On the early evolution of aspherical Wolf-Rayet bubbles

Abstract

We present several models of the evolution of young Wolf—Rayet (WR) ring 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 free RSG wind and sweeps up an accelerating shell. This shell experiences a strong Rayleigh-Taylor instability and fragments, producing dense knots and filaments. Note that this happens before it interacts with the shell swept up by the RSG wind. We also assume an anisotropic RSG wind. Initially the outer shock of the nebula is isothermal, given its low velocity and the high density of the RSG wind close to the star. Later on the shock becomes adiabatic and the transition to the fast adiabatic regime occurs earlier along the polar direction, where the density is lower. In this phase the post-shock gas of the adiabatic shock is 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 present in opposite zones along the major axis. We find that our simple model may qualitatively explain these features. Our hypotheses are not specific for NGC 6888 and we expect that the ‘two-lobe’ X-ray morphology is a common attribute of young ring nebulae.