Ram pressure stripping in planetary nebulae

We present two-dimensional numerical simulations of the evolution of a low-mass star moving supersonically through its surrounding interstellar medium (ISM). We show that the ejecta of a moving star with a systemic velocity of 20 km s(-1) will interact with the ISM and will form bow shock structures qualitatively similar to what is observed. We find that, owing to ram pressure stripping, most of the mass ejected during the asymptotic giant branch (AGB) phase is left downstream of the moving star. As a consequence, the formation of the planetary nebula is highly influenced, even at the low relative velocity of the star. The models are based on the predictions of stellar evolution calculations. Therefore, the density and velocity of the AGB and post-AGB winds are time dependent and give rise to the formation of shock regions inside the cavity formed by the previous winds. As a result, the stand-off distance is also time dependent and cannot be determined by simple analytical arguments.