Numerical simulations of wind bow shocks produced by runaway OB stars

We present the results of a study focused on the morphology and dynamical evolution of wind bow shocks produced by massive runaway stars in the diffuse interstellar medium. A revision of current analytical models describing the shape and physical parameters of a wind bow shock is proposed, motivated by the recognition that the shocked stellar wind does not need to incorporate into the bow shock as a result of its long cooling time. The semi-analytical expressions we derive, suggest that actual surface densities and velocity patterns in wind bow shocks differ significantly from those previously predicted using the assumption of instantaneous cooling of the shocked gas. To test the general validity of these simplified models and to obtain a more realistic picture of the bow shock characteristics, we have carried out a number of two-dimensional numerical simulations. We have examined the influence of the stellar wind strength, the ambient interstellar density, and the relative velocity of the star on the resulting structures. Particular attention is paid to the role of the physical ingredients used to model the dynamics of the gas, especially the effects of finite cooling times and thermal conduction. The numerical simulations, while confirming in general terms the validity of our simplified, semi-analytical approach, reveal a wealth of details in the structure and evolution of wind bow shocks. By exploring a representative range of the parameter space, we show that the interaction of the stellar wind with the ambient medium can give rise to a remarkable diversity of structures, even with moderate changes in the basic input parameters. Depending on these, a bow shock may be stable or unstable, have a simple or a layered structure with different density and temperature domains, or may not form at all. In this context, we briefly discuss the potential of observations of wind bow shocks as diagnostic tools of the stellar wind of the runaway star and the medium in which it travels.