ON THE MOTIONS OF GAS AND DUST IN NGC 2068

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Hubble, working with slitless spectrograms, long ago noticed a relationship between the spectrum of a diffuse nebula and the spectral type of the star that illuminates it (Hubble 1922). Nebulae associated with stars of spectral type Bl or later were found to have continuous spectra, while those associated with stars of earlier spectral types showed emission lines but little or no visible continuum. Slipher (1912) had earlier shown that a nebula with a strong continuum generally has an absorption spectrum matching that of its illuminating star; this observation clearly indicated that virtually all of the light from such nebulae is simply starlight scattered by solid particles. The lack of emission lines in the spectra of nebulae associated with stars of the later spectral classes is readily explained by the rapidly decreasing size of the ionization regions around such stars. On the other hand, dust in the vicinity of stars with large ionization regions will, under certain circumstances (see e.g., Mathews 1969), tend to be destroyed or expelled very rapidly. The combination of these two effects results in a rather abrupt transition between emission nebulae and reflection nebulae; nevertheless it is possible to find examples of nebulae showing both characteristics. Consider a spherically symmetric nebula in this transition region. We are presented with three distinct spectra: the direct (though