ON THE EARLY EVOLUTION OF ASPHERICAL WOLF-RAYET BUBBLES

We present several models of the evolution of young Wolf-Rayet (WR) ri ng nebulae. We focus on the transient but non-negligible phase during which the stellar wind is accelerated, from a velocity of about 10 km s(-1) in the red supergiant (RSG) stage to velocities of the order of 2000 km s(-1), typical of WR winds. The WR wind interacts with the fre e RSG wind and sweeps up an accelerating shell. This shell experiences a strong Rayleigh-Taylor instability and fragments, producing dense k nots and filaments. Note that this happens before it interacts with th e shell swept up by the RSG wind. We also assume an anisotropic RSG wi nd. Initially the outer shock of the nebula is isothermal, given its l ow velocity and the high density of the RSG wind close to the star. La ter on the shock becomes adiabatic and the transition to the fast adia batic regime occurs earlier along the polar direction, where the densi ty is lower. In this phase the post-shock gas of the adiabatic shock i s hot enough to emit soft X-rays. We compare our results with the well studied WR ring nebula NGC 6888. It shows an ellipsoidal, filamentary shell at optical wavelengths, while in the X-ray two lobes are presen t in opposite zones along the major axis. We find that our simple mode l may qualitatively explain these features. Our hypotheses are not spe cific for NGC 6888 and we expect that the 'two-lobe' X-ray morphology is a common attribute of young ring nebulae.