Clump morphology and evolution in MHD simulations of molecular cloud formation

By Robi Banerjee, E. Vázquez-Semadeni, P. Hennebelle, Ralf Klessen. 2008

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We study the properties of clumps formed in three-dimensional weakly magnetized magnetohydrodynamic simulations of converging flows in the thermally bistable, warm neutral medium (WNM). We find the following. (1) Similarly to the situation in the classical two-phase medium, cold, dense clumps form through dynamically triggered thermal instability in the compressed layer between the convergent flows, and are often characterized by a sharp density jump at their boundaries though not always. (2) However, the clumps are bounded by phase-transition fronts rather than by contact discontinuities, and thus they grow in size and mass mainly by accretion of WNM material through their boundaries. (3) The clump boundaries generally consist of thin layers of thermally unstable gas, but these layers are often widened by the turbulence, and penetrate deep into the clumps. (4) The clumps are approximately in both ram and thermal pressure balance with their surroundings, a condition which causes their internal Mach numbers to be comparable to the bulk Mach number of the colliding WNM flows. (5) The clumps typically have mean temperatures 20 ≲〈T〉≲ 50 K, corresponding to the wide range of densities they contain (20 ≲n≲ 5000 cm−3) under a nearly isothermal equation of state. (6) The turbulent ram pressure fluctuations of the WNM induce density fluctuations that then serve as seeds for local gravitational collapse within the clumps. (7) The velocity and magnetic fields tend to be aligned with each other within the clumps, although both are significantly fluctuating, suggesting that the velocity tends to stretch and align the magnetic field with it. (8) The typical mean field strength in the clumps is a few times larger than that in the WNM. (9) The magnetic field strength in the densest regions within the clumps (n∼ 104 cm−3) has a mean value of B∼ 6 μG but with a large scatter of nearly two orders of magnitude, implying that both sub- and supercritical cores are formed in the simulation. (10) In the final stages of the evolution, the clumps' growth drives them into gravitational instability, at which point star formation sets in, and the pressure in the clumps' centres increases even further.

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

View Paper: https://www.semanticscholar.org/paper/29c88e6f3a8d30d04857d504765f669af939ef7e

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Citations in the corpus

Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “…gas suffers a phase transition from the warm to the cold phase upon entering the clouds, and thus the cloud boundaries are phase transition fronts (Banerjee et al. 2009), but no restriction to the flow of gas across these fronts exists (Ballesteros-Paredes et al. 1999a; Hennebelle & Pérault…”
Gravity or turbulence? Velocity dispersion–size relation (2010, background): “…times in a GMC, the densest clumps collapse significantly sooner than the GMC at large, and begin forming stars before the bulk of the GMC has collapsed, as observed in simulations (e.g. Vázquez-Semadeni et al. 2007; Heitsch et al. 2008; Banerjee et al. 2009; Vázquez-Semadeni et al. 2010).”
H II REGIONS: WITNESSES TO MASSIVE STAR FORMATION (2010, background): “…pressure, a refinement criterion to resolve the Jeans length (Banerjee et al. 2004), various cooling processes of relevance for protostellar collapse (Banerjee et al. 2004; Banerjee & Pudritz 2006; Banerjee et al. 2006); and Lagrangian sink particles (Banerjee et al. 2009; Federrath et al. 2009).”
TOWARD UNDERSTANDING THE COSMIC-RAY ACCELERATION AT YOUNG SUPERNOVA REMNANTS INTERACTING WITH INTERSTELLAR CLOUDS: POSSIBLE APPLICATIONS TO RX J1713.7−3946 (2011, background): “…clouds are formed as a complex of clumps fragmented by the thermal instability that are embedded in diffuse gas (Koyama & Inutsuka 2002; Hennebelle et al. 2008; Inoue & Inutsuka 2008; 2009; Banerjee et al. 2009; Heitsch et al. 2009; VázquezSemadeni et al. 2006, Audit & Hennebelle 2010).”
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).”
The Formation and Destruction of Molecular Clouds and Galactic Star Formation (2015, background): “…density, this formation timescale is shorter than that of molecular cloud formation solely through the accumulation of diffuse WNM (see, e.g., Koyama & Inutsuka 2002; Hennebelle et al. 2008; Heitsch & Hartmann 2008; Banerjee et al. 2009; Vázquez-Semadeni et al. 2011, for the cases of WNM flows).”
Modelling CO formation in the turbulent interstellar medium (2009, background): “…& Pérault 1999, 2000; Koyama & Inutsuka 2000, 2002; Bergin et al. 2004) or on a detailed treatment of the dynamics, but without any treatment of the chemistry (e.g. Balsara et al. 2004; Kritsuk & Norman 2004; Slyz et al. 2005; Hennebelle & Audit 2007; Hennebelle et al. 2008; Banerjee et al. 2009).”
Turbulent molecular clouds (2012, background): “5 (e.g. Hennebelle et al. 2008, Banerjee et al. 2009), which accords well with the analytical predictions deduced above.”
Importance of the Initial Conditions for Star Formation - I. Cloud Evolution and Morphology (2010, background): “Banerjee et al. (2009), Peters et al. (2011) and Hennebelle et al. (2011) noted reduced fragmentation in high-mass cores.”
A NEW JEANS RESOLUTION CRITERION FOR (M)HD SIMULATIONS OF SELF-GRAVITATING GAS: APPLICATION TO MAGNETIC FIELD AMPLIFICATION BY GRAVITY-DRIVEN TURBULENCE (2011, background): “…massive star clusters (Mac Low & Klessen 2004; Schneider et al. 2010), and the galactic spiral shock, producing large-scale converging flows (Vishniac 1994; Walder & Folini 1996; VázquezSemadeni et al. 2006; Heitsch et al. 2006; Hennebelle et al. 2008; Banerjee et al. 2009; Folini et al. 2010).”
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).”
Planck intermediate results: XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds (2015, unknown): “…(sub-Alfv´enic), the ﬁelds produce a clear anisotropy in MHD turbulence (Sridhar & Goldreich 1994; Goldreich & Sridhar 1995; Matthaeus et al. 2008; Banerjee et al. 2009) and compression by shocks that is favored to occur along the magnetic ﬁeld lines, creating structures perpendicular to the…”
Is molecular gas necessary for star formation (2011, methodology): “…simple two-parameter cooling function introduced by Koyama & Inutsuka (2002) and used in a number of later works (e.g. Vázquez-Semadeni et al. 2007; Banerjee et al. 2009), to the more detailed treatment used in e.g. Audit & Hennebelle (2005) that accounts explicitly for photoelectric heating from…”
Probing the evolution of molecular cloud structure: From quiescence to birth (2009, unknown): “This can be directlyaddressed by a comparison of our data to simulations that include self-gravity and follow the evolution of cloud structure asa function of time (e.g. Offner et al. 2008; Banerjee et al. 2009).”
AN IMPRINT OF MOLECULAR CLOUD MAGNETIZATION IN THE MORPHOLOGY OF THE DUST POLARIZED EMISSION (2013, result): “…with respect to B. Alignment between B and the gas velocity field has been reported in previous works (Ostriker et al. 2001; Ossenkopf & Mac Low 2002; Banerjee et al. 2009; Vázquez-Semadeni et al. 2011) but the exact process which causes this relative orientation is not well understood yet.”
MODELING COLLAPSE AND ACCRETION IN TURBULENT GAS CLOUDS: IMPLEMENTATION AND COMPARISON OF SINK PARTICLES IN AMR AND SPH (2010, background): “…(about 10−14 g cm−3), when the gas is still in the roughly isothermal regime, as for instance in the large-scale simulations of colliding flows by Banerjee et al. (2009) and in the turbulent box calculations by Padoan & Nordlund (2009), where sink particles are formed at much lower densities…”
LIMITING ACCRETION ONTO MASSIVE STARS BY FRAGMENTATION-INDUCED STARVATION (2010, background): “Numerical calculations 9 of the dynamical evolution of supersonically turbulent clouds show that transient cores forming at the stagnation points of convergent flows exhibit similar morphology (Ballesteros-Paredes et al. 2003; Klessen et al. 2005; Banerjee et al. 2009).”
The SILCC (SImulating the LifeCycle of molecular Clouds) project – I. Chemical evolution of the supernova-driven ISM (2014, background): “As the bubble(s) expand and collide (similar to a typical colliding flow scenario, e.g. Banerjee et al. 2009; Heitsch, Naab & Walch 2011) the H2 distribution in case of peak driving is dispersed and becomes very filamentary.”