Molecular Cloud Evolution. I. Molecular Cloud and Thin Cold Neutral Medium Sheet Formation

By E. Vázquez-Semadeni, D. Ryu, T. Passot, R. González, A. Gazol. 2005

207 citations recorded by the source corpus.

We discuss molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). Initially, a shocked layer forms, and within it, a thin cold layer. An analytical model and high-resolution one-dimensional simulations predict the thermodynamic conditions in the cold layer. After ~1 Myr of evolution, the layer has column density ~2.5 × 1019 cm-2, thickness ~0.03 pc, temperature ~25 K, and pressure ~6650 K cm-3. These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the one-dimensional simulations, the inflows into the sheets produce line profiles with a central line of width ~0.5 km s-1 and broad wings of width ~1 km s-1. Three-dimensional numerical simulations show that the cold layer develops turbulent motions and increases its thickness until it becomes a fully three-dimensional turbulent cloud. Fully developed turbulence arises on times ranging from ~7.5 Myr for inflow Mach number M1,r = 2.4 to >80 Myr for M1,r = 1.03. These numbers should be considered upper limits. The highest density turbulent gas (HDG, n > 100 cm-3) is always overpressured with respect to the mean WNM pressure by factors of 1.5-4, even though we do not include self-gravity. The intermediate-density gas (IDG, 10 < n/cm-3 < 100) has a significant pressure scatter that increases with M1,r, so that at M1,r = 2.4 a significant fraction of the IDG is at a higher pressure than the HDG. Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compressive process that forms the clouds themselves and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic.

Subfields: Applied Mathematics, Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/040df935c5feae0f7491f31f3f1096a7555a4381

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Citations in the corpus

Gravity or turbulence? II. Evolving column density PDFs in molecular clouds (2011, background): “…the same cloud at different evolutionary times appears as if it were different kind of objects: at earlier times, the compressed layer could be classified as a diffuse H I cloud, with low column densities and thus, very little CO content (see Heitsch et al. 2006; Vázquez-Semadeni et al. 2006).”
Gravity or turbulence? Velocity dispersion–size relation (2010, background): “…& Pérault 1999) with the result that the gas cools rapidly, producing a dense, cold, turbulent cloud (Walder & Folini 1998; Koyama & Inutsuka 2002; Heitsch et al. 2005, 2006; Vázquez-Semadeni et al. 2006) which soon becomes Jeans unstable (Vázquez-Semadeni et al. 2007; Heitsch et al. 2008).”
The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows (2006, background): “One of the main differences to the present study is that they impose velocity perturbations on their incoming gas flow – as do Vázquez-Semadeni et al. (2006), although as “random noise” at a percent-level – , whereas we defer the structure generation to the actual flow collision site.”
The Global Evolution of Giant Molecular Clouds. I. Model Formulation and Quasi-Equilibrium Behavior (2006, unknown): “…is that they are gravitationally unbound transient fluctuations in the atomic ISM; only local subregions constituting a small fraction of the cloud mass are bound and can collpase (Mac Low & Klessen 2004; Clark & Bonnell 2004; Clark et al. 2005; Vazquez-Semadeni et al. 2006; Dobbs et al. 2006).”
From the warm magnetized atomic medium to molecular clouds (2008, background): “Indeed, this gas has been previously shocked and it is in the process of cooling as it moves towards the dense cold regions (Vázquez-Semadeni et al. 2006; Hennebelle & Audit 2007).”
Theory of Star Formation (2007, background): “One conception of GMCs is that they are dynamic, transient entities in which the turbulence is driven by large-scale colliding gas flows that create the cloud (e.g. Heitsch et al. 2005; Vázquez-Semadeni et al. 2006; BallesterosParedes et al. 2007).”
Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region (2013, background): “This however is not necessarily so if the L1495/B213 complex was formed by the convergence of large-scale flows, a mechanism favored by a number of theoretical considerations and numerical simulations (e.g., Elmegreen 1993; Vázquez-Semadeni et al. 2006; Hennebelle et al. 2008; Heitsch et al. 2008).”
Cooling, Gravity, and Geometry: Flow-driven Massive Core Formation (2007, background): “…the converging flow models demonstrate with ease that flows provide a natural mechanism for the generation of structure and turbulence in clouds (Audit & Hennebelle 2005; Heitsch et al. 2005; Vázquez-Semadeni et al. 2006; Heitsch et al. 2006; Hennebelle et al. 2007; Hennebelle & Audit 2007).”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “…of the velocity dispersion is only moderately supersonic (sonic Mach numbers Ms of a few; e.g., Koyama & Inutsuka 2002; Audit & Hennebelle 2005; Vázquez-Semadeni et al. 2006, 2007; Banerjee et al. 2009), significantly weaker than the highly supersonic regime (Ms & 10) known to exist in GMCs…”
Clump morphology and evolution in MHD simulations of molecular cloud formation (2008, methodology): “This process also causes the newly formed dense gas to be turbulent, with a transonic velocity dispersion with respect to its own sound speed (Koyama & Inutsuka 2002; Heitsch et al. 2005; Vázquez-Semadeni et al. 2006; Hennebelle et al. 2008).”
Comparing the statistics of interstellar turbulence in simulations and observations - Solenoidal versus compressive turbulence forcing (2009, background): “5 http://www.ita.uni-heidelberg.de/ chfeder/videos.shtml?lang=en (e.g., Heitsch et al. 2006; Vázquez-Semadeni et al. 2006; Hennebelle et al. 2008; Banerjee et al. 2008) are more suitable for studying the boundary effects during the formation of molecular clouds.”
Accretion-driven turbulence as universal process: galaxies, molecular clouds, and protostellar disks (2009, background): “…accretion energy is converted into turbulent energy we resort to numerical simulations of converging flows (e.g. Audit & Hennebelle 2005; Heitsch et al. 2005, 2006a; Folini & Walder 2006; Vázquez-Semadeni et al. 2006, 2007; Hennebelle et al. 2008; Banerjee et al. 2009; Audit & Hennebelle 2010).”
Simulating the Formation of Molecular Clouds. II. Rapid Formation from Turbulent Initial Conditions (2006, background): “…a similar effect in unbound atomic regions in which significant turbulence is present (such as the thin, dense sheets formed in the simulations of Audit & Hennebelle 2005; Vázquez-Semadeni et al. 2006) provided that enough gas is present to allow for effective self-shielding of the resulting H2.”
Physical Processes in the Interstellar Medium (2014, background): “…leading to the rapid production of a cloud of much denser, cooler gas (see e.g. Hennebelle & Pérault, 1999, 2000; Koyama & Inutsuka, 2002; Audit & Hennebelle, 2005; Heitsch et al., 2005; Vázquez-Semadeni et al., 2006; Hennebelle & Audit, 2007; Heitsch & Hartmann, 2008; Banerjee et al., 2009).”