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P. J. Huggins (NYU), A. Frank (Univ. of Rochester)
We present a new model for the formation of globules in planetary nebulae (PNe) based on the magnetic Rayleigh-Taylor (MRT) instability. We show that the presence of a magnetic field in the circumstellar gas can play an important role in the development of structure in PNe and affect the fragmentation of PN shells which are formed by fast winds or ionization fronts. Using field strengths measured in the precursor AGB envelopes, we find that close to the central star where the fields are relatively strong, the wavelengths of unstable MRT modes are larger than the shell dimensions, and the development of small scale structure is suppressed. The wavelength of the most unstable MRT mode decreases with increasing distance from the star as the fields become weaker, and when the wavelength becomes comparable to the shell thickness, it leads to the sudden, rapid break-up of the shell. For typical PN parameters and magnetic fields comparable to those observed in AGB envelopes, the model predicts shell break-up into numerous fragments with a mass scale and a separation scale similar to those of globules observed in PNe. Our results provide a link between MHD models of global PN shaping and the formation of small scale structures in the nebulae. This work is supported in part by NSF AST 03-07277 and NSF AST 05-07519
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.