Enrique Vazquez-Semadeni

Molecular Cloud Evolution. II. From Cloud Formation to the Early Stages of Star Formation in Decaying Conditions

We study the formation of giant dense cloud complexes and of stars within them using SPH numerical simulations of the collision of gas streams (‘‘inflows’’) in the WNM at moderately supersonic velocities. The collisions cause compression,cooling,andturbulencegenerationinthegas,formingacloudthatthenbecomesself-gravitatingandbeginsto collapse globally. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulationsshowthat(1)Thecloudsarenotinastateofequilibrium.Instead,theyundergosecularevolution.Duringits early stages, the cloud’s mass and gravitational energy jEgj increase steadily, while the turbulent energy Ek reaches a plateau.(2)When jEgjbecomescomparabletoEk,globalcollapsebegins,causingasimultaneousincreasein jEgjandEk that maintains a near-equipartition condition jEg j� 2Ek. (3) Longer inflow durations delay the onset of global and local collapsebymaintainingahigherturbulentvelocitydispersioninthecloudoverlongertimes.(4)Thestarformationrate islargefrom the beginning,without any periodofslow and acceleratingstar formation.(5) The column densities of the local star-forming clumps closely resemble reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. The MC formation mechanism discussedherenaturallyexplainstheapparent‘‘virialized’’stateofMCsandtheubiquityofHihalosaroundthem.Also, within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (k15 Myr) accumulation periods do occur during which the clouds build up their gravitational energy, and which are expected to be spent in the atomic phase.

Turbulent flow-driven molecular cloud formation: a solution to the post-t tauri problem?

We suggest that molecular clouds can be formed on short timescales by compressions from large scale streams in the interstellar medium (ISM). In particular, we argue that in the Taurus-Auriga complex, with filaments of 10-20 × 2-5 pc, most have been formed by H I flows in 3 Myr, explaining the absence of post-T Tauri stars in the region with ages 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 12CO) 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 km s-1) than are 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 supernova explosion.

Density probability distribution in one-dimensional polytropic gas dynamics

We discuss the generation and statistics of the density fluctuations in highly compressible polytropic turbulence, based on a simple model and one-dimensional numerical simulations. Observing that density structures tend to form in a hierarchical manner, we assume that density fluctuations follow a random multiplicative process. When the polytropic exponent g is equal to unity, the local Mach number is independent of the density, and our assumption leads us to expect that the probability density function ~PDF! of the density field is a log-normal. This isothermal case is found to be special, with a dispersion s s scaling as the square turbulent Mach number M ˜2 , where s[ ln r and r is the fluid density. Density fluctuations are stronger than expected on the sole basis of shock jumps. Extrapolating the model to the case gfi1, we find that as the Mach number becomes large, the density PDF is expected to asymptotically approach a power-law regime at high densities when g,1, and at low densities when g.1. This effect can be traced back to the fact that the pressure term in the momentum equation varies exponentially with s, thus opposing the growth of fluctuations on one side of the PDF, while being negligible on the other side. This also causes the dispersion s s to grow more slowly than M ˜2 when gfi1. In view of these results, we suggest that Burgers flow is a singular case not approached by the

Molecular Cloud Evolution. I. Molecular Cloud and Thin Cold Neutral Medium Sheet Formation

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 one-dimensional simulations predict the thermodynamic conditions in the cold layer. After ~1 Myr of evolution, the layer has column density ~2.5 × 1019 cm-2, thickness ~0.03 pc, temperature ~25 K, and pressure ~6650 K cm-3. These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the one-dimensional simulations, the inflows into the sheets produce line profiles with a central line of width ~0.5 km s-1 and broad wings of width ~1 km s-1. Three-dimensional 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 ~7.5 Myr for inflow Mach number M1,r = 2.4 to >80 Myr for M1,r = 1.03. These numbers should be considered upper limits. The highest density turbulent gas (HDG, n > 100 cm-3) is always overpressured with respect to the mean WNM pressure by factors of 1.5-4, even though we do not include self-gravity. The intermediate-density gas (IDG, 10 < n/cm-3 < 100) has a significant pressure scatter that increases with M1,r, so that at M1,r = 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.

CLOUDS AS TURBULENT DENSITY FLUCTUATIONS: IMPLICATIONS FOR PRESSURE CONFINEMENT AND SPECTRAL LINE DATA INTERPRETATION

We examine the idea that diffuse H I and giant molecular clouds and their substructure form as density fluctuations induced by large-scale interstellar turbulence. We do this by closely investigating the topology of the velocity, density, and magnetic fields within and at the boundaries of the clouds emerging in high-resolution two-dimensional simulations of the interstellar medium (ISM) including self-gravity, magnetic fields, parameterized heating and cooling, and a simple model for star formation. We find that the velocity field is continuous across cloud boundaries for a hierarchy of clouds of progressively smaller sizes. Cloud boundaries defined by a density-threshold criterion are found to be quite arbitrary, with no correspondence to any actual physical boundary, such as a density discontinuity. Abrupt velocity jumps are coincident with the density maxima, which indicates that the clouds are formed by colliding gas streams. This conclusion is also supported by the fact that the volume and surface kinetic terms in the Eulerian virial theorem for a cloud ensemble are comparable in general and by the topology of the magnetic field, which exhibits bends and reversals where the gas streams collide. However, no unique trend of alignment between density and magnetic features is observed. Both sub- and super-Alfvenic motions are observed within the clouds. In light of these results, 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, unless it is strongly gravitationally bound. Turbulent pressure confinement appears self-defeating because turbulence contains large-scale motions that necessarily distort Lagrangian cloud boundaries or equivalently cause flux through Eulerian boundaries. We then discuss the compatibility of the present scenario with observational data. We find that density-weighted velocity histograms are consistent with observational line profiles of comparable spatial and velocity resolution, exhibiting similar FWHMs and similar multicomponent structure. An analysis of the regions contributing to each velocity interval indicates that the histogram features do not come from isolated clumps but rather from extended regions throughout a cloud, which often have very different total velocity vectors. Finally, we argue that the scenario presented here may also be applicable to small scales with larger densities (molecular clouds and their substructure, up to at least n~103-105 cm-3) and conjecture that quasi-hydrostatic configurations cannot be produced from turbulent fluctuations unless the thermodynamic behavior of the flow becomes nearly adiabatic. We demonstrate, using appropriate cooling rates, that this will not occur except for very small compressions (10-2 pc) or until protostellar densities are reached for collapse.

On the Probability Density Function of Galactic Gas. I. Numerical Simulations and the Significance of the Polytropic Index

We investigate the form of the one-point probability density function (pdf) for the density field of the interstellar medium using numerical simulations that successively reduce the number of physical processes included. Two-dimensional simulations of self-gravitating supersonic MHD turbulence, of supersonic self-gravitating hydrodynamic turbulence, and of decaying Burgers turbulence produce in all cases filamentary density structures and evidence for a power-law density pdf at large densities with logarithmic slope between -1.7 and -2.3. This suggests that a power-law shape of the pdf and the general filamentary morphology are the signature of the nonlinear advection operator. These results do not support previous claims that the pdf is lognormal. A series of one-dimensional simulations of forced supersonic polytropic turbulence is used to resolve the discrepancy. They suggest that the pdf is lognormal only for effective polytropic indices γ = 1 (or nearly lognormal for γ ≠ 1 if the Mach number is sufficiently small), while power laws develop for densities larger than the mean if γ < 1. We evaluate the polytropic index for conditions relevant to the cool interstellar medium using published cooling functions and different heating sources, finding that a lognormal pdf should probably occur at densities around 103 and is possible at larger densities, depending strongly on the role of gas-grain heating and cooling. Several applications are examined. First, we question a recent derivation of the initial mass function from the density pdf by Padoan, Nordlund, & Jones because (1) the pdf does not contain spatial information and (2) their derivation produces the most massive stars in the voids of the density distribution. Second, we illustrate how a distribution of ambient densities can alter the predicted form of the size distribution of expanding shells. Finally, a brief comparison is made with the density pdfs found in cosmological simulations.

A TURBULENT MODEL FOR THE INTERSTELLAR MEDIUM. II. MAGNETIC FIELDS AND ROTATION

We present results from two-dimensional numerical simulations of a supersonic turbulent flow in the plane of the galactic disk, incorporating shear, thresholded and discrete star formation (SF), self-gravity, rotation and magnetic fields. A test of the model in the linear regime supports the results of the linear theory of Elmegreen (1991). In the fully nonlinear turbulent regime, while some results of the linear theory persist, new effects also emerge. Some exclusively nonlinear effects are: a) Even though there is no dynamo in 2D, the simulations are able to maintain or increase their net magnetic energy in the presence of a seed uniform azimuthal component. b) A well-defined power-law magnetic spectrum and an inverse magnetic cascade are observed in the simulations, indicating full MHD turbulence. Thus, magnetic field energy is generated in regions of SF and cascades up to the largest scales. c) The field has a slight but noticeable tendency to be aligned with density features. d) The magnetic field prevents HII regions from expanding freely, as in the recent results of Slavin \& Cox (1993). e) A tendency to exhibit {\it less} filamentary structures at stronger values of the uniform component of the magnetic field is present in several magnetic runs. f) For fiducial values of the parameters, the flow in general appears to be in rough equipartition between magnetic and kinetic energy. There is no clear domination of either the magnetic or the inertial forces. g) A median value of the magnetic field strength within clouds is

Influence of Cooling-Induced Compressibility on the Structure of Turbulent Flows and Gravitational Collapse

\sim 12\mu

Gravity or turbulence? Velocity dispersion–size relation

G, while for the intercloud medium a value of

Gravity or turbulence? – II. Evolving column density probability distribution functions in molecular clouds

\sim 3\mu