Comet-Tail Structures in Emission Nebulae.

Abstract

A catalogue is given of a large number of "comet-tail" or "elephant-trunk" structures in several emission nebulae whose distances are known from spectroscopic parallaxes The dimensions of the comet- tail structures and the orientations of their axes with respect to the directions to the 0 stars exciting the nebulae are listed The forms of these structures are in general agreement with the theory of their origin presented by van de llulst, Spitzer, Frieman, Layzer, and others. The observational evidence suggests that in the final stages of development of these structures their tips become larger in diameter than their necks and eventually break off, forming separate droplets Several different observations show that the densities in the bright ionized rims of the comet-tail structures are considerably higher than the mean densities of the emission nebulae to which they belong Finally, the application of these structures to the galactic-structure problems of locating distant 0 stars and of determining the distances of H ii regions is discussed