Model Simulations of a Shock-cloud Interaction in the Cygnus Loop

We present optical observations and 2D hydrodynamic modeling of an isolated shocked ISM cloud. Hα 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 ∼ 0. 1 yr for surrounding Balmer dominated emission filaments and 0. 025 − . 055 yr for internal cloud emission features. These measurements imply transverse velocities of ≃ 250 km s and ≃ 80 – 140 km s for ambient ISM and internal cloud shocks respectively. A lack of observed turbulent gas stripping at the cloud– ISM boundary in the Hα 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 which is populated by higher density clumps. Guided by the Hα 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 ∼ 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 KelvinHelmholtz instabilities, consistent with the cloud’s appearance. Our model results also suggest that cloud clumps have densities ∼ 10 times the ambient ISM density and account for ∼ 30% of the total cloud volume. Moreover, the observed spacing of internal cloud shocks and model simulations indicate that the distance between clumps is ∼ 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. Subject headings: ISM: individual (Cygnus Loop) — supernova remnants — ISM: kinematics and dynamics — shock waves — hydrodynamics