On the ring nebulae around runaway Wolf–Rayet stars

Abstract

ABSTRACT 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 $60\mbox{-}\rm M_{\odot }$ 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 $24\, \rm \mu \rm m$ 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.