The Influence of Compressibility on the Interaction of a Clump with an Inhomogeneous Gaseous Layer

This work is devoted to one possible mechanism for forming inhomogeneous structures at the periphery of HII regions: compression of primary condensations under radiation. According to the model used, as the HII region expands in an inhomogeneous medium, between the shock and ionization fronts a dense, rapidly moving layer of neutral gas is formed. The gas in front of the layer is heated by the shock wave, and the layer itself acquires acceleration under the action of the difference in pressures of ionized hydrogen in the HII region and neutral gas in front of the shock wave. In this work, we investigate the influence of compressibility on the dynamics of immersion of a clump into a layer, as well as the inhomogeneous structure of the density, temperature, and entropy fields. Particular attention is paid to the stage of the clump penetration into the layer. It was found that with a decrease in the polytropic index and a corresponding increase in the compressibility coefficient, Archimedes’ force, which affects the collapse of the cavity, weakens. To study the distribution of the cloud substance over time and identify the cloud material from the layer, the calculations of two-dimensional unsteady motions of an ideal perfect gas in a constant gravity field in the presence of local inhomogeneity were performed. It was found that when a clump interacts with an inhomogeneous layer at the stage of cavity formation, the heated gas moves in the direction of the clump, shifting the extremum in the distribution of parameters from the symmetry axis. The cloud itself is strongly deformed when penetrating a dense cold gas layer.