Collapse and Fragmentation in Finite Sheets

By A. Burkert, L. Hartmann. 2004

147 citations recorded by the source corpus.

We present two-dimensional simulations of finite, self-gravitating gaseous sheets. Unlike the case of infinite sheets, such configurations do not constitute equilibrium states but instead are subject to global collapse unless countered by pressure forces or rotation. The initial effect of finite geometry is to promote concentrations of material at the edges of the sheet. If the sheet is not perfectly circular, gravitational focusing results in enhanced concentrations of mass. In the second-most simple geometry, that of an elliptical outer boundary, the general result is collapse to a filamentary structure with the densest concentrations of mass at the ends of the filament. We suggest that these simple calculations have interesting implications for the gravitational evolution of overall molecular cloud structure, envisioning that such clouds might originate as roughly sheetlike sections of gas accumulated as a result of large-scale flows in the local interstellar medium. We show some examples of local clouds with overall filamentary shape and denser concentrations of mass and star clusters near the ends of the overall extended structure, suggestive of our simple ellipse collapse calculations. We suggest that cluster-forming gas is often concentrated as a result of gravity acting on irregular boundaries; this mechanism can result in very rapid infall of gas, which may be of importance to the formation of massive stars. This picture suggests that much of the supersonic "turbulence" observed in molecular clouds might be gravitationally generated. Our results may provide impetus for further theoretical explorations of global gravitational effects in molecular clouds and their implications for generating the substructure needed for fragmentation into stars and clusters.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Computational Mechanics

View Paper: https://www.semanticscholar.org/paper/384d0c95adc137fc3a46c0f3014e0f7538178940

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Citations in the corpus

The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows (2006, background): “By this fragmentation mechanism star formation is “localized” in the sense that before the whole cloud can collapse, the non-linear density perturbations caused by supersonic turbulence have formed stars (see also Burkert & Hartmann 2004).”
Spitzer Observations of NGC 1333: A Study of Structure and Evolution in a Nearby Embedded Cluster (2007, background): “A ring-like morphology in dense gas has been detected before in the Coalsack (Lada et al. 2004), and some models of collapsing clouds have yielded various ring-like morphologies (e.g. Li & Nakamura 2002; Burkert & Hartmann 2004).”
Formation of Structure in Molecular Clouds: A Case Study (2005, background): “Global (i.e., large-scale) grav-l 1 Lt itational effects could still have a crucial effect on the system (Burkert & Hartmann 2004), especially once the inflow stops.”
ON THE STAR FORMATION EFFICIENCY OF TURBULENT MAGNETIZED CLOUDS (2012, background): “There, mass is transported to the protostars more efficiently, because it is funneled through multiple filaments toward their intersections where gravitational focusing occurs (Burkert & Hartmann 2004).”
FILAMENTARY STRUCTURE OF STAR-FORMING COMPLEXES (2009, background): “This paper applies the basic idea that a self-gravitating layer can develop and preserve filamentary structure, as a consequence of its equilibrium (e.g., Nagai, Inutsuka, & Miyama 1998) or of its collapse (e.g. Burkert & Hartmann 2004).”
Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “Typically, a cloud contains two or three long filaments that are either parallel or converge at a low angle in a massive condensation that often contains an active cluster-forming site (Tachihara et al. 2002; Burkert & Hartmann 2004).”
Gravity or turbulence? Velocity dispersion–size relation (2010, background): “From this point of view, molecular clouds are in a global state of collapse, with an internal distribution of free-fall times due to the fluctuations in the density field induced by the initial turbulence (Burkert & Hartmann 2004; Vázquez-Semadeni et al. 2006; Heitsch & Hartmann 2008).”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “…the pressureless collapse is expected to produce filamentary structures, as verified in numerical simulations of collapsing sheet-like clouds (e.g. Burkert & Hartmann 2004; Hartmann & Burkert 2007; Vázquez-Semadeni et al. 2007; Heitsch et al. 2008b, 2009; Gómez & Vázquez-Semadeni 2014;…”
Cooling, Gravity, and Geometry: Flow-driven Massive Core Formation (2007, methodology): “Non-linear density perturbations collapse at a higher rate than the global cloud (Burkert & Hartmann 2004), thus allowing stars to form locally before the whole-sale collapse of the cloud.”
Clump morphology and evolution in MHD simulations of molecular cloud formation (2008, background): “5 of Vázquez-Semadeni et al. 2007), and the formation of a ring at the periphery of the cloud (Burkert & Hartmann 2004).”
Dense core formation by fragmentation of velocity-coherent filaments in L1517 (2011, background): “…state of the cloud described in the following sections indicates that the gas in L1517 cannot be too far from a state of equilibrium, as otherwise it would quickly develop supersonic motions, (e.g., Burkert & Hartmann 2004) and would not fragment into well-separated cores (Inutsuka & Miyama 1997).”
Thermal physics, cloud geometry and the stellar initial mass function (2005, background): “A related possibility is that a flattened configuration with an elongated or irregular boundary can collapse to form an irregular filamentary shape (Burkert & Hartmann 2004).”