On the Rapid Collapse and Evolution of Molecular Clouds

By Bruce G. Elmegreen. 2007

184 citations recorded by the source corpus.

Stars generally form faster than the ambipolar diffusion time, suggesting that several processes short-circuit the delay and promote a rapid collapse. These processes are considered here, including turbulence compression in the outer parts of giant molecular cloud (GMC) cores and GMC envelopes, GMC core formation in an initially supercritical state, and compression-induced triggering in dispersing GMC envelopes. The classical issues related to star formation timescales are addressed: high molecular fractions, low efficiencies, long consumption times for CO and HCN, rapid GMC core disruption and the lack of a stable core, long absolute but short relative timescales with accelerated star formation, and the slow motions of protostars. We consider stimuli to collapse from changes in the density dependence of the ionization fraction, the cosmic ray ionization rate, and various dust properties at densities above ~105 cm-3. We favor the standard model of subcritical GMC envelops and suggest they would be long-lived if not for disruption by rapid star formation in GMC cores. The lifecycle of GMCs is illustrated by a spiral arm section in the Hubble Heritage image of M51, showing GMC formation, star formation, GMC disruption with lingering triggered star formation, and envelope dispersal. There is no delay between spiral arm dust lanes and star formation; the classical notion results from heavy extinction in the dust lane and triggered star formation during cloud dispersal. Differences in the IMF for the different modes of star formation are considered.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1cd9683746f6429ef108287c329362cc0b2d1ef3

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Citations in the corpus

Before the first supernova: combined effects of H II regions and winds on molecular clouds (2014, unknown): “Krumholz & Tan (2007) measured SFERff(t) in a variety of systems of different densities and inferred typical values of ≈ 0.02, essentially independent of density (although see Elmegreen (2007) for further discussion of these measurements).”
Analytical Theory for the Initial Mass Function: CO Clumps and Prestellar Cores (2008, background): “First of all, the N (L) / L 3 relation is not rigorously justified (see Elmegreen 2007).”
STELLAR FEEDBACK IN MOLECULAR CLOUDS AND ITS INFLUENCE ON THE MASS FUNCTION OF YOUNG STAR CLUSTERS (2009, background): “We assume that the timescale for ISM removal is a few crossing times: ∆t ∼ (1−10)×τc (Elmegreen 2000, 2007; Hartmann et al. 2001; Tan et al. 2006; Krumholz & Tan 2007).”
Magnetically Regulated Star Formation in Three Dimensions: The Case of the Taurus Molecular Cloud Complex (2008, background): “Elmegreen (2007) argued that the envelopes of GMCs, where most of the molecular gas resides, are probably magnetically critical or even subcritical, and can be long-lived if not for disruption by rapid star formation in GMC cores, which are probably supercritical, with internal dynamics dominated…”
Structural Analysis of Molecular Clouds: Dendrograms (2008, background): “However, much of this debate has centered on considering disparate sets of observations and Elmegreen (2007) has presented a synthesis that argues relatively long-lived (20-30 Myr) self-gravitating clouds can ccommodate local, rapid star formation within them accounting for the sets of observations…”
Physical Processes in the Interstellar Medium (2014, background): “The star formation process in molecular clouds appears to be fast (Hartmann et al., 2001; Elmegreen, 2007).”
FIRE-2 simulations: physics versus numerics in galaxy formation (2017, background): “…repeatedly shown that almost all star formation occurs in the largest GMC complexes in galaxies (see Evans 1999; Rosolowsky et al. 2003; Blitz & Rosolowsky 2005; Bolatto et al. 2008; Elmegreen 2007; Tasker & Tan 2009; Feldmann & Gnedin 2011; Murray 2011; Harper-Clark & Murray 2011; Hopkins 2012a).”
STAR FORMATION EFFICIENCIES AND LIFETIMES OF GIANT MOLECULAR CLOUDS IN THE MILKY WAY (2010, background): “…considerable and intense debate, with estimates ranging from a single dynamical or free-fall time (Elmegreen 2000; Hartmann et al. 2001), to a few (Elmegreen 2007), up to ten or more (Scoville & Hersh 1979; Scoville & Wilson 2004), although it is important to note that the last authors were…”
Is molecular gas necessary for star formation (2011, background): “An obvious interpretation of these observations is that the formation of stars depends on the presence of molecular gas (Krumholz & McKee 2005; Elmegreen 2007; Krumholz et al. 2009).”
Young Massive Star Clusters (2010, background): “The evolution of the cluster during the first phase is a complex mix of gas dynamics, stellar dynamics, stellar evolution, and radiative transfer, and is currently incompletely understood (see Elmegreen 2007, Price & Bate 2009).”
The effect of magnetic fields on star cluster formation (2008, background): “One of the key areas of uncertainty is whether or not star formation is a rapid (Mac Low & Klessen 2004; Hartmann et al. 2001; Elmegreen 2007) or slow (Shu et al. 1987; Tan et al. 2006) process, central to which is the relative importance of magnetic fields to the star formation process.”
Importance of the Initial Conditions for Star Formation - I. Cloud Evolution and Morphology (2010, background): “…star formation is of the order of a few dynamical times (Ballesteros-Paredes, Hartmann & Vázquez-Semadeni 1999; Elmegreen 2000; Hartmann, Ballesteros-Paredes & Bergin 2001; Mac Low & Klessen 2004; Elmegreen 2007), the star-forming core has only little time to interact with the surrounding medium.”
How do massive black holes get their gas (2009, background): “…arguments that suggest that they quickly dissolve on a few local dynamical times, probably via feedback from some combination of stellar winds, H II regions and radiation pressure (Larson 1981; Blitz 1993; Bonnell et al. 2006; Krumholz, Matzner & McKee 2006; Allen et al. 2007; Elmegreen 2007).”
THE LONG-TERM EVOLUTION OF THE GALACTIC DISK TRACED BY DISSOLVING STAR CLUSTERS (2010, background): “There is considerable debate about whether star cluster formation occurs in a single crossing time (Elmegreen 2000; Hartmann et al. 2001; Elmegreen 2007; Pflamm-Altenburg & Kroupa 2007) or over 3 − 4 crossing times (Tan et al. 2006; Huff & Stahler 2006; Krumholz & Tan 2007; Jeffries 2007; Nakamura…”
The mass function of young star clusters in spiral galaxies (2008, background): “In general, the ISM in galaxies is hierarchically structured also on scales above the scale of individual GMCs (Elmegreen & Falgarone 1996; Elmegreen 2007) and SGMC-like structures may appear conspicuous in the Antennae simply because a larger fraction of the gas is in molecular form there (Gao et…”
On the fraction of star formation occurring in bound stellar clusters (2012, background): “For this, it is needed to assume a star formation law, which has been the topic of extensive debate in the recent literature (e.g. Elmegreen 2002; Krumholz & McKee 2005; Elmegreen 2007; Krumholz & Tan 2007; Padoan & Nordlund 2011).”
Clump morphology and evolution in MHD simulations of molecular cloud formation (2008, background): “After some 20 Myr of evolution, some regions have already undergone local collapse and started to form stars, while other clumps do not yet show signs of star formation, similarly to the suggestion by Elmegreen (2007) for clouds behind the spiral arms of the Galaxy.”
Why are most molecular clouds not gravitationally bound (2011, background): “However turbulence can also promote collapse on small scales (Klessen et al. 2000; Mac Low & Klessen 2004), (including MHD turbulence Heitsch et al. 2001; Vázquez-Semadeni et al. 2005; Elmegreen 2007).”
Cooling, Gravity, and Geometry: Flow-driven Massive Core Formation (2007, background): “The issue of cloud lifetimes is currently a matter of debate (e.g. Krumholz & Tan 2007; Elmegreen 2007, McKee & Ostriker 2007), and seems to depend strongly on the galactic environment (Hartmann et al. 2001; Heitsch et al. 2006).”