Small scale structure in circumstellar envelopes and the origin of globules in planetary nebulae

Abstract

We analyze the small scale structure in the circumstellar envelopes of NGC 7027 and IRC+10216, using high resolution optical images in dust scattered light. We use the observations to test the proposal that globules in planetary nebulae, typified by globules in the Helix nebula, originate in high density contrast proto-globules in the atmosphere of the progenitor AGB star and are carried out in the circumstellar wind (Dyson et al. 1989). We find no evidence for the presence of proto- globules in the extended envelopes of NGC 7027 and IRC+10216 with the expected sizes and masses>10 5 M which are needed to produce globules like those in the Helix nebula. We do find azimuthal structure in the envelopes on size scales of l> 0:1d where d is the radial distance, consistent with the smoothing out of small scale structure by thermal motions in the wind acceleration region. Unless globules require special conditions not found in NGC 7027 or IRC+10216, which are among the most likely precursors of Helix-like nebulae, our results argue against their origin in the atmosphere of the central star. We suggest alternative scenarios for globule formation, including the fragmentation of the neutral circumstellar gas at the transition phase by directed outflows or jets.