Astrophys.J.620:786-794,2005 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.
Astrophys.J.527:285-297,1999 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.
Under the assumptions that molecular clouds are nearly spatially and temporally isothermal and that the density peaks (``cores'') within them are formed by turbulent fluctuations, we argue that cores ...cannot reach a hydrostatic (or magneto-static) state as a consequence of their formation process. In the non-magnetic case, stabilization requires a Bonnor-Ebert truncation at a finite radius, which is not feasible for a single-temperature flow, unless it amounts to a shock, which is clearly a dynamical feature. Instead, in this case, cores must be dynamical entities that can either be pushed into collapse, or else ``rebound'' towards the mean pressure and density as the parent cloud. Nevertheless, rebounding cores are delayed in their re-expansion by their own self-gravity. We give a crude estimate for the re-expansion time as a function of the closeness of the final compression state to the threshold of instability, finding typical values \(\sim 1\) Myr, i.e., of the order of a few free-fall times. Our results support the notion that not all cores observed in molecular clouds need to be on route to forming stars, but that instead a class of ``failed cores'' should exist, which must eventually re-expand and disperse, and which can be identified with observed starless cores. In the magnetic case, recent observational and theoretical work suggests that all cores are critical or supercritical, and are thus qualitatively equivalent to the non-magnetic case. Our results support the notion that the entire star formation process is dynamical, with no intermediate hydrostatic stages.
Phase changes in the Galactic interstellar medium (ISM) are to a large extent controlled by the formation of massive stars. The cycling of the ISM from mostly neutral atomic (Hi) clouds into ...molecular (H2) clouds, traced by low-excitation species such as OH and CO, leads to the formation of dense, self-gravitating clumps and cores traced by high-density species such as NH3, CS, HCN, HCO+, and N2H+, and by other even higher dipole-moment molecules in regions where stars form.
Dust continuum emission in the mid-infrared (IR), far-IR, submillimeter, and millimeter ranges of the spectrum reveal progressively cooler and higher column density
Stars form in a cold, dense, molecular phase of the interstellar medium (ISM) that appears to be organized into coherent, localized volumes or clouds. The star formation history of the universe, the ...evolution of galaxies, and the formation of planets in stellar environments are all coupled to the formation of these clouds, the collapse of unstable regions within them to stars, and the clouds’ final dissipation. The physics of these regions is complex, and descriptions of cloud structure and evolution remain incomplete and require continued exploration. Here we review the current status of observations and theory of molecular clouds, focusing
We study the instantaneous virial balance of clumps and cores (CCs) in 3D simulations of driven, MHD, isothermal molecular clouds (MCs). The models represent a range of magnetic field strengths in ...MCs from subcritical to non-magnetic regimes. We identify CCs at different density thresholds, and for each object, we calculate all the terms that enter the Eulerian form of the virial theorem (EVT). A CC is considered gravitationally bound when the gravitational term in the EVT is larger than the amount for the system to be virialized, which is more stringent than the condition that it be large enough to make the total volume energy negative. We also calculate, quantities commonly used in the observations to indicate the state of gravitational boundedness of CCs such as the Jeans number J_c, the mass-to magnetic flux ratio mu_c, and the virial parameter alpha_vir. Our results show that: a) CCs are dynamical out-of-equilibrium structures. b) The surface energies are of the same order than their volume counterparts c) CCs are either in the process of being compressed or dispersed by the velocity field. Yet, not all CCs that have a compressive net kinetic energy are gravitationally bound. d) There is no 1-to-1 correspondence between the state of gravitational boundedness of a CC as described by the virial analysis or as implied by the classical indicators. In general, in the virial analysis, we observe that only the inner regions of the objects are gravitationally bound, whereas J_c, alpha_vir, and mu_c estimates tend to show that they are more gravitationally bound at the lowest threshold levels and more magnetically supercritical. g) We observe, in the non-magnetic simulation, the existence of a bound core with structural and dynamical properties that resemble those of the Bok globule Barnard 68 (B68).
(Abrigded) We discuss the column density profiles of "cores" in 3D SPH numerical simulations of turbulent molecular clouds. The SPH scheme allows us to perform a high spatial resolution analysis of ...the density maxima (cores) at scales between ~0.003 and 0.3pc. We analyze simulations in three different physical conditions: large scale driving, small scale driving, and random Gaussian initial conditions without driving; each one at two different timesteps: just before self-gravity is turned-on, and when gravity has been operating such that 5% of the total mass in the box has been accretted into cores. For this dataset, we perform Bonnor-Ebert fits to the column density profiles of cores. We find that 65% of the cores can be matched to Bonnor-Ebert profiles, and of these, 47% correspond to stable equilibrium configurations with xi_max < 6.5, even though the cores analyzed in the simulations are not in equilibrium, but instead are dynamically evolving. We also find in some cases substantial superposition effects when we analyze the projection of the density structures. As a consequence, different projections of the same core may give very different values of the BE fits. Finally, we briefly discuss recent results claiming that Bok globule B68 is in hydrostatic equilibrium, stressing that they imply that this core is unstable by a wide margin. We conclude that fitting BE profiles to observed cores is not an unambiguous test of hydrostatic equilibrium, and that fit-estimated parameters of the BE sphere may differ significantly from the actual values in the cores.
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.
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 review recent results on the nonlinear development of thermal instability in the context of the turbulent atomic interstellar medium (ISM). First, we pre- sent a brief summary of the linear ...theory, remarking that, in the atomic ISM, the wave mode is stable at small scales. Next, we revisit the growth of isolated entropy perturbations in initially unstable gas, as a function of the ratio \(\eta\) of the cooling to the dynamical crossing times. Third, we consider the evolution of {\it velocity} perturbations. These correspond to the wave mode, and are stable at moderate amplitudes and small scales, as confirmed numerically. Fourth, we consider the behavior of magnetic pressure in turbulent regimes. We propose that recent findings of a poor B-rho correlation at low rho are due to the different B-rho scalings for the slow and fast modes of non- linear MHD waves. This implies that, in fully turbulent regimes, the magnetic field may not be a very efficient source of pressure, and that polytropic de- scriptions of magnetic pressure are probably not adequate. Finally, we discuss simulations of the ISM (and resolution issues) concerned with the possibility of significant amounts of gas being in the ``lukewarm'' temperature range be- tween the warm and cold stable phases. The mass fraction in this range in- creases, and the phase segregation decreases, as smaller scales are considered. We attribute this to the enhanced stability of moderate, adiabatic-like veloc- ity fluctuations with \(\eta \gg 1\), to the recycling of gas from the dense to the diffuse phase by stellar energy injection, and to the magnetic field not being strongly turbulent there, possibly providing additional stability. Final- ly, we suggest that the lukewarm gas can be observationally distinguished through simultaneous determination of two of its thermodynamic variables.
