Abstract
We present a unified description of the scenario of global hierarchical collapse (GHC). GHC constitutes a flow regime of (non-homologous) collapses within collapses, in which all scales ...accrete from their parent structures, and small, dense regions begin to contract at later times, but on shorter time-scales than large, diffuse ones. The different time-scales allow for most of the clouds’ mass to be dispersed by the feedback from the first massive stars, maintaining the cloud-scale star formation rate low. Molecular clouds (MCs), clumps, and cores are not in equilibrium, but rather are either undergoing contraction or dispersal. The main features of GHC are as follows: (1) The gravitational contraction is initially very slow, and begins when the cloud still consists of mostly atomic gas. (2) Star-forming MCs are in an essentially pressureless regime, causing filamentary accretion flows from the cloud to the core scale to arise spontaneously. (3) Accreting objects have longer lifetimes than their own free-fall time, due to the continuous replenishment of material. (4) The clouds’ total mass and its molecular and dense mass fractions increase over time. (5) The clouds’ masses stop growing when feedback becomes important. (6) The first stars appear several megayears after global contraction began, and are of low mass; massive stars appear a few megayears later, in massive hubs. (7) The minimum fragment mass may well extend into the brown-dwarf regime. (8) Bondi–Hoyle–Lyttleton-like accretion occurs at both the protostellar and the core scales, accounting for an IMF with slope dN/dM ∝ M−2. (9) The extreme anisotropy of the filamentary network explains the difficulty in detecting large-scale infall signatures. (10) The balance between inertial and gravitationally driven motions in clumps evolves during the contraction, explaining the approach to apparent virial equilibrium, from supervirial states in low-column density clumps and from subvirial states in dense cores. (11) Prestellar cores adopt Bonnor–Ebert-like profiles, but are contracting ever since when they may appear to be unbound. (12) Stellar clusters develop radial age and mass segregation gradients. We also discuss the incompatibility between supersonic turbulence and the observed scalings in the molecular hierarchy. Since gravitationally formed filaments do not develop shocks at their axes, we suggest that a diagnostic for the GHC scenario should be the absence of strong shocks in them. Finally, we critically discuss some recent objections to the GHC mechanism.
Abstract
We discuss the mechanism of cluster formation in a numerical simulation of a molecular cloud (MC) undergoing global hierarchical collapse, focusing on how the gas motions in the parent cloud ...control the assembly of the cluster. The global collapse implies that the star formation rate (SFR) increases over time. The collapse is hierarchical because it consists of small-scale collapses within larger scale ones. The latter culminate a few Myr later than the first small-scale ones and consist of filamentary flows that accrete on to massive central clumps. The small-scale collapses consist of clumps that are embedded in the filaments and falling on to the large-scale collapse centres. The stars formed in the early, small-scale collapses share the infall motion of their parent clumps, so that the filaments feed both gas and stars to the massive central clump. This process leads to the presence of a few older stars in a region where new protostars are forming, and also to a self-similar structure, in which each unit is composed of smaller scale subunits that approach each other and may merge. Because the older stars formed in the filaments share the infall motion of the gas on to the central clump, they tend to have larger velocities and to be distributed over larger areas than the younger stars formed in the central clump. Finally, interpreting the initial mass function (IMF) simply as a probability distribution implies that massive stars only form once the local SFR is large enough to sample the IMF up to high masses. In combination with the increase of the SFR, this implies that massive stars tend to appear late in the evolution of the MC, and only in the central massive clumps. We discuss the correspondence of these features with observed properties of young stellar clusters, finding very good qualitative agreement.
We report on the filaments that develop self-consistently in a new numerical simulation of cloud formation by colliding flows. As in previous studies, the forming cloud begins to undergo ...gravitational collapse because it rapidly acquires a mass much larger than the average Jeans mass. Thus, the collapse soon becomes nearly pressureless, proceeding along its shortest dimension first. This naturally produces filaments in the cloud and clumps within the filaments. The filaments are not in equilibrium at any time, but instead are long-lived flow features through which the gas flows from the cloud to the clumps. The filaments are long-lived because they accrete from their environment while simultaneously accreting onto the clumps within them; they are essentially the locus where the flow changes from accreting in two dimensions to accreting in one dimension. Moreover, the clumps also exhibit a hierarchical nature: the gas in a filament flows onto a main, central clump but other, smaller-scale clumps form along the infalling gas. Correspondingly, the velocity along the filament exhibits a hierarchy of jumps at the locations of the clumps. Two prominent filaments in the simulation have lengths ~15 pc and masses ~600M sub(middot in circle) above density n ~ 10 super(3) cm super(-3) (~2x10 super(3)M sub(mid dot in circle) at n > 50 cm super(-3)). The density profile exhibits a central flattened core of size ~0.3 pc and an envelope that decays as r super(-2.5) in reasonable agreement with observations. Accretion onto the filament reaches a maximum linear density rate of ~30M sub(middot in circle) Myr super(-1) pc super(-1).
ABSTRACT
We study the gravitationally dominated, accretion-driven evolution of a prestellar core. In our model, as the core’s density increases, it remains immersed in a constant-density environment ...and so it accretes from this environment, increasing its mass and reducing its Jeans length. Assuming a power-law density profile ρ ∝ r−p, we compute the rate of change of the slope p, and show that the value p = 2 is stationary, and furthermore, an attractor. The radial profile of the Jeans length scales as rp/2, implying that, for p < 2, there is a radius below which the region is smaller than its Jeans length, thus appearing gravitationally stable and in need of pressure confinement, while, in reality, it is part of a larger scale collapse and is undergoing compression by the infalling material. In this region, the infall speed decreases towards the centre, eventually becoming subsonic, thus appearing ‘coherent’, without the need for turbulence dissipation. We present a compilation of observational determinations of density profiles in dense cores and show that the distribution of their slopes peaks at p ∼ 1.7–1.9, supporting the notion that the profile steepens over time. Finally, we discuss the case of magnetic support in a core in which the field scales as B ∝ ρβ. For the expected value of β = 2/3, this implies that the mass to magnetic flux ratio also decreases towards the central parts of the cores, making them appear magnetically supported, while, in reality, they may be part of larger collapsing supercritical region. We conclude that local signatures of either thermal or magnetic support are not conclusive evidence of stability, that the gravitational instability of a region must be established at the large scales, and that the prestellar stage of collapse is dynamic rather than quasi-static.
Abstract
We investigate the generation of turbulence during the prestellar gravitational contraction of a turbulent spherical core. We define the ratio
g
of the one-dimensional turbulent velocity ...dispersion
to the gravitational velocity
to then analytically estimate
g
under the assumptions of (a) equipartition or virial equilibrium between the gravitational (
) and turbulent kinetic (
) energies and (b) stationarity of transfer from gravitational to turbulent energy (implying
cst). In the equipartition and virial cases, we find
and
, respectively; in the stationary case we find
, where
η
is an efficiency factor,
is the energy injection scale of the turbulence, and
R
is the core’s radius. Next, we perform AMR simulations of the prestellar collapse of an isothermal, transonic turbulent core at two different resolutions, and a nonturbulent control simulation. We find that the turbulent simulations collapse at the same rate as the nonturbulent one, so that the turbulence generation does not significantly slow down the collapse. We also find that (a) the simulations approach near balance between the rates of energy injection from the collapse and of turbulence dissipation; (b)
, close to the “virial” value (turbulence is 30% ∼ 40% of nonthermal linewidth); (c) the injection scale is
, and (d) the “turbulent pressure”
scales as
, an apparently nearly adiabatic scaling. We propose that this scaling and the nearly virial values of the turbulent velocity dispersion may be reconciled with the nondelayed collapse rate if the turbulence is dissipated as soon as it is generated.
It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (N-PDF). Instead, clouds without star formation seem to possess ...a lognormal distribution, while clouds with active star formation develop a power-law tail at high column densities. The lognormal behaviour of the N-PDF has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behaviour occurs when the cloud is dominated by gravity. In the present contribution, we use thermally bi-stable numerical simulations of cloud formation and evolution to show that, indeed, these two regimes can be understood in terms of the formation and evolution of molecular clouds: a very narrow lognormal regime appears when the cloud is being assembled. However, as the global gravitational contraction occurs, the initial density fluctuations are enhanced, resulting, first, in a wider lognormal N-PDF, and later, in a power-law N-PDF. We thus suggest that the observed N-PDF of molecular clouds are a manifestation of their global gravitationally contracting state. We also show that, contrary to recent suggestions, the exact value of the power-law slope is not unique, as it depends on the projection in which the cloud is being observed.
From Diffuse Gas to Dense Molecular Cloud Cores Ballesteros-Paredes, Javier; André, Philippe; Hennebelle, Patrick ...
Space science reviews,
08/2020, Letnik:
216, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Molecular clouds are a fundamental ingredient of galaxies: they are the channels that transform the diffuse gas into stars. The detailed process of how they do it is not completely understood. We ...review the current knowledge of molecular clouds and their substructure from scales
∼
1
kpc
down to the filament and core scale. We first review the mechanisms of cloud formation from the warm diffuse interstellar medium down to the cold and dense molecular clouds, the process of molecule formation and the role of the thermal and gravitational instabilities. We also discuss the main physical mechanisms through which clouds gather their mass, and note that all of them may have a role at various stages of the process. In order to understand the dynamics of clouds we then give a critical review of the widely used virial theorem, and its relation to the measurable properties of molecular clouds. Since these properties are the tools we have for understanding the dynamical state of clouds, we critically analyse them. We finally discuss the ubiquitous filamentary structure of molecular clouds and its connection to prestellar cores and star formation.
Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, ...stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.
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