Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario

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.

Dense gas formation in the Musca filament due to the dissipation of a supersonic converging flow

“…In numerical simulations of the ISM, filaments are omnipresent and can form in various ways: through shocks in (magnetic) supersonic turbulent colliding flows (e.g. Padoan et al 2001;Jappsen et al 2005;Smith et al 2016;Federrath 2016;Clarke et al 2017;Inoue et al 2018), during the global gravitational collapse of a cloud (Gómez & Vázquez-Semadeni 2014;Vázquez-Semadeni et al 2019), through velocity shear in a magnetised medium (Hennebelle 2013), or via the gravitational instability of a sheet (Nagai et al 1998).…”

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

“…[1][2][3][4] Density variations are seeded and amplified by gas motion, but the exact nature of this motion is unknown across spatial scale and galactic environment. 5 Although dense star-forming gas likely emerges from a combination of instabilities, 6,7 convergent flows, 8 and turbulence, 9 establishing the precise origin is challenging because it requires quantifying gas motion over many orders of magnitude in spatial scale. Here we measure [10][11][12] the motion of molecular gas in the Milky Way and in nearby galaxy NGC 4321, assembling observations that span an unprecedented spatial dynamic range (10 −1 −10 3 pc).…”

Statistical model for filamentary structures of molecular clouds

“…This difference can be a sign that the multiplicative cascade alone cannot explain the highest density structures even for a low stellar activity region such as Polaris. This high density compared to the multiplicative cascade model might be explained by the effect of a global collapse and/or accretion mechanisms, which in addition to multiplicative processes have the ability to concentrate more the mass of the coherent structures (Vázquez-Semadeni et al 2019).…”

“…It is likely, however, that the multifractal nature of one may affect the statistical properties of the other. There is also a possibility that because of the non-linear nature and the hierarchical structures involved in this process, the mechanism of hierarchical collapse of molecular clouds such as described by Vázquez-Semadeni et al (2019) has the capacity of enhancing the multifractal nature of star-forming regions.…”