Date : 7/21 (Thu) [15:00 - 17:00] @ F227
Speaker: Prof. Kazunari Iwasaki (Doushisha University)
Title : A Mechanism for Turbulence Generation inside Interstellar Clouds
Abstract: It is well known that the interstellar medium consists of two thermal equilibrium states, i.e., a clumpy cold phase and a diffuse warm phase as a result of the balance of radiative cooling and heating. Koyama and Inutsuka (2002) and many authors have shown that cold clouds have supersonictranslational velocity in a surrounding diffuse warm gas. The supersonic translational motion of cold clouds can be origin of the observed "supersonic" turbulence. However, the detailed physical processes in the multi-phase turbulence are still unknown. In this study, we investigate the turbulence generation mechanism inside the cold clouds that is important in star formation. One of the promising mechanisms is the Kelvin-Helmholtz (KH) instability induced by a velocity shear between the cold clouds and surrounding warm gas. We performed multi-dimensional numerical simulations of the KH instability with/without a radiative cooling/heating and the thermal conduction. As an initial condition, a static cold layer sandwiched by a warm gas with velocity shear is considered. Without cooling/heating and thermal conduction, the KH instability drives only a weak turbulence with a velocity dispersion of 0.1-0.2 km/s because of the high density contrast. On the other hand, from the simulations with cooling/heating and thermal conduction, we found that the phase transition significantly enhances the velocity dispersion inside the cold layer. The mechanism is as follows: the roll-up of the interface induces the phase transition from the warm gas to the cold gas. The phase transition increases the velocity dispersion in the cold layer because the warm gas streams with high velocity. As a result, the KH instability with the phase transition drives a transonic turbulence.

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