Molecular Cloud Turbulence and Star FormationJavier Ballesteros-ParedesUniversidad Nacional Aut´onoma de M´exicoRalf S. KlessenAstrophysikalisches Institut PotsdamMordecai-Mark Mac LowAmerican Museum of Natural History at New YorkEnrique V´azquez-SemadeniUniversidad Nacional Aut´onoma de M´exicoWe review the properties of turbulent molecular clouds (MCs), focusing on the physicalprocesses that influence star formation (SF). MC formation appears to occur during large-scalecompression of the diffuse ISM driven by supernovae, magnetorotational instability, or gravita-tional instability in galactic disks of stars and gas. The compressions generate turbulence thatcan accelerate molecule production and produce the observed morphology. We then review theproperties of MC turbulence, including density enhancements observed as clumps and cores,magnetic field structure, driving scales, the relation to observed scaling relations, and the in-teraction with gas thermodynamics. We argue that MC cores are dynamical, not quasistatic,objects with relatively short lifetimes not exceeding a few megayears. We review their morphol-ogy, magnetic fields, density and velocity profiles, and virial budget. Next, we discuss how MCturbulence controls SF. On global scales turbulence prevents monolithic collapse of the clouds;on small scales it promotes local collapse. We discuss its effects on the SF efficiency, and crit-ically examine the possible relation between the clump mass distribution and the initial massfunction, and then turn to the redistribution of angular momentum during collapse and how itdetermines the multiplicity of stellar systems. Finally, we discuss the importance of dynamicalinteractions between protostars in dense clusters, and the effect of the ionization and winds fromthose protostars on the surrounding cloud. We conclude that the interaction of self-gravity andturbulence controls MC formation and behavior, as well as the core and star formation processeswithin them.1. INTRODUCTIONStar formation occurs within molecular clouds (MCs).These exhibit supersonic linewidths, which are interpretedas evidence for supersonic turbulence (Zuckerman andEvans, 1974). Early studies considered this property mainlyas a mechanism of MC support against gravity. In more re-cent years, however, it has been realized that turbulence isa fundamental ingredient of MCs, determining propertiessuch as their morphology, lifetimes, rate of star formation,etc.Turbulence is a multiscale phenomenon in which kineticenergy cascades from large scales to small scales. The bulkof the specific kinetic energy remains at large scales. Tur-bulence appears to be dynamically important from scales ofwhole MCs down to cores (e.g., Larson, 1981; Ballesteros-Paredes et al., 1999b; Mac Low and Klessen, 2004). Thus,early microturbulent descriptions postulating that turbu-lence only acts on small scales did not capture major effectsat large scales such as cloud and core formation by the tur-bulence.Turbulence in the warm diffuse interstellar medium(ISM) is transonic, with both the sound speed and the non-thermal motions being ∼ 10 km s−1(Kulkarni and Heiles,1987; Heiles and Troland, 2003), while within MCs it ishighly supersonic, with Mach numbers M ≈ 5–20 (Zuck-erman and Palmer, 1974). Both media are highly com-pressible. Hypersonic velocity fluctuations in the roughlyisothermal molecular gas produce large density enhance-ments at shocks. The velocity fluctuations in the warmdiffuse medium, despite being only transonic, can stilldrive large density enhancements because they can pushthe medium into a thermally unstable regime in which thegas cools rapidly into a cold, dense regime (Hennebelleand P´erault, 1999). In general, the atomic gas respondsclose to isobarically to dynamic compressions for densities0.5 . n/cm−3. 40 (Gazol et al., 2005). We argue in thisreview that MCs form from dynamically evolving, high-density features in the diffuse ISM. Similarly, their internalsubstructure of clumps and cores are also transient densityenhancements continually changing their shape and even1the material contained in the turbulent flow, behaving assomething between discrete objects and waves (V´azquez-Semadeni et al., 1996). Because of their higher density,some clumps and cores become gravitationally unstableand collapse to form stars.We here discuss the main advances in our understandingof the turbulent properties of MCs and their implications forstar formation since the reviews by V´azquez-Semadeni et al.( 2000) and Mac Low and Klessen (2004), proceeding fromlarge (giant MCs) to small (core) scales.2. MOLECULAR CLOUD FORMATIONThe questions of how MCs form and what determinestheir physical properties have remained unanswered untilrecently (e.g., Elmegreen, 1991; Blitz and Williams, 1999).Giant molecular clouds (GMCs) have gravitational energyfar exceeding their thermal energy (Zuckerman and Palmer,1974), although comparable to their turbulent (Larson,1981) and magnetic energies (Myers and Goodman, 1988;Crutcher, 1999; Bourke et al., 2001; Crutcher et al., 2004).This near equipartition of energies has traditionally beeninterpreted as indicative of approximate virial equilibrium,and thus of general stability and longevity of the clouds(e.g., McKee et al., 1993; Blitz and Williams, 1999). Inthis picture, the fact that MCs have thermal pressures ex-ceeding that of the general ISM by roughly one order ofmagnitude (e.g., Blitz and Williams, 1999) was interpretedas a consequence of their being strongly self-gravitating(e.g., McKee, 1995), while the magnetic and turbulent en-ergies were interpreted as support against gravity. Becauseof the interpretation that their overpressures were due toself-gravity, MCs could not be incorporated into globalISM models based on thermal pressure equilibrium, suchas those by Field et al. (1969); McKee and Ostriker (1977);Wolfire et al. (1995).Recent work suggests instead that MCs are likely to betransient, dynamically evolving features produced by com-pressive motions of either gravitational or turbulent origin,or some combination thereof. In what follows we first dis-cuss these two formation mechanisms, and then discusshow they can give rise to the observed physical and chemi-cal properties of MCs.2.1. Formation mechanismsLarge-scale gravitational instability in the combinedmedium of the collisionless stars (Toomre, 1964), and thecollisional gas (Goldreich and Lynden-Bell, 1965) appearslikely to
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