Control of star formation by supersonic turbulence

By M. Mac Low, Ralf Klessen. 2000

1,230 citations recorded by the source corpus.

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/294eee64e89deca51028769763217dd71f042146

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Citations in the corpus

Dust coagulation and fragmentation in molecular clouds. I. How collisions between dust aggregates alter the dust size distribution (2009, background): “This is the gravo-turbulent fragmentation picture of turbulence-dominated molecular clouds, where the (supersonic) turbulence is driven at large scales, but also reaches the scales of quiescent (subsonic) cores (Mac Low & Klessen 2004; Klessen et al. 2005).”
Simulating the Formation of Molecular Clouds. I. Slow Formation by Gravitational Collapse from Static Initial Conditions (2006, background): “…see Kim, Ostriker & Stone 2002 for a recent view of its importance), or may simply be part of the general turbulent velocity field, which itself is probably driven primarily by some combination of energy input from supernovae and from the magnetorotational instability (Mac Low & Klessen 2004).”
On the fraction of star formation occurring in bound stellar clusters (2012, background): “Again, the Mach number plays a central role.”
Simulating the Formation of Molecular Clouds. II. Rapid Formation from Turbulent Initial Conditions (2006, background): “…that molecular clouds are transient objects, formed and then dispersed on a timescale of only a fewmillion years by the action of large-scale turbulent flows in the ISM, which are themselves driven by energy input from supernovae and from the magnetorotational instability (Mac Low & Klessen 2004).”
MAXIMALLY STAR-FORMING GALACTIC DISKS. I. STARBURST REGULATION VIA FEEDBACK-DRIVEN TURBULENCE (2011, background): “Because the density PDF in turbulent regions is a lognormal function of M (McKee & Ostriker 2007), Krumholz & McKee (2005) show that the resulting dependence of the star-forming efficiency on the Mach number is weak, εff(ρ̄) ∝ M−0.3 .”
Magnetically Regulated Star Formation in Three Dimensions: The Case of the Taurus Molecular Cloud Complex (2008, background): “Recent numerical simulations have focused more on the role of turbulence in the dynamics of larger clouds and core formation in them (as reviewed in, e.g., Mac Low & Klessen 2004).”
A primer on hierarchical galaxy formation: the semi-analytical approach (2006, methodology): “We do not attempt to go into the details of these processes here; instead we refer the interested reader to the review articles by Elmegreen & Scalo (2004) and Mac-Low & Klessen (2004).”
The formation of disc galaxies in a ΛCDM universe (2010, background): “Note that SNe explosions can regulate star formation in galaxies without expelling gas, being a driver of galactic turbulence (Mac Low & Klessen 2004).”
Cooling, Gravity, and Geometry: Flow-driven Massive Core Formation (2007, methodology): “2007, ApJ, 665, 445 Heitsch, F., Zweibel, E. G., Slyz, A. D., & Devriendt, J. E. G. 2004, ApJ, 603, 165 Hennebelle, P. & Audit, E. 2007, A&A, 465, 431 Hennebelle, P., Audit, E., & Miville-Deschênes, M.-A. 2007, A&A, 465, 445 Hueckstaedt, R. M. 2003, New Astronomy, 8, 295 Keto, E., Ho, L. C., & Lo, ”
Interstellar Turbulence I: Observations and Processes (2004, background): “Readers are referred to reviews of some of these problems and more detailed discussions of statistical properties in Vázquez-Semadeni et al. (2000), Ballesteros-Paredes (2003), Mac Low (2003), Nordlund & Padoan (2003), Ostriker (2003), and Mac Low & Klessen (2004).”
Modelling CO emission – I. CO as a column density tracer and the X factor in molecular clouds (2010, background): “As turbulence is now considered an important aspect of the dynamics in molecular clouds and star formation (Mac Low & Klessen 2004; Ballesteros-Paredes et al. 2007; McKee & Ostriker 2007, and references therein), its role in influencing molecule formation needs to be understood.”
Star formation in galaxy mergers with realistic models of stellar feedback and the interstellar medium (2012, background): “…from ultraviolet (UV) and IR photons, protostellar jets, cosmic rays, supernovae at H averford C ollege L ibrary on O ctober 2, 2014 http://m nras.oxfordjournals.org/ D ow nloaded from (SNe), and gravitational cascades from large scales (e.g. Mac Low & Klessen 2004, and references therein).”
ATLASGAL - The APEX telescope large area survey of the galaxy at 870 μm (2009, background): “On the other, a scenario is advocated in which the star formation rate is governed by supersonic turbulence (Mac Low & Klessen 2004).”
Thermal physics, cloud geometry and the stellar initial mass function (2005, background): “However, turbulence is predicted to decay rapidly as a cloud evolves, even if it consists predominantly of magnetohydrodynamic (MHD) waves (Mac Low & Klessen 2004).”
Density Fluctuations in MHD Turbulence: Spectra, Intermittency, and Topology (2006, background): “Magnetohydrodynamic (MHD) turbulence is a key element for understanding star formation (see McKee & Tan 2002; Mac Low & Klessen 2004, and references therein).”
On the universality of supersonic turbulence (2013, background): “This kind of turbulence is relevant for the highly compressible interstellar medium (Elmegreen & Scalo 2004; Mac Low & Klessen 2004; McKee & Ostriker 2007), because it controls the rate of star formation triggered by gas compression in shocks (Krumholz & McKee 2005; Hennebelle & Chabrier 2011;…”
TOWARD UNDERSTANDING THE COSMIC-RAY ACCELERATION AT YOUNG SUPERNOVA REMNANTS INTERACTING WITH INTERSTELLAR CLOUDS: POSSIBLE APPLICATIONS TO RX J1713.7−3946 (2011, background): “Molecular line emissions from clouds are always observed with supra-thermal line widths that are considered to be the outcome of supersonic turbulence (Larson 1981, Heyer & Brunt 2004, Mac Low & Klessen 2004).”
Magnetism in the spiral galaxy NGC 6946: magnetic arms, depolarization rings, dynamo modes, and helical fields (2007, background): “While supersonic turbulence in molecular gas clouds of the Milky Way is observed and helps to support the clouds (Mac Low & Klessen 2004), supersonic turbulence in the diffuse ISM would lead to widespread shocks and hence is hard to maintain.”