Hydrodynamics of Cloud Collisions in Two Dimensions: The Fate of Clouds in a Multiphase Medium

We have studied head-on collisions between equal-mass, mildly supersonic H I clouds (Mach number 1.5 with respect to the background medium) through high-resolution numerical simulations in two dimensions. We explore the role of various factors, including the radiative cooling parameter, η = τrad/τcoll (τcoll = Rc/vc), evolutionary modifications on the cloud structure, and the symmetry of the problem. Self-gravity is not included. Radiative losses are taken into account explicitly and not approximated with an isothermal adiabatic index γ ≈ 1, which, in fact, leads to very different results. We assume a standard two-phase interstellar medium (ISM) model where clouds are characterized by a temperature Tc = 74 K and number density nc = 22 cm-3 and are in pressure equilibrium with the surrounding warm intercloud medium (WIM), with a density contrast χ = ρc/ρi = 100. In particular, we study collisions for the adiabatic (η ≫ 1) and radiative (η = 0.38) cases that may correspond to small (Rc ≤ 0.4 pc for an assumed WIM) or large (Rc ~ 1.5 pc) clouds, respectively. In addition to a standard case of identical "nonevolved" clouds, we also consider the collision of identical clouds, "evolved" through independent motion within the intercloud gas, over one crushing time before collision. This turns out to be about the mean collision time for such clouds in the ISM. The presence of bow shocks and ram pressure from material in the cloud wake alters these interactions significantly with respect to the standard case. In some cases, we removed the mirror symmetry from the problem by colliding initially identical clouds "evolved" to different ages before impact. In those cases, the colliding clouds have different density and velocity structures, so that they provide a first insight on the behavior of more complex interactions. In our adiabatic collisions, the clouds are generally disrupted and convert their gas into the warm phase of the ISM. Although the details depend on the initial conditions, the two colliding clouds are converted into a few low-density contrast (χ ~ 5) clumps at the end of the simulations. By contrast, for symmetric radiative cases, we find that the two clouds coalesce, and there are good chances for a new massive cloud to be formed. Almost all the initial kinetic energy of the two clouds is radiated away during such collisions. On the other hand, for both adiabatic and radiative collisions, symmetry breaking leads to major differences. Most importantly, asymmetric collisions have a much greater tendency to disrupt the two clouds. Portions of individual clouds may be sheared away, and instabilities along the interfaces between the clouds and with the intercloud medium are enhanced. In addition, radiative cooling is less efficient in our asymmetric interactions, so that those parts of the clouds that initially seem to merge are more likely to reexpand and fade into the warm intercloud medium. Since the majority of real cloud collisions should be asymmetric for one reason or another, we conclude that most gasdynamical diffuse cloud collisions will be disruptive, at least in the absence of significant self-gravity or a significant magnetic field.