FORMATION OF THE DARK BAYS IN THE CRAB OPTICAL SYNCHROTRON NEBULA - IS THE CRAB PULSAR WIND BUBBLE INTERACTING WITH ITS PROGENITOR WIND

We present models for the formation of the two dark bays in the Crab optical synchrotron nebula, from which the optical synchrotron-emitting particles seem to be excluded. This investigation is motivated by the recent proper motion observations of the bays reported by Fesen, Martin, & Shull. We propose that the bays are formed by the advance of the Crab pulsar's wind bubble into an ambient medium possessing a stratified density distribution with higher density in the plane of the bays than in other directions. The ambient density and degree of stratification required to produce the bays depends on whether the medium consists of general interstellar medium, a progenitor stellar wind, or fast-moving supernova ejecta. In the case of interstellar medium, the bays would be expected to move apart much more slowly than observed, and the required density would have to be implausibly high, given that the Crab Nebula is located approximately 200 pc below the Galactic plane. For fast-moving ejecta, the required densities inside the bays are well beyond the upper limits set by the lack of optical line emission from the bays, and the required density stratification is very large. We conclude that the pulsar wind bubble is probably interacting with the slow wind from its progenitor, and we show that bays formed in this way agree with the observations reasonably well.