Structure of the Barnard Loop Nebula as Determined from Gemini 11 Photographs

Abstract

Ultraviolet objective-prism photographs in the 220() 4900 A wavelength region made from the Gemini 11 spacecraft have revealed an unexpected brightness of the Barnard Loop Nebula. Microphotometer tracings of the film have been reduced to absolute energy units by reference to stars of known flux. The form of the nebula differs significantly in the ultraviolet from that surface brightness distribution seen in the recombination line Ha By comparison with optical wavelength measurements it is seen that in the optically brightest region the observed surface brightness is slightly greater than that predicted for pure atomic emission and that the discrepancy is usually even larger for other regions. It is hypothesized that the excess radiation is due to starlight scattered by interstellar grains in the nebula. Owing to its low optical depth, this radiation should clearly outline the structure of the nebula in both the ionized and un-ionized regions. It is shown that the surface brightness distribution requires a nonhomogeneous density distribution, with the total atomic density increasing outward. Such a density distribution may be the result of a dynamical radial force due to stellar radiation pressure acting on the grains mixed with the gas.