On the ring nebulae around runaway Wolf-Rayet stars

Wolf-Rayet stars are advanced evolutionary stages of massive stars. Despite their large mass-loss rates and high wind velocities, none of them displays a bow shock, although a fraction of them are classified as runaway. Our 2.5-D numerical simulations of circumstellar matter around a 60-M-circle dot runaway star show that the fast Wolf-Rayet stellar wind is released into a wind-blown cavity filled with various shocks and discontinuities generated throughout the preceding evolutionary phases. The resulting fast-wind-slow-wind interaction leads to the formation of spherical shells of swept-up dusty material similar to those observed in the near-infrared at 24 mu m with Spitzer, which appear to be comoving with the runaway massive stars, regardless of their proper motion and/or the properties of the local ambient medium. We interpret bright infrared rings around runaway Wolf-Rayet stars in the Galactic plane as an indication of their very high initial masses and complex evolutionary history. Stellar-wind bow shocks become faint as stars run in diluted media, therefore our results explain the absence of bow shocks detected around Galactic Wolf-Rayet stars, such as the high-latitude, very fast-moving objects WR71, WR124 and WR148. Our results show that the absence of a bow shock is consistent with the runaway nature of some Wolf-Rayet stars. This questions the in situ star formation scenario of high-latitude Wolf-Rayet stars in favour of dynamical ejection from birth sites in the Galactic plane.