steep enough. Furthermore, the selection of scattered fluid parcels along the line of sight (LOS) by their LOS-velocity inherent to the construction of spectroscopic data may introduce spurious small-scale structure in high spectral resolution channel maps.Comment: 15 pages, no figures. To appear in the Proceedings of "The Chaotic Universe", Roma/Pescara, Italy, 1-5 Feb. 1999, eds. V. Gurzadyan and L. Bertone. Uses included .cls fil

Dependence of the Star Formation Efficiency on the Parameters of Molecular Cloud Formation Simulations

We investigate the response of the star formation efficiency (SFE) to the main parameters of simulations of molecular cloud formation by the collision of warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and magnetic fields. The parameters we vary are the Mach number of the inflow velocity of the streams, Msinf, the rms Mach number of the initial background turbulence in the WNM, and the total mass contained in the colliding gas streams, Minf. Because the SFE is a function of time, we define two estimators for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of star formation in each simulation (sferel). The latter is close to the "star formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both estimators decrease with increasing Minf, although by no more than a factor of 2 as Msinf increases from 1.25 to 3.5. Increasing levels of background turbulence similarly reduce the SFE, because the turbulence disrupts the coherence of the colliding streams, fragmenting the cloud, and producing small-scale clumps scattered through the numerical box, which have low SFEs. Finally, the SFE is very sensitive to the mass of the inflows, with sferel decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding streams increases from ~0.15 to ~1.5. This trend is in partial agreement with the prediction by Krumholz & McKee (2005), since the latter lies within the same range as the observed efficiencies, but with a significantly shallower slope. We conclude that the observed variability of the SFE is a highly sensitive function of the parameters of the cloud formation process, and may be the cause of significant scatter in observational determinations.Comment: 19 pages, submitted to MNRA