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 timescales than large, diffuse ones. The different timescales 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 (SFR) low. MCs, clumps, and cores are not in equilibrium, but rather are either undergoing contraction or dispersal. The main features of GHC are: 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.

IRAS 09002-4732: A Laboratory for the Formation of Rich Stellar Clusters

“…There is evidence for delayed O star formation in the ONC (O'Dell et al 2009), W3 Main (Tieftrunk et al 1997;Feigelson, & Townsley 2008;Bik et al 2014), and M 17 SW (Povich et al 2016). This picture is consistent with the global hierarchical gravitational collapse by Vázquez-Semadeni et al (2019) who explain late massive star formation in the central potential well and later destruction of filamentary gas feeding by OB stellar feedback.…”

“…More commonly, either the cluster is sparse (dozens of stars) or the rich cluster has already destroyed its natal molecular environment. The stellar spatial distribution, individual properties (ages, extinction) and collective properties (population, IMF, cluster structure) directly support contemporary numerical calculations of cluster formation in clouds that involve the hierarchical formation and merging of groups in molecular filaments such as those described by Vázquez-Semadeni et al (2019) and Krumholz et al (2019).…”

“…This latter effect is also reflected in the size of the ionized H II region: Orion and Eagle Table 5 provides a more quantitative comparison of the IRAS 09002-4732 stellar population to the Orion Nebula Cluster region. As outlined in §1, the Orion region has motivated much of the astrophysical discussions of cluster formation (Krumholz et al 2019;Vázquez-Semadeni et al 2019). Combining the ellipsoidal structural parameters obtained in §3.4 above with the estimates of total population in §4.4, cluster physical parameters can be derived following Kuhn et al (2015a,b).…”

Investigating the physical conditions in extended system hosting mid-infrared bubble N14

“…It is also reported that the global collapse leads in a chaotic and hierarchical manner, yielding gravitationally driven fragmentation in star-forming molecular clouds (e.g., Burkert & Hartmann 2004;Heitsch et al 2009;Galván-Madrid et al 2009;Peretto et al 2013;Beuther et al 2015;Liu et al 2015Liu et al , 2016bFriesen et al 2016;Jin et al 2016;Hacar et al 2017;Csengeri et al 2017;Yuan et al 2018;Jackson et al 2019;Barnes et al 2019;Vázquez-Semadeni et al 2019). The modeling results based on the global hierarchical gravitational collapse in molecular clouds by Vázquez-Semadeni et al (2019) indicate late birth of massive OB-stars and later loss of molecular gas feeding by feedback of massive stars. It also suggests that after the onset of global collapse, one can expect local collapse events in molecular clouds.…”

“…In recent years, it has been suggested that star-forming clouds seem to be in a state of global gravitational contraction (e.g., Hartmann et al 2012;Vázquez-Semadeni et al 2019). It is also reported that the global collapse leads in a chaotic and hierarchical manner, yielding gravitationally driven fragmentation in star-forming molecular clouds (e.g., Burkert & Hartmann 2004;Heitsch et al 2009;Galván-Madrid et al 2009;Peretto et al 2013;Beuther et al 2015;Liu et al 2015Liu et al , 2016bFriesen et al 2016;Jin et al 2016;Hacar et al 2017;Csengeri et al 2017;Yuan et al 2018;Jackson et al 2019;Barnes et al 2019;Vázquez-Semadeni et al 2019). The modeling results based on the global hierarchical gravitational collapse in molecular clouds by Vázquez-Semadeni et al (2019) indicate late birth of massive OB-stars and later loss of molecular gas feeding by feedback of massive stars.…”