Model simulations of a shock-cloud interaction in the Cygnus loop

We present optical observations and two-dimensional hydrodynamic modeling of an isolated shocked ISM cloud. H alpha images taken in 1992.6 and 2003.7 of a small optical emission cloud along the southwestern limb of the Cygnus Loop were used to measure positional displacements of similar to 0."1 yr(-1) for surrounding Balmer-dominated emission filaments and 0."025-0."055 yr(-1) for internal cloud emission features. These measurements imply transverse velocities of similar or equal to 250 and similar or equal to 80-140 km s(-1) for ambient ISM and internal cloud shocks, respectively. A lack of observed turbulent gas stripping at the cloud-ISM boundary in the H alpha images suggests that there is not an abrupt density change at the cloud-ISM boundary. Also, the complex shock structure visible within the cloud indicates that the cloud's internal density distribution is two-phased-a smoothly varying background density that is populated by higher density clumps. Guided by the H alpha images, we present model results for a shock interacting with a non-uniform ISM cloud. We find that this cloud can be well modeled by a smoothly varying power-law core with a density contrast of similar to 4 times the ambient density, surrounded by a low-density envelope with a Lorentzian profile. The lack of sharp density gradients in such a model inhibits the growth of Kelvin-Helmholtz instabilities, consistent with the cloud's appearance. Our model results also suggest that cloud clumps have densities similar to 10 times the ambient ISM density and account for similar to 30% of the total cloud volume. Moreover, the observed spacing of internal cloud shocks and model simulations indicate that the distance between clumps is similar to 4 clump radii. We conclude that this diffuse ISM cloud is best modeled by a smoothly varying, low-density distribution coupled to higher density, moderately spaced internal clumps.