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P.J. Huggins (NYU), N. Mauron (Univ. de Montpellier)
We analyze the small scale structure of the archetypical 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 of Dyson et al. (1989 MNRAS 241, 625) that globules in planetary nebulae, typified by the globules in the Helix nebula, originate in high density condensations in the atmosphere of the progenitor AGB star and are carried out in the circumstellar wind. We find no evidence for the presence of ``proto-globules'' in the envelopes of NGC 7027 and IRC+10216 with masses > 10-5 solar masses which are expected for the precursors of Helix-type globules. We do find azimuthal structure on size scales of d > 0.1 r where r is the radial distance, consistent with the smoothing out of small scale structure in the ejected material by thermal motions or turbulence in the wind acceleration region. Unless the proposed formation and ejection of proto-globules requires special circumstances that are not represented in the envelopes examined, our results provide strong evidence against this mechanism as the origin of Helix-like globules in planetary nebulae.
This work was supported in part by NSF AST 99-86159.