Flow-driven cloud formation and fragmentation: results from Eulerian and Lagrangian simulations

Abstract

The fragmentation of shocked flows in a thermally bistable medium provides a natural mechanism to form turbulent cold clouds as precursors to molecular clouds. Yet because of the large density and temperature differences and the range of dynamical scales involved, following this process with numerical simulations is challenging. We compare two‐dimensional simulations of flow‐driven cloud formation without self‐gravity, using the Lagrangian smoothed particle hydrodynamics (SPH) code vine and the Eulerian grid code proteus. Results are qualitatively similar for both methods, yet the variable spatial resolution of the SPH method leads to smaller fragments and thinner filaments, rendering the overall morphologies different. Thermal and hydrodynamical instabilities lead to rapid cooling and fragmentation into cold clumps with temperatures below 300 K. For clumps more massive than 1 M⊙ pc−1, the clump mass function has an average slope of −0.8. The internal velocity dispersion of the clumps is nearly an order of magnitude smaller than their relative motion, rendering it subsonic with respect to the internal sound speed of the clumps but supersonic as seen by an external observer. For the SPH simulations most of the cold gas resides at temperatures below 100 K, while the grid‐based models show an additional, substantial component between 100 and 300 K. Independent of the numerical method, our models confirm that converging flows of warm neutral gas fragment rapidly and form high‐density, low‐temperature clumps as possible seeds for star formation.

First Results from BISTRO: A SCUBA-2 Polarimeter Survey of the Gould Belt

“…Theoretical studies have shown that both B-fields (e.g., Li & Nakamura 2004;Basu et al 2009) and turbulence (e.g., Klessen et al 2000;Heitsch et al 2011) can significantly affect how dense structures form, collapse, and evolve in the interstellar medium. For example, one paradigm of low-mass star formation suggests that collapse is guided by B-fields, producing flattened cores and disks (e.g., Mouschovias 1991).…”

Profiles of interstellar cloud filaments

“…The filaments are naturally formed as a result of interstellar turbulence (e.g., Padoan & Nordlund 2011), but significant contribution must exist from immediate triggering by supernova explosions and the radiation and stellar winds from massive stars (e.g., Peretto et al 2012). The filaments are expected to undergo gravitational fragmentation that will lead to star formation (e.g., Inutsuka & Miyama 1997;Myers 2009;Heitsch et al 2011).…”

The onset of large-scale turbulence in the interstellar medium of spiral galaxies

“…This phenomenon has been identified in numerical simulations (e.g. Hunter et al 1986;Walder & Folini 2000;Audit & Hennebelle 2005;Heitsch et al 2005;Vázquez-Semadeni et al 2006Heitsch, Naab & Walch 2011;Inoue & Fukui 2013;Folini et al 2014, and others), being promptly related to the collapse of dense strucutures in the ISM and star formation, and analytically described by Vishniac (1994) as the nonlinear evolution of perturbations in shock-bounded slabs.…”

“…Converging flows have been considered as one of the main mechanisms for the formation of molecular clouds (Audit & Hennebelle 2005;Heitsch et al 2006;Hennebelle et al 2007;Banerjee et al 2009;Heitsch, Naab & Walch 2011), and could also be the cause of their turbulence. Strong shocks combined to efficient cooling of the downstream gas result in very dense and cold thin layers.…”