EPoS Contribution 
Gravitational Fragmentation of Expanding Shells
Kazunari Iwasaki Nagoya University, Nagoya, Japan  
We investigate the gravitational fragmentation of expanding shells driven by HII regions using the threedimensional Lagrangian simulation codes based on the Riemann solver, called Godunov smoothed particle hydrodynamics. The ambient gas is assumed to be uniform. In order to attain high resolution to resolve the geometrically thin dense shell, we calculate not the whole but a part of the shell. We find that perturbations begin to grow earlier than the prediction of the linear analysis under the thinshell approximation. The wavenumber of the most unstable mode is larger than that in the thinshell linear analysis. The development of the gravitational instability is accompanied by the significant deformation of the contact discontinuity. We derive useful analytic formulae for the fragment scale and the epoch when the gravitational instability begins to grow.  
Collaborators: S. Inutsuka, Nagoya Univ., Japan T. Tsuribe, Osaka Univ., Japan 
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