Clump morphology and evolution in MHD simulations of molecular cloud formation

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.

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