HYDRODYNAMICS OF CLOUD COLLISIONS IN 2 DIMENSIONS - THE FATE OF CLOUDS IN A MULTIPHASE MEDIUM

We have studied head-on collisions between equal-mass, mildly superson ic H I clouds (Mach number 1.5 with respect to the background medium) through high-resolution numerical simulations in two dimensions. We ex plore the role of various factors, including the radiative cooling par ameter, eta = tau(rad)/tau(coll) (tau(coll) = R-c/upsilon(c)), evoluti onary modifications on the cloud structure, and the symmetry of the pr oblem. Self-gravity is not included. Radiative losses are taken into a ccount explicitly and not approximated with an isothermal adiabatic in dex gamma approximate to 1, which, in fact, leads to very different re sults. We assume a standard two-phase interstellar medium (ISM) model where clouds are characterized by a temperature T-c = 74 K and number density n(c) = 22 cm(-3) and are in pressure equilibrium with the surr ounding warm intercloud medium (WIM), with a density contrast chi = rh o(c)/rho(i) = 100. In particular, we study collisions for the adiabati c (eta much greater than 1) and radiative (eta = 0.38) cases that may correspond to small (R-c less than or equal to 0.4 pc for an assumed W IM) or large (R-c similar to 1.5 pc) clouds, respectively. In addition to a standard case of identical ''nonevolved'' clouds, we also consid er the collision of identical clouds, ''evolved'' through independent motion within the intercloud gas, over one crushing time before collis ion. This turns out to be about the mean collision time for such cloud s in the ISM. The presence of bow shocks and ram pressure from materia l in the cloud wake alters these interactions significantly with respe ct 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 adi abatic collisions, the clouds are generally disrupted and convert thei r gas into the warm phase of the ISM. Although the details depend on t he initial conditions, the two colliding clouds are converted into a f ew low-density contrast (chi similar to 5) clumps at the end of the si mulations. By contrast, for symmetric radiative cases, we find that th e two clouds coalesce, and there are good chances for a new massive cl oud to be formed. Almost all the initial kinetic energy of the two clo uds is radiated away during such collisions. On the other hand, for bo th adiabatic and radiative collisions, symmetry breaking leads to majo r differences. Most importantly, asymmetric collisions have a much gre ater tendency to disrupt the two clouds. Portions of individual clouds may be sheared away, and instabilities along the interfaces between t he clouds and with the intercloud medium are enhanced. In addition, ra diative cooling is less efficient in our asymmetric interactions, so t hat those parts of the clouds that initially seem to merge are more li kely to reexpand and fade into the warm intercloud medium. Since the m ajority of real cloud collisions should be asymmetric for one reason o r another, we conclude that most gasdynamical diffuse cloud collisions will be disruptive, at least in the absence of significant self-gravi ty or a significant magnetic field.